Mendel's hybridisation experiments with other plants than Pisum


Cetl, I.

Folia Facultatis Scientiarum Naturalium Universitatis Purkynianae Brunensis, Biologia 14(7): 3-42

1973


Mendel's mostly unpublished hybridisation experiments with other plants than Pisum were reconstructed and analysed on the basis of available documents. Special attention was payed to two series of these experiments. In the series of crossings with species and varieties from 14 plant genera, it was searched for proving the general validity of th regularities discovered in Pisum, especially of the segregation principle. In the species crossings with Ilieracium, the role of hybridisation in taxonomy and evolution of this genus was studied. In both series of experiments, Mendel endeavoured to solve the puzzle of the „constant" hybrids described by Gärtner.

FOLIA
FACULTATIS
SCIENTIARUM
NATURALIUM
UNIVERSITATIS
PURKYNIANAE
BRUNENSIS
TOMUS
XIV
BIOLOGIA
41
OPUS
7
1973
MENDEL'S
HYBRIDISATION
EXPERIMENTS
WITH
OTHER
PLANTS
THAN
PISUM
I.
CETL
.Department
of
Genetics,
Faculty
of
Science,
J.
E.
Purkyni
University,
Brno
CONTENTS
01
Ornamental
plants
6
02
Phaseolus
7
03
Species
and
varieties
of
14
plant
gener•t
11
03.01
Attempted
reconstruction
11
03.02
Intentions,
material,
and
experimental
methods
13
03.03
Evaluation
of
results
17
03.04
Appreciation
20
04
Ifieracium
21
04.01
Attempted
reconstruction
21
04.02
Choice
of
Hieracium
24
04.03
Initial
intentions
26
04.04
Floristical,
taxonomical
and
grower's
experience
26
04.05
Technique
of
crossing
28
04.06
Nageli's
influence
28
04.07
Evaluation
of
result
31
04.08
Appreciation
37
05
Decline
of
investigation
39
06
Summary
40.
07
Literature
41
3
„The
picture
that
emerges
is
of
a
man
very
actively
and
effectively
experimenting,
aware
of
the
importance
of
his
discovery,
and
testing
and
extending
it
on
a
wide
variety
of
forms.
None
of
these
results
were
published;
it
is
difficult
to
suppose
that
his
work
would
have
been
so
completely
ignored
if
he
had
presented
this
confirmatory
evidence...”
(Sturtevant,
1965,
p.
12).
In
addition
to
the
Pisum
experiments
Mendel
carried
out
hybridisations
with
many
other
plant
genera,
species,
and
varieties.
Except
his
Phaseolus
crosses
mentioned
in
the
Pisum
paper
and
a
short
Ilieracium
report,
nothing
was
published
by
him
in
that
field.
Iltis'
biography
(This,
1924,
p.
103-108)
gives
some
information
about
Mendel's
crosses
with
other
plants.
Nevertheless,
Mendel's
scientif►c
intentions
and
achieved
results
as
well
as
the
significance
of
these
experiments
in
the
development
of
his
way
of
thinking
are
often
omitted
or
contradictorily
presented
in
literature.
On
the
occasion
of
the
Gregor
Mendel
Colloquium
held
in
Brno
1970,
Orel
(1971)
subjected
Mendel's
classical
Pisum
paper
to
a
penetrating
reconstruction
and
analysis
front
the
viewpoint
of
Mendel's
way
of
presentation.
A
new
conception
resulting
from
it
was
examined
in
the
discussion;
it
was
suggested
that
it
is
necessary
to
reconstruct
and
analyse
also
all
other
Mendel's
experiments.
Sources
for
reconstructing
Mendel's
experiments
with
other
plants
compared
with
those
of
Pisum
are
limited.
Fortunately,
just
after
publishing
the
Pisuin
paper,
Mendel
opened
correspondence
with
Carl
Nageli.
In
his
letters;
he
mentioned
in
detail
the
majority
of
his
experiments
performed
after
1863.
These
letters
were
published
by
Correns
(1905)
and
later
edited
also
in
English
by
Piternick
and
Piternick
(1950)
and
newly
by
Stern
and
Sherwood
(1966),
in
Russian
by
Gaissinovitch
(1965);
cf.
also
Witte
(1971).
Additional
important
facts
are
rendered
also
in
the
fi
rst
nine
volumes
of
the
Proceedings
of
the
Brno
Natural
Science
Society
issued
during
the
period
of
Mendel's
activity
in
plant
hybridisation
(1863
to
1871).
Also
the
preserved
fragments
of
Niigeli's
letters
to
Mendel
(Hoppe,
1971)
arc
of
significance.
Of
no
lesser
importance
is
Mendel's
copy
of
Gartner's
book
Versuche
and
Beobachtungen
fiber
die
Bastard-
erzeugung
im
Pflanzenreich
(Stuttgart,
1849)
giving
a
summarised
information
of
the
then
state
of
knowledge
in
the
field
of
plant
hybridisation.
The
copy
is
full
of
notes
and
marginalia
in
Mendel's
handwriting.
The
same
goes
for
the
German
version
of
Darwin's
book
The
Variation
in
Animals
and
Plants
under
Domestication
which
appeared
in
Stuttgart
in
1868
containing
Mendel's
hand-
written
notes
especially
to
the
experiments
with
Mirabilis.
In
the
present
paper,
the
data
about
these
Mendel's
experiments
are
compiled
on
the
level
of
individual
combinations,
independent
crosses
within
the
same
combination,
and
single
operations
(crosses,
backcrosses,
growing
of
plants
of
individual
hybrid
generations)
within
the
same
combination
and
cross.
Mendel's
unfinished
experiments
are
also
included
as
they
could
show
Mendel's
intentions,
failures,
changes
in
his
plans,
etc.
Mendel
carried
out
his
crosses
wtih
other
plants
than
Pisum
in
different
periods
of
his
experimental
activity.
These
.experiments
can
be
divided
into
the
following
independent
series:
1.
Some
preliminary
experience
with
hybridisation
of
ornamental
plants
before
opening
the
Pisunt
experiments.
4
2.
Experiments
with
Phaseolus
and
a
small
trial
carried
out
with
Dianthus
simultaneously
with
Pisum.
3.
A
series
of
crosses
with
species
and
varieties
of
14
plant
genera
in
the
period
from
1864
to
1870.
4.
Hybridising
experiments
with
ll
ieraciunt
done
from
1866
to
1871,
or
to
1872.
5.
Some
crossings
with
several
fruit
trees
carried
out
at
the
close
of
his
life,
most
probably
after
1871.
5
01
ORNAMENTAL
PLANTS
Mendel
acquired
some
experience
in
artificial
crossings
with
ornamental
plants
before
opening
the
Piston
experiments
(Pisum
paper
p.
3
and
10).
The
goal
of
these
hybridisations
which
gave
impulse
to
his
Pisum
programme
was
to
obtain
new
colour
varieties.
These
crosses
could
he
realised
in
any
time
since
Mendel's
boyhood
up
to
1856.
Iltis
(1924,
p.
68)
assumed
that
they
were
carried
out
"long
before"
the
Pisum
crosses
were
opened.
The
period
of
Mendel's
stay
in
the
Monastery
seems
to
be
the
most
probable.
The
ornamental
plants
used
in
the
experiments
might
have
been
Lathyrus
or
Phaseolus
whose
hybrids
were
also
given
in
the
Pisurn
paper
(p.
9).
Lathyrus
was
not
mentioned
later
elsewhere
while
the
Phaseolus
experiments
were
de-
scribed
along
Pisum.
\Veiling
(1970,
p.
72)
admitted
that
these
experiments
were
either
Mendel's
own
fertilisations
or
they
were
carried
out
by
some
Brno
gardeners
who
were
in
contact
with
Mendel.
Vavra
(1970)
drew
attention
to
the
crossings
of
orna-
mental
plants
by
Moravian
gardeners.
E.g.,
J.
Tvr4
-
was
especially
interested
in
the
breeding
of
Fuchsia,
Pelargonium,
Verbena,
Sanchezia,
etc.
just
from
1851
up
to
his
death
in
1883.
It
is
known
that
Mendel
associated
with
Tvrq.
It
may
be
assumed
that
Mendel
occupied
himself
with
Fuchsia.
There
is
a
photo
of
the
Augustinians,
given
also
by
This
(1924,
Plate
4)
showing
Mendel
with
a
Fuchsia
flower
in
his
hand.
Finally,
Mendel
might
have
carried
out
some
crosses
with
ornamental
plants
in
the
Monastery
after
1843
with
his
friar
M.
Klacel
who
took
care
of
the
Monastery
experimental
garden
before
Mendel.
6
02
PIIASEOLUS
Results
of
the
crosses
with
Phaseolus
were
given
by
Mendel
in
the
chapter
Experiments
on
Hybrids
of
Other
Plant
Species
of
the
Pisum
paper
(p.
32-38).
The
aim
of
the
experiments
was
"to
determine
whether
the
law
of
development
discovered
for
Pisum
is
also
valid
for
hybrids
of
other
plants"
(p.
32).
Mendel
carried
out
these
first
verifying
crosses
contemporaneously
with
his
last
Pisum
experiments.
According
to
\Veiling
(1970)
Mendel
opened
those
experiments
in
1859
or
1860.
They
were
followed
up
to
the
third
or
fourth
generation
and
were
closed
simultaneously
with
the
Pisum
ones.
The
organisation
and
the
most
probable
time
sequence
of
individual
experiments
are
given
in
Table
1.
Table
1
Mendel's
experiments
with
Phaseolus
Combination/Year
1860
1861
1862
.
1863
1864
Phaseolus
vulgaris
X
Ph.
nanus
C
I
F
1
F2
F3
Ph.
nanus
x
Ph.
multiflorus
C
F
1
F
2
F
3
1
4-''4
No.
of
individual
operations
1
2
2
2
2
crossing
F
i
to
1''
4
hybrid
generation
The
impulse
to
Mendel's
choice
of
Phaseolus
for
his
experiments
might
have
been
similar
with
Pisum.
Phaseolus
hybrids
had
been
already
investigated,
e.g.,
by
Wiegmann
(1828)
and
they
were
mentioned
also
by
Gartner
(1849).
The
diversity
of
flower
colours
evidently
attracted
Mendel
in
choosing
this
common
garden
plant
for
hybridisation
experiments.
Mendel
was
apparently
not
aware
of
difficulties
connected
with
partial
sterility
of
hybrids
occuring
when
rather
distant
species
were
intercrossed.
The
first
combination,
Phaseolus
vulgaris
x
Ph.
nanus,
represented
a
tri-
kybrid
cross.
Unfortunately,
Mendel
gave
neither
the
extent
nor
detailed
results
of
this
experiment.
He
satisfied
himself
with
a
statement
that
"The
nume-
rical
relationships
in
which
different
forms
occured
in
individual
generations
were
the
same
as
in
Pisum"
(p.
32).
In
a
tabular
survey,
eight
constant
combinations
were
given.
If
constant
combinations
could
be
detected
then
at
least
progenies
of
individual
F2
plants
(i.e.,
F3
families)
must
have
been
observed
by
Mendel.
It
can
be
supposed
that
the
number
of
F2
plants
was
not
large
enough
to
give
convincing
ratios.
Most
probably,
Mendel
considered
the
detection
of
six
new
constant
combinations
sufficient
for
proving
the
coincidence
with
Pisum.
More
information
was
given
by
Mendel
about
the
cross
Ph.
nanus
x
Ph.
multillorus.
This
experiment
was
also
based'
on
a
relatively
limited
number
of
plants
due
to
lowered
fertility;
from
17
F
t
plants
with
many
hundreds
of
flowers
only
49
seeds
were
harvested.
Similarly
in
the
F2,
only
15
plants
of
30
gave
well
-developed
seeds.
According
to
Weiling
(1970),
this
cross
was
interspecific
and
thus
the
limited
fertility
was
comprehensible.
The
parents
differed
in
several
characters.
Mendel
paid
attention
particularly
7
to
three
pairs
of
them.
As
the
number
of
F
2
plants
was
low,
the
ratio
of
individuals
with
dominant
and
recessive
characters
was
fluctuating.
Only
in
the
stem
height
and
in
the
pod
shape,
the
ratios'
were
exactly
as
in
Pisum
3
dominant
to
1
recessive.
Mendel
gave:
"Despite
of
many
obstacles
with
which
the
observations
had
to
contend,
this
experiment
still
establishes
that
develop-
ment
of
hybrids
follows
the
same
law
as
in
Pisum
with
respect
to
those
traits
concerned
with
the
shape
of
the
plant"
(p.
34).
The
fl
ower
colour
(purple
red
vs.
white)
and
the
seed
colour
(pink
with
black
flecks
vs.
white)
gave,
however,
unexpected
results.
At
first
sight,
it
seemed
difficult
to
find
an
agreement
with
peas.
In
the
F
2
,
a
whole
colour
range
from
purple
red
to
pale
violet
and
white
appeared,
in
an
enigmatic
ratio
30
coloured
to
1
white.
The
coloration
of
seeds
was
similarly
diverse.
From
this
observation,
Mendel
deduced:
"But
these
puzzling
phenomena,
too,
could
probably
be
explained
by
the
law
valid
for
Pisum
if
one
might
assume
that
in
Ph.
multiflorus
the
colour
of
fl
owers
and
seeds
is
composed
of
two
or
more
totally
independent
colours
that
behave
individually
exactly
like
any
other
constant
trait
in
the
plant"
(p.
35).
An
idea
of
"composite"
traits
arose
in
Mendel's
mind.
The
observed
30
:
1
ratio
fitted
with
an
assumption
that
the
purple
red
fl
ower
colour
of
Ph.multiflorus,
A,
consisted
of
.two
or
more
independent
colour
traits
A
l
+
A
2
...
After
a
fertilisation
with
Ph.
minus
carrying
the
differing
trait
of
white
colour,
a,
cor-
responding
hybrid
associations
A
l
a
+
A
2
a
+
...
would
have
been
formed.
If
really
A
=
A,
+
A
2
(the
trait
A
is
composed
only
of
two
particular
traits)
two
series
would
correspond
to
the
hybrids
A
l
a
and
A
2
a:
Al
+
2A
1
a
+
a,
A2
+
2A
2
a
+
a.
The
members
of
these
series
would
enter
into
9
different
combinations,
each
of
them
representing
a
designation
for
another
colour,
and
only
one
among
16
plants
(a
a)
would
be
white
fl
owering:
1
A
l
A2
2
A
l
a
A2
1
A
2
a
2
A
1
A
2
a
4
A
l
a
A
2
a
2
A
2
a
a
1
A
I
a
2
A
i
a a
la
a.
Mendel
obviously
did
not
venture
to
classify
his
30
F
2
plants
according
to
the
flower
colour
as
only
a
half
of
the
F
2
plants
had
given
well
-developed
seeds.
The
attempted
analysis
can
start
from
Mendel's
statement
that
in
the
F2
among
30
coloured
plants,
16
were
predominantly
red
fl
owering.
As
no
else
basic
colours
than
purple
red,
violet,
and
white
were
named,
the
remaining
14
coloured
plants
must
have
been
of
violet
fl
ower
colour.
In
this
way
the
ratio
16
purple
:
14
violet
:
1
white
appears
what
fits
closely
to
9
:
6
:
1
(x
2
=
1.04;
.5
<
P
<
.7)
and
differs
from
12
:
3
:
1
(z
2
14.31;
P
<
.001).
This
fact
suggests
that
there
were
involved
rather
identical
genes
with
a
cumulative
effect
than
epistatic
ones;
it
seems
to
correspond
with
a
similar
explanation
suggested
by
Tschermak
(19110,
p.
68-69).
At
the
same
time,
Mendel
mentioned
not
only
the
three
basic
colours
but
he
emphasised
that
there
was
a
large
multiformity
in
the
flower
coloration.
Thus
the
dominance
was
apparently
incomplete.
Yet
it
was
supposed,
in
agreement
with.
Mendel,
that
there
existed
two
independent
cumulative
non
-allelic
genes.
Mendel,
however,
admitted
that
the
colour
A
could
be
composed
also
of
three
independent
traits
(A
=
Al
+
A
2
+
A
3
)
8
and
estimated
that
in
such
a
case,
one
white
fl
owering
plant
would
appear
once
only
among
64
plants.
Weiling
(1970)
really
found
that
the
observed
30
:
1
ratio
was
in
a
good
agreement
(at
.4
<
P
<
.5)
with
15
:
1
(two
active
genes)
as
well
as
with
63
:
1
(three
genes).
Mendel's
conception
of
"composite"
traits
anticipated
certain
features
of
the
modern
theory
of
gene
interaction
as
it
was
already
suggested
by
Iltis
(1924,
p.
103-105),
Gaissinovitch
(1965,
p.
143-144),
Olby
(1966,
p.
117),
I
-Minims
(1969,
p.
24-25),
etc.
Unfortunately,
Mendel
here
met
only
with
one
type
of
gene
interaction
where
a
simple
addition
of
partial
effects
of
individual
traits
(A
1
-F
A2
A)
was
sufficient
for
an
adequate
explanati
on
.
H
e
i
mans
(1971,
p.
97)
emphasises
that
"The
idea
of
"factor
interaction"
such
as
"epistasy"
"cryptomery"
etc.
did
not
enter
Mendel's
mind,
because
of
the
fundamental
thesis
of
absolute
independence
of
characters".
It
is
probable
that
the
multi-
formity
of
mutual
relations
between
interacting
"traits"
caused
Menders
difficulties
in
analysing
the
results
of
his
later
"colour"
experiments
in
Matthiola
as
it
is
shown
in
part
03.03.
It
seems
that
Mendel's
methodical
approach
was
different
from
that
used
in
Pisum
if
he
met
with
F2
ratios
which
were
uneasy
to
interpret
and
to
bring
into
agreement
with
those
known
in
Pisum
due
gene
interaction
or
simply
to
a
low
number
of
available
1
4'
2
plants.
In
such
eases,
Mendel
satisfied
himself
with
determining
the
number
of
constant
combinations
without
giving
any
ratios.
It
was,
however,
necessary
to
grow
the
1
7
3
or
14
progenies.
Indeed,
in
both
Phaseolas
crosses
and
also
in
extended
Matthiola
experiments,
the
high
hybrid
generations
were
investigated.
Still
another
complication
was,
however,
connected
with
it.
In
Pisum
(p.
23),
the
number
of
constant
combinations
corresponded
to
the
number
of
gametic
combinations
and,
at
the
same
time,
to
the
corresponding
power
of
2
of
the
number
of
trait
pairs.
If,
on
the
other
hand,
"composite"
hybrid
series
were
in
operation,
the
given
relations
were
not
more
valid,
and
the
number
of
different
constant
combinations
must
have
been
lower.
When
rather
cumulative
identical
factors
were
present
as
it
was
deduced
from
the
results
of
Mendel's
cross
Ph.
nanus
x
Ph.
multiflorus,
the
number
of
constant
combinations
would
be
3
with
two
trait
pairs,
4
with
three
trait
pairs,
etc.,
generally
n
H-
1
with
n
trait
pairs,
instead
of
4,
8,
etc.,
generally
2n
with
n
trait
pairs
as
it
could
be
expected
in
the
absence
of
interaction.
Nevertheless,
Mendel
postulated
the
validity
of
the
"Pisum
law"
even
in
such
complicated
situations.
His
analysis
of
the
second
bean
experiment
shows
that
he
regarded
the
"Pisan"
law"
as
the
existence
of
independent
paired
carriers
of
traits
which
enter
different
germ
cells
and
join
in
the
fertilised
egg.
Thus
the
term
"the
law
valid
for
Pistil!'"
is
more
substantial
than
it
would
seem
at
first
sight.
At
the
close
of
his
description
of
the
bean
experiments,
Mendel
pointed
out
the
significance
of
similar
experiments
for
understanding
the
diversity
in
the
flower
colour
of
ornamental
plants.
In
this
way,
Mendel's
former
interest
(cf.
part
01)
recurred
with
a
new
urgency.
In
Mendel's
mind,
the
question
of
flower
colour
was
connected
with
a
more
general
problem
of
the
variability
of
cultivated
and
also
of
wild
plants.
According
to
Mendel
(Pisum
paper
p.
37),
variation
of
cultivated
plants
is
not
a
consequence
of
changed
external
conditions
but
it
is
mainly
caused
by
the
fact
that
the
plants
are
members
of
different
hybrid
series.
The
higher
the
number
9
of
hybrid
series
involved,
the
larger
the
diversity
of
a
plant
population.
At
the
same
time,
not
only
different
"simple"
series
of
trait
pairs
(A
+
2Aa
+
a;
B
2Bb
b;
...)
can
be
present
but
also
different
"composite"
ones
(A
1
+
2A
i
a
+
a;
A2
+
2A
2
a
+
a;
...).
It
is
then
comprehensible
why
Mendel
placed
his
account
on
the
plant
variability
after
the
report
on
bean
experiments:
in
analysing
the
causes
of
variability,
lie
obviously
had
in
mind
that
just
the
"composite"
hybrid
series
significantly
contribute
to
the
genetic
complexity
of
a
plant
population.
10
03
SPECIES
AND
VARIETIES
OF
14
PLANT
GENERA
Already
at
the
beginning
of
his
Piston
paper,
Mendel
expressed
his
conviction
that
the
fi
nal
solution
in
searching
for
a
generally
valid
law
governing
the
formation
of
hybrids
and
their
development
.
in
the
progeny
"can
be
reached
only
when
the
results
of
detailed
experiments
from
the
most
diverse
plant
families
are
available"
(p.
3).
In
this
sense,
his
experiments
with
Pisum
represented
only
the
first
of
such
detailed
experiments.
On
page
4.2-43,
he
added:
"Whether
variable
hybrids
of
other
plant
species
show
complete
agree-
ment
in
behaviour
also
remains
to
be
decided
experimentally..."
The
circumstances
under
which
Mendel's
verifying
experiments
were
started
were
later
mentioned
in
his
second
letter
to
Nageli
of
April
18,
1867.
Unfortu-
nately,
his
effort
in
initial
searching
for
convenient
plants
making
possible
as
large
experimental
series
as
in
Pisum
was
futile
in
1863
and
1864.
He
strove
to
attract
scientific
interest
to
such
experiments
in
his
memorable
lectures
of
1865,
but
without
success.
Finally,
he
was
compelled
to
undertake
such
experiments
by
himself.
Hence
a
larger
series
of
new
experiments
followed.
Thus
two
groups
of
these
experiments
are
to
be
distingished:
one,
rather
small
and
unsuccessful].
(with
Verbascum,
Tropaeolum,
and
Campanula),
and
another,
more
extensive,
opened
later.
03.01
ATTEMPTED
RECONSTRUCTION
An
attempted
reconstruction
of
these
experiments
is
given
in
Table
2
where
fourteen
plant
genera
with
33
different
species
are
treated.
Six
different
species
were
proved
in
Cirsium,
per
four
in
Linaria
and
Aquilegia,
per
two
in
Verbascurn,
Tropaeolum,
Campanula,
Geum,
Antirrhinum,
Calceolaria,
Mirabilis,
while
in
Matthiola,
Ipomoea,
and
Zea,
varieties
of
the
same
species
rather
being
used.
Some
crosses
were
interspecific,
other
intraspecific.
In
the
latter,
it
.was
now
impossible
to
detect
individual
varieties
so
-that
the
crosses
with
different
varieties
of
the
same
species
were
considered
,a
single
combination.
Hence
the
total
number
of
combinations
was
30.
Twenty
five
of
them,
or
83.3
per
cent,
were
interspecific,
and
only
five,
or
16.7
per
cent,
intraspecific.
Evidently
the
species
crosses
prevailed.
In
27
combinations,
the
crosses
were
carried
out
by
Mendel
while
in
three
remaining
Mendel
investigated
natural
hybrids.
Indepen-
dent
crosses
in
the
same
combination
were
done
by
Mendel
only
in
one
case.
The
F
1
plants
were
obtained
in
27
combinations,
the
F2
in
11,
the
F3
in
3,
and
the
F4
plants
in
one
combination.
The
experiments
were
closed
with
the
F
1
in
16
combinations,
with
the
Fy
in
8,
with
the
F3
in
2,
and
with
the
F4
in
one
combination.
Only
in
eleven
combinations,
or
in
36.7
per
cent,
the
F2
plants
were
available
for
the
evaluation
of
numerical
ratios.
In
two
cases,
or
in
6.7
per
cent,
the
F
1
plants
were
backcrossed.
For
a
better
illustration
of
the
dynamics
of
the
experiments
an
individual
operation
was
considered
as
a
unit.
The
sum
of
these
operations
was
76.
Their
distribution
over
individual
vegetation
periods
is
given
in
Table
2,
last
row.
According
to
this
index,
the
peak
was
reached
in
1867.
These
data
show
a
large
diversity
of
genera,
species,
and
varieties
used
in
the
crosses.
Thus
rather
an
extensive
than
an
intensive
character
of
these
experiments
seems
to
be
suggested.
In
some
cases,
the
experiments
show
rather
preliminary
11
character.
The
fact
that
more
than
60
per
cent
of
them
were
unfinished
gives
evidence
of
it.
Table
2
Mendel's
experiments
with
14
plant
genera
Combination/Year
1864
1865
1866
1867
.
1868
I
1869
F
3
F2
F
1
F3
F2
F
2
F
1
1870
Verbascum
phoeniceurn
x
V.
Blattaria
Tropaeolum
majus
x
T.
minus
Campanula
medium
x
C.
pyramidalis
Cirsium
arvense
x
C.
oleraceum
C.
arvense
x
C.
canum
C.
canum
x
C.
oleraceunt
C.
lanceolaturn
x
C.
oleraceum
C.
premorsunt
(=
C.
oleraceum
x
C.
rivulare)
C.
premorsunt
x
C.
palustre
C.
premorsunt
X
C.
canum
C.
canum
x
C.
palustre
C.
rivulare
x
C.
palustre
Geum
urbanum
x
G.
rivale
I,inaria
vulgaris
x
L.
striata
L.
vulgaris
x
L.
genistaefolia
L.
vulgaris
x
L.
triphylla
Aquilegia
canadensis
x
A.
vulgaris
A.
canadensis
X
A.
atropurpurea
A.
canadensis
X
A.
Witimaniana
Matthiola
annoy
(varieties)
M.
annua
x
M.
glabra
(?)
Iponwea
purpurea
(varieties)
Antirrhinum
majus
(varieties)
A.
majus
x
A.
rupestre
(?)
Calceolaria
salicifolia
x
C.
rugosa
L'
ea
Mays
major
x
Z.
M.
minor
Z.
M.
major
X
Z.
Cuzko
(?)
Mirabilis
jalapa
x
M.
longiflora
M.
jalapa
(varieties)
Lychnis
diurna
x
L.
vespertina
(
--
Melandrium
rubrum
x
M.
album)
C
C
C
F
1
F
1
F
1
F2
C
C
C
C
N
C
C
N
N
C
C
C
C
C
C
C
C
C
C
C
Fi
F
1
Fi
F
1
F1
F
l
F
1
C
Fi
F
1
F
1
BC
F
1
BC
C
C
F
1
F
t
F
1
F
i
C
F
1
F
i
.
F
i
C
C
F2
F2
F2
F2
F2
F
i
F
2
Fi
C
F2
Fl
F
1
C
F4
F
3
1F2
No.
of
individual
operations
3 3
21
26
13
7
3
C
crossing
Fi
to
h'.
1
hybrid
generation
BC
backcrossing
N
a
natural
hybrid
In
the
second
letter
to
Niigeli,
Mendel
gave
two
different
reasons
why
he
left
so
many
of
his
experime
n
ts
unfi
n
ished.
Sortie
plants
showed
unfit
for
hybridising
experiments
owing
to
the
hybrid
sterility
or
to
the
difficult
protection
of
flowers
from
the
foreign
pollen.
Another
reason
was
the
shortage
of
room
in
Mendel's
experimental
garden.
Particularly
the
latter
obstacle
was
serious.
E.g.,
in
his
seventh
letter,
Mendel
complained
that
Cirsium
would
be
a
very
good
experimental
plant
if
it
were
not
so
exacting
on
space;
in
the
eighth
12
letter,
he
wrote
that
he
grew
about
1500
plants
of
Matthiola
annua
in
1870
whereby
his
experiments
with
Melandriunt
and
Mirabilis
continued,
and
16
individual
operations
with
Hieracium
were
done
simultaneously.
These
facts
give
evidence
for
Mendel's
well
-thought-out
methodical
approach;
lie
investigated
many
plants
as
far
as
they
might
have
been
useful
in
his
design.
The
loss
of
a
part
of
the
material
studied
was
thus
unavoidable.
On
the
other
hand,
important
results
were
obtained
in
some
crosses,
e.g.,
in
Cirsum
canum
x
C.
oleraceum,
C.
premorsum
x
C.
canum,
Geum
urbanum
x
G.
rivale,
Linaria
vulgaris
x
L.
striala,
Melandrium
rubrum
x
M.
album,
and
in
the
crosses
among
varieties
of
./Ilatthiola,
Zea,
and
Mirabilis.
03.02
INTENTIONS,
MATERIAL,
AND
EXPERIMENTAL
METHODS
According
to
Mendel's
own
words,
these
experiments
were
to
verify
the
results
and
conclusions
obtained
with
Pisum.
From
this
point
of
view,
each
cross
of
this
series
should
represent
one
"detailed
experiment".
Unfortunately,
only
few
species,
for
example,
Matthiola
annua,
possessed
as
many
of
closely
related
forms
with
a
number
of
discernible
traits
as
Pismo
did.
Mendel's
ideas
about
the
character
of
his
verifying
experiments
were
suggested
in
his
first
letter
to
Niigeli.
After
analysing
the
data
given
by
Gartner
(1849),
Mendel
came
to
the
conclusion
that,
in
the
majority
of
cases,
they
did
not
disagree
with
his
own
results
obtained
with
Pisum
or
that,
at
least,
they
might
be
brought
into
agreement
with
them.
It
is
known
that
already
in
his
main
paper
Mendel
opposed
categorically
to
appreciation
of
the
diversity
of
the
hybrid
progeny
according
to
their
total
appearance
("Gesammteindruck")
as
practiced
by
Gartner
and
other
hybridisers
of
the
preceding
period.
On
the
contrary,
he
would
have
that
the
degree
of
kinship
between
the
hybrid
forms
and
their
parents
should
be
determined
diagnostically;
first
a
complete
analysis
of
the
differences
in
individual
traits
in
the
parents
should
be
done
and
then
the
occurence
of
these
traits
in
the
F
t
and
especially
in
the
F
2
should
be
observed.
Arid
even
if
Gartner
and
other
investigators
did
not
precisely
determine
those
differences,
the
essential
conformity
with
Mendel's
results
consisted
in
the
multiformity
of
descendants
of
the
hybrids
testifying
to
the
existence
of
the
segregation
process.
Further,
Mendel
wrote:
"In
order
to
determine
the
agreement,
if
any,
with
Pisuin,
a
study
of
those
forms
which
occur
in
the
first
generation
1=1
2
1
should
be
sufficient.
If,
for
two
differing
traits,
the
same
ratios
and
series
which
exist
in
Pisum
can
be
found,
the
whole
matter
would
be
decided".
It
seems
that
Mendel
centred
his
interest
on
the
problem
of
general
existence
of
the
segregation
process
while
the
analysis
of
the
second
basic
mechanism
of
heredity,
namely
combination,
was
put
aside
for
the
moment.
If
Mendel
stressed
in
this
experimental
series
the
verification
of
segregation,
he
must
have
had
in
mind
the
cases
in
which
the
segregation
expressed
in
the
diversity
of
the
hybrid
progeny
was
denied
at
all.
To
these
eases
belong
Gartner's
"constant".
hybrids
mentioned
by
Mendel
already
in
the
close
of
his
Pisum
paper:
"We
meet
an
essential
difference
in
those
hybrids
which
remain
constant
in
their
progeny
and
propagate
themselves
as
truly
as
the
pure
species.
According
to
Gartner,
to
this
class
belong
the
remarkably
fertile
hybrids
A
quilega
13
atropurpurea-canadensis,
Lavatera
pseudolbia-thuringiaca,
Geum
urbano-rivale,
and
some
Dianthus
hybrids;
and,
according
to
Wichura,
the
hybrids
of
the
Willow
family...
The
correctness
of
the
facts
is
guaranteed
by
eminent
observers,
and
cannot
be
doubted.
Gartner
had
an
opportunity
of
following
up
Dianthus
Armeria-deltoides
to
the
tenth
generation,
since
it
regularly
propagated
itself
in
the
garden"
(p.
40).
Mendel's
scientific
consicientiousness
did
not
permit
him
to
treat
roughly
the
problem
of
the
"constant"
or
"non
-segregating"
hybrids.
Thus
the
extensive
crosses
done
after
those
with
Pisum
were
not
a
heap
of
acidentally
grouped
more
or
less
important
experiments
but
a
concentrated
effort
to
analyse
the
problem
of
the
general
validity
of
the
segregation
"law";
the
"constant"
hybrids
seemed
to
be
the
only
exception
that
needed
serious
reinvestigation.
Some
facts
bear
witness
of
it.
The
majority
of
combinations
of
this
series
was
interspecific.
If
Mendel
strove
only
for
the
proof
of
the
results
obtained
with
peas,
it
would
be
incomprehensible
why
he
performed
so
many
crosses
where
several
difficulties
might
have
arisen:
lowered
fertility
or
even
sterility
of
hybrids,
large
numbers
of
difficult
definable
differing
traits
and,
consequently,
of
members
of
the
combination
series.
Furthermore,
Mendel
repeated
some
combinations
described
by
Gartner
as
leading
to
"constant"
hybrids,
e.g.,
in
Geum
and
Aquilegia,
what
suggests
that
Mendel's
endeavour
was
to
convince
himself
of
the
existence
of
"constant"
hybrids.
Also
the
majority
of
Mendel's
references
to
individual
pages
of
Gartner's
book
was
connected
with
the
problem
of
"constant"
hybrids
(Fig.
1).
Mendel
produced
experimentally
not
only
the
mentioned
hybrids
held
by
Gartner
for
"constant".
In
many
other
cases,
too,
where
rather
variable
hybrids
were
expected,
Mendel's
genera
and
species
were
often
coincident
with
those
given
by
Gartner,
e.g.,
Verbascum,
Tropaeolum,
Linaria,
Matthiola,
Antirrhinum,
Calceolaria,
Zea,
Mirabilis,
and
Melandrium
.
At
least
some
of
Mendel's
experi-
'/
7
v
4,
/
f,
;
,A
"":''t
,
)
Fig.
1
Mendel's
references
to
individual
pages
relating
to
the
problem
of
"constant"
hybrids
in
his
copy
of
Gartner's
book
14
ments
with
Mirabilis
might
also
have
been
stimulated
by
Darwin's
Variation
under
Domestication.
Among
the
plant
material
of
this
series,
wild
species
were
present
as
well.
The
choice
of
these
species
corresponded
to
the
pecularities
of
Moravia's
vegetation,
especially
in
the
case
of
Linaria,
Verbascuni,
Cirsium,
and
Melandrium.
On
the
other
hand,
experiments
with
many
common
garden
plants
were
carried
out
(Tropacolum,
Matthiola,
Iponwea,
A
ntirrhinum).
A
part
of
Mendel's
plant
material
used
in
this
series
was
also
in
close
connection
with
the
activities
of
the
botanists
of
Brno.
Iltis
(1924,
p.
108)
called
attention
to
the
fact
that
"Mendel
found
that
there
was
much
more
interest
in
his
crossings
of
wild
plants
than
in
his
study
of
the
crossings
of
the
edible
peas.
No
doubt
it
was
largely
for
this
reason
that
from
1866
onwards
he
devoted
most
of
his
energy
to
the
former".
Iltis
also
named
some
natural
plant
hybrids
which
were
collected
by
the
members
of
the
Brno
Natural
Science
Society
and
then
presented
in
the
Society's
sessions
and
in
the
papers
published
in
the
Proceedings.
\Veiling
(1969a)
extended
substantially
Iltis'
survey.
In
order
to
give
a
full
picture,
we
revised
all
volumes
that
went
back
to
the
period
of
Mendel's
activity
in
plant
hybridisation,
i.e., volumes
1
(1863)
to
9
(1871).
The
total
number
of
plant
hybrids
presented
in
the
Society
was
48
during
the
given
period.
Three
of
them
were
Menders
artificial
hybrids
(
Verbascum
phoeni-
ceum
x
Blattaria,
Campanula
medium
X
C.
pyramidalis,
and
Gell771
urba-
num
x
G.
rivale),
the
others
were
natural
hybrids
originating
mainly
from
the
collections
of
C.
Theimer,
A.
Makowsky,
G.
von
Niessl,
and
C.
Romer.
Especially
in
Verbascum
and
Cirsium,
Mendel
might
have
been
influenced
in
his
choice
by
natural
hybrids
presented
and
discussed
in
the
Natural
Science
Society.
Moreover,
Mendel
also
made
use
of
three
natural
Cirsium
hybrids
obtained
from
the
botanists
of
Brno.
He
was
somewhat
sceptical
to
the
possiblity
of
making
investigations
with
these
forms.
In
his
first
letter
to
Nagai
he
gave:
"Those
hybrids
which
are
colected
in
the
wild
can
be
used
as
secondary
evidence
only,
as
long
as
their
origin
is
not
unequivocally
known".
In
all
three
natural
Cirsium
hybrids
investigated
by
him,
this
requirement
was
apparently
met.
Mendel
got
his
natural
hybrid
C.
premorsum
(=
C.
oleraceum
x
C.
rivulare)
probably
from
C.
Theimer
(Verhandlungen
des
naturforschenden
Vereines
in
Briinn
1
(Sitzungs-Berichte):
19-20,
1863);
C.
canum
x
C.
palustre
either
from
A.
Makowsky
(Verhandlungen
1
(Abhandlungen),
p.
114,
1863)
or
from
G.
von
Niessl
(Verhandlungen
5
(Sitzungs-Berichte):
61-62,
1867);
and
C.
rivulare
X
C.
palustre
from
G.
von
Niessl
(Verhandlungen
2
(Sitzungs-Beriehte):
43-44,
1864).
Some
of
these
natural
hybrids
were
carefully
described
as
compared
with
their
parent
species.
In
Theimer's
C.
premorsum
and
Niessl's
C.
rivulare
X
C.
palustre,
accurate
diagnoses
were
reported
which
must
have
been
convincing
even
for
Mendel.
Besides,
Theimer
(l.c.)
found
that
"the
sterility
which
was
often
acribed
to
the
hybrid
plants
was
not
proved
in
many
new
experiments,
and
these
hybrids
were
found
to
give
germinable
seeds".
Niessl
(Verhandlungen
5,
l.c.)
gave
interesting
facts
about
his
C.
rivulare
x
C.
palustre.
Not
only
some
metric
data
but
also
three
different
hybrid
forms
found
in
the
population
were
described.
One
form
was
near
to
C.
rivulare,
another,
which
was
the
most
frequent
and
fertile,
represented
probably
the
proper
hybrid,
.and
the
third
was
"Near
to
C.
palustre,
possibly
a
hybrid
of
the
just
described
[second]
form
with
C.
palustre
or
a
reverting
form".
At
the
same
time,
the
second
form
was
handed
over
to
Mendel.
15
In
the
allusions
of
both
authors,
conformity
with
Mendel's
views
might
he
foreshadowed.
Niessl
also
said
that
he
took
side
with
the
view
which
was
in
agreement
with
Mendel
that
"hybrids
collected
at
different
places
should
be
completely
described
without
exception,
in
order
to
find,
finally,
certain
more
constant
characters
among
the
usually
fluctuating
ones"
(Verhandlungen
4
(Sitzungs-Berichte):
82,
1866).
Later,
Niessl
reported
on
the
hybrids
of
crypt°.
gamous
plants
discussing
them
from
the
viewpoint
of
their
sexual
reproduction
(Verhandlungen
5
(Sitzungs-Berichte):
20-27,
1867).
A
mutual
interchange
of
materials
as
well
as
of
ideas
regarding
plant
hybridisation
took
place
between
Mendel
and
the
botanists
of
Brno
headed
by
G.
von
Niessl,
the
most
active
of
them.
Natural
hybrids
collected
and
discussed
by
them
inspired
Mendel
not
only
in
using
them
in
his
investigations
but
also
in
his
attempts
to
obtain
their
artificial
pendants.
A
picture
emerges
of
the
friendly
relations
and
of
the
working
contacts
within
the
botanical
group
of
the
Brno
Natural
Science
Society.
In
his
methodical
approach
to
the
investigation
of
the
above
mentioned
plants,
Mendel
was
largely
dependent
on
his
own
experience
gained
in
his
preceding
experiments.
1Iis
first
task
in
each
new
cross,
similarly
with
Pisum
and
Phasenlas,
was
to
analyse
the
parental
forms
regarding
the
particular
traits.
A
document
has
survived,
viz.,
a
survey
of
alternative
trait
pairs
in
Geunt
urbanum
and
G.
rivale,
proving
this
procedure
as
an
obligatory
constituent
of
his
programme
(Fig.
2).
Subsequently,
Mendel
examined
the
appearance
of
hybrids
and
the
segregation
of
the
trait
pairs
in
the
progenies.
At
the
same
time,
the
presence
or
absence
of
dominance
was
studied
ill
individual
pairs.
In
general,
dominate
was
prevailing,
Sometimes,
a
"Zwischenbildung"
was
mediated
by
two
different
traits
appearing
side
by
side,
as
in
some
characters
of
seeds
from
L.
valgaris
x
L.
striata.
Only
in
crosses
between
varieties
of
Mirabilis
jalapa,
dominance
was
lacking
in
the
fl
ower
colour.
Mendel
did
not
regard
this
as
a
departure
from
the
regularities
found
in
Pisani,
and
his
analysis
of
such
a
situation
perhaps
still
more
objectively
substantiated
his
concept
of
discrete
and
independent
hereditary
units.
According
to
some
remarks
(e.g.,
in
the
seventh
letter),
progenies
of
individual
F,
plants
were
grown
separately
from
each
other.
This
precondition
was
of
significance
in
resolving
whether
the
progenies
were
variable
or
"constant",
especially
if
in
some
cases
(in
Linaria
vulgaris
x
L.
striata
and
in
Ceuta
'latana'
X
G.
rivale),
the
individual
F
1
plants
were
different.
Regarding
the
form
in
which
the
results
obtained
in
the
F2
generation
were
presented,
no
numerical
ratios
were
given
anywhere.
in
some
eases
where
the
parents
differed
from
each
other
in
a
rather
small
number
of
trait
pairs,
Mendel
only
emphasised
that
the
hybrid
behaved
in
exactly
the
same
manner
as
Pisani.
11,
however,
analysis
of
separate
traits
was
difficult
or
incomplete,
as
in
species
crosses,
be
satisfied
himself
with
the
statement
that
the
l
plants
were
different
or
vari-
able.
Few
remarks
wer
e
given
about
the
crossing
technique.
Apparently,
his
strict
rules
of
crossing,
so
well
-tried
i
n
pe
a
s
and
beans,
permitted
him
to
be
successful
also
in
new
objects,
whose
flowers
were
rather
large
in
the
majority
of
cases.
Only
in
Cirsium,
he
attempted
to
search
for
Several
new
simplified
methods
making
possible
to
cross
effectively
alSo
plants
with
small
flowers.
He
examined
the
possibility
of
using
dioecious
species
as
a
mother
plant;
further,
a
crossing
without
emasculation
with
the
premature
uncovering
of
stigmas;
finally,
the
16
physiological
male
sterility
of
late
developing
flower
heads
was
employed.
These
attempts
represented
probably
his
technical
preparation
for
the
very
exacting
experiments
with
Hieracium.
0
3.0
3
EVALUATION
OF
RESULTS
Although
some
interesting
facts
were
also
found
in
the
unfinished
experiments
(e.g.,
the
different
crossability
between
some
Linaria
species;
the
luxuriance
of
hybrids
in
Verbascum,
Linaria,
and
Calceolaria;
interesting
chages
of
the
biennial
and
perennial
types
in
Verbascum
and
Cirsium
hybrids,
etc.),
the
most
significant
results
were
obtained
in
the
completed
experiments.
Regarding
the
main
question
about
the
variability
or
constancy
of
one
or
of
the
other
hybrid,
the
same
answer
was
invariably
given:
"constant"
hybrids
were
not
observed
at
all.
Often,
this
conclusion
was
given
in
a
hidden
form.
E.g.,
in
Cirsium,
Geum,
and
Linaria,
Mendel
was
not
explicit
in
formulationg
the
results.
In
an
accomp-
anying
letter
(the
seventh
letter
to
Niigeli)
to
his
packet
of
parent
forms,
F
1
and
F2
plants
appertaining
to
the
crosses
with
the
species
of
the
named
genera,
no
direct
data
but
only
short
comments
to
individual
specimens
were
given.
It
seems
that
Mendel's
aim
was,
above
all,
to
demonstrate
the
recognised
variability
among
his
1
7
2
plants,
and
this
demonstration
was
realised,
in
these
cases,
by
submitting
the
samples
to
Niigeli.
The
cross
Geum
urbanism
x
G.
rivale
seems
to
Mendel
to
be
the
most
deciding
for
solving
the
puzzle
of
"constant"
hybrids.
Ile
made
regular
and
frequent
mentions
of
this
cross
in
his
letters
to
Niigeli
and
similarly
numerous
marginalia
and
notes
relating
to
it
in
his
copy
of
Gartner's
book.
In
the
seventh
letter
to
which
also
five
F
2
plants
from
this
combination
were
added,
the
result
was,
unfortunately,
given
in
a
somewhat
peculiar
form:
"According
to
Gartner,
progeny
of
this
Geum
hybrid
show
no
variation".
This
formulation
seems
to
involve
the
obtained
result.
It
is
not
difficult
to
imagine
that
the
sample
comprised
several
F2
specimens
that
were
different
at
first
sight;
then
only
a
few
words
were
sufficient
to
remind
it
in
such
an
accompanying
letter.
Otherwise,
we
might
come
across
another
text,
for
example:
"The
hybrid
gave
variable
progeny
even
if
according
to
Gartner,
progeny
of
this
Geum
hybrid
show
no
variation".
Also
his
notes
in
the
copy
of
Gartner's
book
(Fig.
2)
seem
to
suggest
that
he
obtained
variability
at
least
in
two
trait
pairs
(D,
d,
and
E,
e)
in
the
1
7
2
.
At
the
same
time,
hybrid
variability
was
also
proved
by
him
in
Cirsium
cattail'
X
C.
oleraceum,
C.
premorsum
X
C.
canum,
and
Linaria
vulgaris
X
L.
striata;
in
the
last
named
cases,
Mendel
gave
the
result
in
a
more
open
form.
On
the
basis
of
this
experience,
Mendel
arrived
at
the
conclusion
that
the
idea
of
"constant"
hybrids
could
originate
from
a
subjective
estimate
of
the
appearance
of
hybris
and
of
their
progeny
according
to
their
"Gesammtein-
druck".
As
soon
as
an
analysis
of
the
individual
trait
pairs,
even
if
far
incomplete
as,
in
Celan,
was
applied,
the
validity
of
such
a
conception
was
seriously
endangered.
In
all
named
combinations,
the
crosses
were
clearly
interspecific.
Regarding
another
group
of
finished
experiments
which
were
rather
intraspecific,
viz.,
with
Matthiola
glabra
x
M.
anima,
Zea
Mays
major
(with
deeply
red
seeds)
x
Z.
Cuzko
(?)
(with
white
seeds),
Mirabilis
jalapa
(red
x
yellow
and
red
x
white),
17
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,
r
40.0,31
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*7
7
try
Fig.
2
Mender,
rr
lerc
o
ces
t..
;mil\
idual
pages
relating
to
the
crosses
with
Geunz
and
.
(above),
his
,nr%
cv
of
alternative
trait
pairs
in
Crawl
urbanum
and
G.
rivals
(below),
and
his
attempt
to
an.d.se
the
constitution
of
the
hybrid
G.
urbanum
x
G.
rivals
G.
imermedium)
(in
the
centre
on
the
right)
in
his
copy
of
Giirtner's
book
Mendel
summarised
the
results
unambiguously:
"Their
hybrids
behave
exactly
like
those
of
Pisum".
About
extensive
experiments
with
"colour"
varieties
of
Matthiola
annua
Mendel
wrote
in
his
ninth
letter:
"According
to
experinces
thus
far,
an
agreement
with
Pisunt
appears
probable".
Just
in
the
"colour"
experiments
with
Matthiola,
several
difficulties
cropped
up.
An
enormous
abundance
of
colour
shades
was
obtained
so
that
an
assortment
of
36
Matthiola
varieties
with
definable
flower
colours
ordered
from
Erfurt
was
insufficient
for
a
comparison.
Also
sudden
changes
of
colour
intensity
appeared.
Under
such
circumstances,
an
exact
classification
was
difficult,
and
a
determina-
tion
of
numerical
ratios
in
the
F2
and
of
corresponding
series
was
impossible..
These
troubles,
however,
did
not
discourage
Mendel
from
continuing
the
experiments.
He
decide,
in
about
1870,
to
search
for
those
forms
which
would
give
"simpler
series".
Without
doubt,
"composite"
traits
were
involved.
In
such
cases,
his
analyses
were
difficult
as
different
types
of
gene
interaction
might
come
into
play,
and
differences
in
the
number
of
constant
combinations
between
the
"simple"
and
"composite"
traits
under
the
same
number
of
trait
pairs
involved
occured
as
it
was
already
suggested
in
part
02.
Moreover,
unexpected
turns
in
the
flower
colour
shade
were
due
to
frequent
cryptic
effects
of
interacting
genes.
Under
such
circumstances,
die
F2
ratios
were
often
uncomprehensible
to
Mendel.
For
this
reason,
Mendel
apparently
preferred
to
take
into
account
the
number
of
individal
types
of
constant
progenies
derived
from
individual
Fy
or
F3
plants,
as
he
did
it
in
Phaseolus,
but
this
number
did
not
agree
with
the
nuber
of
gametic
combinations
under
such
a
situation
as
it
(lid
in
the
"simple"
traits.
In
spite
of
all
possible
difficulties,
Mendel
was
still
convinced
that
an
agreement
with
Piston
appeared
probable,
and
he
hoped
"finally
to
get
to
the
bottom
of
the
problem"
(the
eighth
letter).
After
his
crosses
with
Phaseolus
nanus
X
Ph.
multillorus,
an
opportunity
offered
itself
here
to
solve
the
problem
of
"composite"
traits.
With
his
intention
to
start
a
new
seris
of
crosses
giving
simpler
series,
he
was
not
far
from
it.
Unfortunately,
Mendel's
research
activity
lessened
in
1871,
and
also
the
extended
experiments
with
Matthiola,
whose
signifiance
could
be
compared
with
that
of
the
Piston
ones,
remained
unfinished.
Also
another
experiment,
viz.,
with
Melandrium
rubrunt
x
M.
album,
where
two
dioccious
species
were
crossed
yielded
an
important
result.
A
clear
numerical
ratio
of
3
female
to
1
male
plant
was
obtained
among
203
hybrids.
Even
if
this
ratio
did
not
agree
with
that
of
1
:
1
as
we
would
expect
today,
there
is
no
doubt
that
Mendel
foreshadowed
the
genetic
determination
of
sex.
He
deduced
that
the
endowment
("Anlage")
for
the
development
of
one
or
another
sex
must
have
been
present
already
in
the
zygote
("Grundzelle").
The
3
:
1
ratio
suggested,
according
to
his
explanation,
that
the
germ
cells
of
both
sexes
were
different.
as
regards
the
relevant
genetic
determinants.
It
is
not
excluded
that.
he
met
here
also
some
sex
-linked
characters.
The
unusual
sex
ratio
does
not
seem
to
have
been
a
product
of
chance.
Mendel's
female
M.
rubrum
probably
was
not
of
the
normal
XXAA
genotype.
Several
complications
could
be
involved
that
prevented
the
manifestation
of
the
male
character
in
one
quarter
of
his
F
1
plants.
Under
normal
conditions,
Mendel
would
have
easily
arrived
at
a
modern
conception
of
sex
determination.
It
can
be
concluded
that
in
Menders
experiments
with
species
and
varieties
of
14.
plant
genera,
results
were
obtained
which
gave
evidence
rather
for
the
general
validity
of
regularities
found
in
Pision
than
against
it.
In
each
case,
19
the
results
of
this
series
of
Mendel's
experiments
even
though
they
represent
only
a
fragment
of
his
original
intention
must
have
strenghtened
his
belief
that
the
principles
governing
in
Pisum
and
especially
the
"law"
of
segregation
are
of
general
validity.
03.04
APPRECIATION
Mendel
published
nothing
about
these
experiments
during
his
lifetime.
Only
three
of
his
hybrids
found
an
echo
in
the
Proceedings
of
the
Natural
Science
Society.
Not
until
the
publication
of
Mendel's
letters
to
Niigeli
by
Correns
(1905)
were
these
experiments
brought
to
light.
Correns
comprehended
well
that
the
notion
of
Mendel's
hybridisations
carried
out
with
other
plants
might
be
important
in
emphasising:
"If
Mendel
published
all
this
material,
it
would
perhaps
be
not
so
unappreciated,
surely
not
for
such
a
long
time"
(Correns,
1905,
p.
191).
Also
Sturtevant
(1965)
suggested
that
these
experiments,
if
published,
would
save
Mendel's
work
from
the
long-lasting
lack
of
understanding.
th
is
(1924,
p.
103)
tried
to
classify
these
experiments
according
to
agreement
or
disagreement
of
their
results
with
Pisum.
He
named
only
Mirabilis
and
Zea,
where
agreement
with
Pisum
was
found,
and
wrote:
"Many
of
hybridiastion
experiments,
however,
gave
results
which
diverged
more
or
less
markedly
from
those
obtained
in
the
original
experiments
on
the
edible
pea,
and
Mendel
was
unable
to
explain
them
in
a
way
satisfactory
to
his
etremely
critical
intelligence.
The
result
was
that
in
the
end
lie
came
to
hold
the
view
that
the
laws
he
had
discovered
in
the
case
of
the
edible
pea
were
not
universally
valid.
Still,
he
tried
to
stretch
his
theory
so
that
it
would
account
for
some
of
the
deviations
from
the
normal
pea
-type
of
inheritance".
Even
if
Iltis
praised
Mendel's
crosses
with
Phaseolus,
Matthiola,
and
Melandrium
and
mentioned
also
some
others,
he,
nevertheless,
failed
to
appreciate
that
all
these
experiments
supported
Mendel's
basic
idea
proved
in
the
Pisum
experiments.
Similar
views
prevailed
for
a
long
a
time
and
led
to
an
understimation
of
this
extended
part
of
Mendel's
work.
On
the
occasion
of
the
Mendel
Centennial
in
1965,
Mendel's
letters
to
Niigeli
were
subjected
to
a
more
profound
analysis.
Dunn
(1965,
p.
14)
wrote
that
"They
were
evidently
written
as
scientific
papers".
Olby
(1966,
p:
119)
was
the
fi
rst
who
referred
to
Mendel's
experiments
of
this
series
in
relation
to
the
problem
of
"constant"
or
"true
breeding"
hybrids.
Weiling's
analysis
of
Mendel's
Ilieracium
crosses
(Weiling,
1969a,
1971a)
showed,
at
least
indirectly,
that
it
is
necessary
to
undertake
a
detailed
analysis
also
of
Mendel's
other
crosses.
Similarly
Heimans
(1970)
gave
an
additional
impulse
to
study
the
question
of
"true
breeding"
hybrids
and
its
significance
in
Mendel's
work.
When
the
experiments
of
this
series
culminated,
Mendel
met
with
a
plant
genus
where
the
behaviour
of
the
hybrid
progenies
was
similar
to
that
of
Gartner's
"constant"
hybrids.
This
genus
was
Hieracium.
20
O4
HIERACIUM
Hieracium
was
one
of
the
plant
genera
hybridised
by
Mendel
along
with
the
majority
of
others
from
1866
on.
For
some
reasons,
the
I
-
Herm:him
experiments
should
not
be
separated
from
those
with
other
14
genera.
When,
however,
a
large
number
of
F,
plants
in
Mendel's
first
successful]
cross
with
Hieraciunt
did
not
show
any
variability
in
1868,
and
"constancy"
of
the
hybrid
in
Giirtner's
sense
might
have
been
suspected,
Mendel
largely
extended
his
hyb-
ridising
programme
with
this
genus.
Also
Mendel's
respect
to
the
role
of
hybridisation
in
solving
taxonomical
and
evolutional
problems
made
the
Hieracium
crosses
distinct
from
the
others.
Mendel's
Hieracium
paper
(1870)
and
his
detailed
notes
to
these
experiments
in
the
letters
to
Niigeli
facilitated
the
reconstruction.
Additional
data
were
published
by
Correns
(1905)
and
Weiling
(1969a,
1971a).
Nevertheless,
con-
tradictory
opinions
of
Mendel's
dependence
in
choosing
Hieraciuni
as
well
as
his
intentions
and
aims
were
accumalated
in
older
literature
in
a
larger
extent
compared
with
his
other
experiments.
04.01
ATTEMPTED
RECONSTRUCTION
The
data
of
the
reconstructed
Hieracium
experiments
are
given
in
Table
3.
In
contrast
to
the
preceding
series,
the
crosses
with
Ilieracium
were
exclusively
interspecifie.
They
comprised
22
combinations;
19
of
them
were
devoted
to
the
subgenus
Pilosella,
in
2
different
species
of
the
subgenus
Euhieracium
were
crossed,
and
in
one
combination
attempt
was
made
to
intercross
two
species
from
both
subgenera.
In
the
combination
[7],
also
a
triple
hybrid
[7a]
was
obtained.
The
F
1
plants
were
proved
in
21
combinations,
the
F2
plants
in
5,
the
F3
plants
in
3,
and
the
F4
plants
in
one
combination.
The
experiments
were
closed
with
the
F
1
in
seventeen
combinations,
with
the
F2
and
F3,
respectively,
in
two
combinations,
and
with
the
F4
in
one
combination.
Only
5
combinations,
or
21.7
per
cent,
yielded
F2
plants,
enabling
the
investigation
of
the
behaviour
of
hybrid
progenies.
In
three
combinations
(13.0
per
cent),
the
F
1
plants
were
baelccrossed,
and
the
BC
r
plants
were
obtained.
The
crosses
were
opened
in
1866
in
6
combinations,
in
1867
also
in
6,
in
1868
in
2,
in
1869
in
4,
and
in
1870
also
in
4
combinations.
It
was
thus
difficult
to
record
any
clear
culmination
point
regarding
the
year
of
opening
of
individual
combinations.
In
the
Hieracium
experiments,
several
independent
crosses
were
done
in
some
combinations
as,
e.g.,
in
combinations
[7],
19],
and
[12]
where
from
2
to
3
independent
experiments
were
sucessively
realised.
The
number
of
individual
operations
was
69.
It
is
not
excluded
that
an
additional
operation
was
performed
in
1872.
Evaluating
the
number
of
individual
operations
realised
in
individual
years
(Table
3,
last
row)
we
can
see
that
the
culmination
point
was
reached
.in
1869
and
1870.
In
Hieracium,
the
total
number
of
combinations
was
lower
than
in
the
preceding
series
of
Mendel's
experiments'
while
the
number
of
individual
operations
was
nearly
the
same.
Also
the
proportion
of
combinations
where
at
least
F2
plants
were
obtained
was
lower
in
Hieracium.
The
dynamics
of
experiments
of
both
series
expressed
in
terms
of
the
number
of
individual
21
operations
per
year
is
illustrated
in
Figure
3.
The
continuance
of
the
Hieracium
experiments
(6
vegetation
periods)
was
approximately
the
same
as
in
other
plants
(7
vegetation
periods).
Figure
3
also
shows
that
the
experiments
with
Hieracium
were
really
preceded
by
those
with
other
plants.
Both
curves
are
40
No.
of
individual
operations
30
20
10
....
H
"4
4
ts
CO
-
(C
(0
(0
.
CO
CO
CO
00
00
(2)
CO
CO
r
—I
7
-1
N
CO
-4
CI
N
N
00
00
•-1
r
—I
Fig.
3
Dynamics
of
Mendel's
experiments
with
Hieracium
(II)
and
with
plants
from
other
14
genera
(0)
characterised
by
a
more
or
less
distinct
peak
between
an
upward
and
a
down-
ward
.
part;
the
inc
rease
an
d
th
e
d
ec
li
ne
was
more
s
h
eer
i
n
o
ther
plants
than
in
Hieracium.
It
can
be
said
that
Mendel
paid
more
permanent
interest
to
Hie-
racium.
At
the
same
time,
Menders
more
profound
affection
to
Hieracium
fol-
lows
also
from
the
repeated
crosses
and
from
the
relatively
frequent
backcrosses
done
in
some
combinations.
Mendel's
interest
in
other
plants
rapidly
decreased
after
1868,
and
this
decrease
might
be
connected
with
Mendel's
recognition
that
the
progeny
of
the
hybrid
[6]
did
not
vary.
Since
that
moment,
he
concentrated
his
:effort
in
the
investigation
of
Hieracium.
From
this
point
of
view,
his
experiments
with
plants
of
other
14
genera
represented
really
only
preliminary
searching
for
"constant"
hybrids
which
finally
led
him
to
systematic
hybridisations
with
Hieracia.
In
spite
of
it,
the
proportion
of
combinations
where
at
least
F2
could
be
studied
was
lower
in
Hieracium
(21.7
per
cent
against
36.7
per
cent
in
other
plants).
This
fact
was,
however,
due
to
a
specific
situation
in
Hieracium
where
already
the
results
obtained
in
the
F
1
were
also
of
significance.
22
Table
3
Mendel's
experiments
with
Hieracium
Combination/Year
1866
1867
1868
1869
1870
1871
1872
[1]
H.
Pilosella
x
H.
pratense
[2]
II.
Pilosella
x
H.
praealtum
[3]
II.
Pilosella
x
H.
Auricula
[4]
H.
murorunt
x
II.
umbellatum
[5]
II.
murorum
x
II.
pratense
[6]
II.
praealtum
var.
obscurum
Reid).
x
x
H.
stoloniflorum
[II.
subcymigerum
x
H.
flagellare]
[7]
II.
praealtunt
(Bauhini)
x
II.
aurantiacum
[II.
magyaricum
X
II.
aurantiacum]
[7a]
(II.
praealtum
(Bauhini)
x
II.
aurantiacum)
x
II.
Pilosella
(B)
[(II.
magyaricuin
x
II.
auran-
tiacum)
x
II.
Pilosella
(B)]
[8]
II.
praealtunt
(?)
echioides
x
II.
aurantiacum
[9]
II.
Auricula
x
II.
Pilosella
(B)
[10]
II.
Pilosella
(B)
x
H.
Auricula
[11]
II.
praealtum
(Bauhini)
x
II.
Pilosella
[II.
magyaricum
x
II.
Pilosella
(B?)]
[12]
II.
Auricula
x
II.
aurantiacum
[13]
II.
cymosum
x
II.
Pilosella
(B?)
[14]
II.
Auricula
x
II.
pratense
(var.)
[15]
II.
Auricula
x
IL
cymosum
[16]
//.
XII
x
II.
Pilosella
(B)
[II.
cymigerurn
x
II.
Pilosella
(B)]
[17]
II.
XII
x
II.
Pilosella
vulgare
(M)
[II.
cyntigerunt
x
II.
Pilosella
vulgare]
[18]
II.
Auricula
x
II.
Pilosella
vulgare
(M)
[19]
II.
Auricula
x
II.
Pilosella
.
niveum
(M)
[20]
II.
praealtum
(M)
x
II.
Pilosella
incanunt
(M)
[H.
jlorentinum
II
obscitrum
X
II.
Pilosella
velutinum]
[21]
II?
x
II.
umbellatum
[II.
barbatum
x
IL
umbellatum]
[22]
II.
vulgatum
X
II.
umbellatum
C
C
C
C
C
C
F
1
F
t
F
1
F
1
F
1
F
1
C
C
C
C
C
C
F
2
F,
BC
C
F
1
BC
F,
BC
F1
Fi
F,
C
C
'
F
3
F
2
BC
'
.
C
F
1
F
2
BC
1
F
2
BC
1
C
C
F
t
F,
C
C
C
C
F
4
F
3
F
t
F
3
F
1
C
F,
C
F
1
F,
C
C
C
C
F
I
F
1
F
1
F2
F1
F
1
F
t
F
t
(F2)
1
No.
of
individual
operations
6
12
13
L
17
16
6
(1)
C
crossing
F,
to
F
4
hybrid
generation
BC
hackcrossing
BC,
backcross
generation
[
]
Nagel
's
redetermination
(B)
Brfinn
(M)
Mfinchen
23
04.02
CIIOICE
OF
IIIERACI
U.111
It
is
evident
from
the
first
letter
to
Nageli
that
Mendel's
first
five
crosses
(combinations
[1]
to
[5]
in
Table
3)
were
carried
out
in
the
summer
of
1866,
about
six
months
before
opening
correspondence
with
Nageli.
Thus
Menders
choice
of
this
object
was
not
directly
influenced
by
Nageli.
Also
Iloppe
(1971,
p.
125)
emphasises
that
Mendel,
"compared
with
a
widespread
opinion,
choose
this
plant
.genus
to
his
experiments
along
with
Cis-sit/in
and
Gem'',
of
his
own
accord
yet
before
opening
correspondence
with
Nageli".
Nevertheless,
rather
different
views
on
Nageli's
participation
in
opening
Mendel's
experiments
with
Ilieraciunt
exist
in
literature.
Mendel's
nephew,
Dr.
Alois
Schindler
said
in
a
memorial
speech
in
1902
that
Mendel
carried
out
his
crossed
with
Pisunt,
Phaseolus,
and
llieraciunt
"in
compliance
with
sugges-
tions
of
experienced
student
of
Hieracium,
Nageli,
Munich"
(cf.
Kii:‘ieneckjr,
1965,
p.
86).
Schindler's
assertion
that
Mendel
got
from
Nageli
directions
to
cross
not
only
Ilieracia
but
also
Pisum,
and
Phaseolus
was
clearly
exaggerated.
Up
to
now,
there
are
quotations
that
Mendel
opened
at
least
his
Hieraciunt
crosses
upon
Nageli's
impulse
or
recommendation
(Hrubj
,;
'1961;
Dunn,
1965,
etc.).
\Veiling
(1969a)
did
not
assume
that
Mendel
was
directly
influenced
by
Nagelni
in
his
choice
but
he
suggested
that
Mendel
mentioned
his
croses
with
Mcrae/um
in
his
first
letter
in
order
to
attract
Nageli's
interest.
Mendel's
custom
was
to
study
any
new
objects
preliminary,
and
it
can
be
assumed
that
also
in
the
case
of
Ilieracium,
he
prepared
his
plant
material,
in
the
latest,
in
1865.
A
real
motive
to
the
choice
of
Hieraciant
was
thus
neither
Nageli's
suggestion
nor
Mendel's
endeavour
to
facilitate
the
contacts
with
The
hawkweeds
belong
to
the
best
known
genera
where
numerous
forms
are
held
for
interspecific
hybrids.
Among
the
48
plant
hybrids
mentioned
in
the
Proceedings
of
the
Brno
Natural
Science
Society
during
the
period
of
Menders
hybridising
activity,
ten
samples
of
natural
hybrids
of
Ilieraciunt
were
pre-
sented:
IL
Pilosella-praealtum
Neilr.
A.
Makowsky:
Die
Flora
des
Brunner
Kreises,
1
(Abhandlungcn):
45-210,
p.
119,
1863
I/.
Pilosella-Auricula
NI.
(II.
Gift:rem:1
M.B.)
A.
Makowsky,
2
(Sitzungs-Berichte):
37,
1864
(session
on
June
10,
1863)
II.
Aurictlla-Pilosella
Fries
G.
von
Niessl,
3
(Sitzungs-I3erichte):
85-86,
1865
(session
on
December
14,
1864)
II.
biforcum
M.B.
C.
Miner,
4.
(Sitzungs-Bericbte):
82,
1866
(session
on
December
13,
1865)
II.
virescens
Sonder
(boreali-recemosum)
II.
echoioidi-Piloselht
II.
bifurcunt
W.K.
(Pilosella-echioides
Schultz)
C.
Romer,
5
(Sitzungs-Berichte):
55-56,
1867
(session
on
December
12,
1866)
II.
Pilosella-praeahunt
G.
von
Niessl,
5
(Sitzungs-Berichte):
57,
1867
(session
on
December
12,
1866)
24
II
.
pracalto-Pilosella
H.
Pilosella-praealtunt
G.
von
Niessl,
6
(Sitzungs-Berichtc):
68,
1868
(session
on
June
6,
1867)
The
chronology
of
their
presentation
suggests
that
Mendel's
decision
to
cross
Hieracia
could
be
stimulated
by
these
findings.
If
Mendel
opened
his
Hieracium
experiments
in
1866
and
his
preparations
dated
back
to
1865
at
least,
the
first
three
Hieraciunt
hybrids
presented
in
tl►e
Natural
Science
Society
by
Makowsky
and
Nicssl
could
represent
the
first
possible
impulse
to
his
choice.
On
the
other
hand,
also
Mendel
acquaintance
with
relevant
literature
might
have
influence
his
choice
of
Hieracium.
In
the
first
place,
Nitgeli's
work
could
play
here
an
important
role.
Nageli
occupied
himself
with
Ilieracium
in
two
periods
of
his
research
activity
as
mentioned
by
him
in
1884
(p.
236-237).
From
1840
to
1846,
he
attempted
to
classify
Hieracia
by
distinguishing
between
the
main
species
and
hybrids,
and
after
1864,
under
Darwin's
influence,
he
returned
to
.1lieracia
with
the
aim
to
study
the
circumstances
of
the
origin
of
species,
partially
by
means
of
extensive
transplantation
experiments.
Nitgeli
also
tried
to
solve
sonic
theoretical
problems
of
plant
hybridisation
in
about
1865.
It
seems
that
Mendel
knew
only
Nageli's
earlier
taxonomical
studies
from
the
'forties
(Nitgeli,
1841,
1845ab)
when
he
wrote
his
first
letter
at
the
very
close
of
1866
while
Nageli's
papers
dealing
with
evolutional
problems
appeared
as
late
as
in
1865
onwards,
and
remained
unknown
to
him
in
that
time.
Correns
(1905,
p.
194)
and
Hoppe
(1971,
p.
125)
are
of
the
same
opinion.
Thus
Mendel
addressed
Niigeli
with
his
first
letter
as
an
outstanding
specialist
in
taxonomy
of
Ilieracium
and
Cirsiunt
who
took
also
the
natural
hybrids
into
account.
Another
source
of
Mendel's
information
of
Hieracium
were
Fries'
papers.
Fries
was
quoted
by
Mendel
in
his
Ilieraciu►n
paper
(p.
28)
and
twice
in
his
second
letter
to
Nfigeli.
According
to
an
index
issued
in
1875
and
added
to
Volume
13
of
the
Brno
Proceedings,
Fries'
Epicrisis
generis
Ilieraciorum
(Fries,
1862)
was
available
in
the
Library
of
the
Brno
Natural
Science
Society.
From
the
Ver-
bandlungen
5
(Sitzungs-Berichtc);
.4,
1867
we
know,
however,
that
this
book
got
into
the
Library
as
late
as
at
the
close
of
1865.
In
his
third
letter,
Mendel
also
named
F.
Schultz
who
attempted
as
the
first
to
cross
Hieracia
(Schultz,
1854-55;
1861;
cf.
also
Focke,
1881).
Finally,
also
Gartner
(1849)
devoted
one
chapter
of
his
book
to
the
problem
of
natural
plant
hybridswhere
mainly
those
of
Manch,'
and
Cirsium
were
discussed.
Thus
the
list
of
papers
which
could
influence
Mendel
in
his
decision
to
experiment
with
Hieracia
was
quite
extended.
Mendel
knew
all
corresponding
papers
about
natural
and
artificial
Hicracium
hybrids,
and
each
of
these
sources
might
have
been
of
the
same
significance
in
stimulating
his
work
with
Ilieracium
as
Nageli's
papers.
It
can
be
concluded
that
the
preparation
and
realisation
of
Mendel's
first
Bieraciunt
crosses
happened
before
he
contacted
Nitgeli.
The
impulses
to
Men-
del's
decision
must
have
come
from
somewhere
else.
His
work
with
Hieracium
might
have
been
stimulated
by
numerous
cases
of
spontaneous
hybrids
presented
in
the
Brno
Natural
Science
Society
since
1863
as
well
as
by
his
study
of
relevant
literature.
25
04.03
INITIAL
INTENTIONS
In
his
Hieracium
report
written
and
read
in
1869,
Mendel
formulated
his
aims
thoroughly.
In
the
period
from.
1866
to
1869,
however,
practically
no
suggestions
of
this
kind
can
be
found
in
the
corresponding
sources.
It
might
be
supposed
that
Mendel
had
in
Ilieracia
similar
aims
to
those
he
had
in
other
plants,
i.e.,
to
verify
the
conclusions
obtained
in
Pisum
and,
at
the
same
time,
to
search
among
them
for
"constant"
hybrids
assumed
by
Gartner.
From
the
very
outset,
Mendel
suspected
the
Hieracium
hybrids
to
be
an
example
of
the
"constant"
ones.
He
wrote
in
his
first
letter
to
Nageli:
"The
surmise
that
some
species
of
Hieracium,
if
hybridised,
would
behave
in
a
fashion
similar
to
Geum,
is
perhaps
not
without
foundation.
It
is,
for
instance,
very
striking
that
the
bifurcation
of
the
stern,
which
must
be
considered
transitional
among
the
Piloselloids,
may
appear
as
a
perfectly
constant
trait
as
I
was
able
to
observe
last
summer
on
seedlings
of
II.
stoloniflorunt
While
in
Geum
Mendel's
assumption
of
the
hybrid
"constancy"
followed
from.
Gartner's
authority,
in
Hieracium
it
was
a
consequence
of
his
own
observation.
Mendel
could
reflect:
If
the
forked
stems
are
a
transitional
constant
character
in
Piloselloidae
then
this
trait
might
arise
by
means
of
crossing
which
led
to
the
hybrid
"constancy".
Thus
Mendel
foreshadowed
his
Hieracium
experiments
for
the
investigation
of
"constant"
hybrids.
On
the
other
hand,
this
project
was
closely
connected
with
taxonomical
aspects
as
the
transitional
forms
are
the
cause
of
nearly
all
difficulties
which
arise
in
the
classification
of
the
highly
polymorphic
genus
Hieracium.
The
sources
of
Mendel's
knowledge
on
this
matter
might
have
been
not
only
his
own
experience
but
also
relevant
literature,
for
example,
Gartner
(1849)
who
summarised
the
then
knowledge
on
transitional
forms
(p.
598).
Even
if
no
Mendel's
marginalia
were
found
in
the
relevant
text
of
his
copy,
yet
Mendel's
approach
appears
to
be
very
close
to
Giirtner's
ideas.
Time
fact
that
Mendel
combined
the
hybrid
"constancy"
in
Gartner's
sense
with
the
phenomenon
of
transitional
forms
shows
Mendel's
originality.
Thus
Mendel's
independence
in
establishing
his
initial
working
programme
with
Hieracium
seems
to
be
clear.
Mendel's
endeavour
to
evaluate
his
hybridisations
with
hawkweeds
in
verifying
the
Pisum
experiments
(cf.
Gaissinovitch,
1965,
p.
128;
Weiling,
1971a,
p.
87)
apparently
decreased
and
an
idea
arose
in
Mendel's
mind
that
Hieracium
gave
a
unique
chance
to
appreciate
the
role
of
hybridisation
in
a
peculiar
.
taxonomic
and
evolutional
situation
as
suggested
also
by
Sinoti)
(1971).
04.04
FLORISTICAL,
TAXONOMICAL
AND
GROWER'S
EXPERIENCE
In
his
Hieracium
experiments,
Mendel
was
compelled
to
overcome
many
difficulties
which
were
incomparable
in
complexity
with
those
arising
in
other
plants.
The
first
problem
was
connected
with
obtaining
experimental
material.
When
he
became
interested
in
Ilieracia,
Mendel
began
collecting
representants
of
this
genus
what
found
art
echo
in
his
letters
to
Niigeli.
Although
he
did
not
realise
26
all
his
plans
suggested
in
the
second
letter
he
gathered
a
considerable
amount
of
Hieracium
forms.
Some
of
them
originated
from
the
nearest
vicinity
of
Brno.
He
visited
also
the
botanically
interesting
territory
south-east
of
Brno
and
the
limestone
territory
northwards
the
town.
During
his
excursions,
his
field
work
did
not
end
with
making
a
record
and
collecting
herbarium
specimens.
Some-
times,
he
collected
the
seeds,
more
frequently,
he
Jigged
up
whole
plants
in
order
to
transplant
them
into
the
garden
or
the
greenhouse.
It
is
unknown
wheteher
Mendel
made
these
excursions
alone
or
in
a
group
of
his
friends
from
the
Natural
Science
Society.
Among
the
reports
on
numerous
botanical
trips
published
in
the
Proceedings,
none
was
found
giving
evidence
of
Mendel's
taking
part
in
them.
Collected
Ilieracium
forms
had
to
be
taxonomically
classified.
Mendel
was
qualified
in
determining
Hieracia
what
can
be
easily
appreciated.
He
sent
to
Nageli
various
Hieracium
forms,
and
his
original
determination
can
be
compared
with
Niigeli's
redeterminations
published
by
Correns
(1905).
The
difference
between
Mendel's
and
Niigeli's
determinations
reflect
rather
terminologically
than
materially
opposite
views.
NageIrs
corrections
concerned
mainly
the
denotations
of
intermediate
and
transitional
forms
where
Mendel
preferred
to
suggest
supposed
relations
to
corresponding
main
species.
In
his
determinations,
Mendel
made
also
use
of
documentary
herbarium
materials
which
were
not
scanty
in
the
Natural
Science
Society.
Already
in
the
first
year
of
Society's
activity,
the
capacity
of
the
herbarium
of
Phanerogantae
was
"2250
Species
in
circa
20,000
Ex."
(Verhandlungen
1
(Sitzungs-Berichte):
91,
1863).
Later,
new
hundreds
of
dried
specimens
delivered
by
the
botanists
of
Brno
and
extensive
shipments
of
foreign
exsiccata
were
recorded
in
the
Proceedings.
Mendel
also
attempted
to
utilise
cultivation
experiments
in
order
to
confirm
an
agreement
between
two
species
(see
his
third
letter).
Thus
the
milieu
of
Brno
Natural
Science
Society
mentioned
already
in
part
03.02
helped
Mendel
in
the
solution
of
complicated
taxonomical
problems
in
Hieracia.
Among
the
scientist
of
Brno,
Mendel
found
fi
rst
impulses
to
cross
Ilieracia,
knowledge
of
their
localities
and
aid
in
their
determination.
He
was
also
compelled
to
adapt
his
gardener's
experience
to
the
new
experi-
mental
object.
Thanks
to
the
perenniality
of
Ilieracia
Mendel
kept
probably
individual
parent
forms
and
hybrids
continuously
for
many
years
so
that
the
yearly
sowing
could
be
omitted.
On
the
other
hand,
they
required
a
permanent
care
and
made
a
spatial
and
time
manoeuvring
difficult.
In
a
short
time,
his
Ilieracium
experiments
grew
up
to
a
large
extent.
Since
1867,
Mendel
frequently
spoke
about
his
"plantage".
It
is
evident
from
one
place
in
the
third
letter
that
at
least
the
majority
of
Hieracium
experiments
was
carried
out
intentionally
in
well-balanced
soil
and
microclimatic
conditions
in
order
to
exclude
the
modifying
influence
of
the
en-
vironment
and
to
make
possible
objective
comparisons
of
different
forms.
In
that
plot,
the
plants
were
grown
"in
unfertilised,
sandy
soil".
It
means
that
the
ma-
jority
of
Hieracia
was
not
cultivated
in
Menders
famous
experimental
garden
but
in
another
place
of
the
Monastery
garden.
A
serious
problem
of
raising
Hieracium
seedlings
was
the
choice
of
a
suitable
sowing
time
which
would
limit
the
damage
of
seedlings
during
the
wintering
period
and,
at
the
same
time,
ensure
not
too
late
fl
owering
in
the
next
vegetation
period.
In
this
respect,
Mendel's
great
practical
experience
in
the
biology
of
deve-
lopment
and
fl
owering
was
an
additional
precondition
to
his
success.
27
Mendel
also
made
use
of
vegetative
reproduction
of
Hieracia
by
means
of
stolons
which
enabled
him
to
send
to
Nfigeli
a
sufficient
amount
of
living
or
dried
material.
Ile
surely
believed
that
this
way
of
reproduction
ensured
the
preserva-
tion
of
the
original
constitution
of
characters
of
an
individual.
In
growing
Ilieracia
in
balanced
environmental
condition
and
in
reproducing
the
documentation
material,
a
wide
parallelism
existed
between
Nageli's
and
Mendel's
growing
methods
(cf.
Nageli,
1884).
04.05
TECHNIQUE
OF
CROSSING
It
must
be
stressed
that
Mendel's
Ilieracium
hybrids
were
the
first
obtained
by
reliable
methods
of
crossing.
Mendel
really
took
up
a
pioneer's
work
in
this
field.
At
the
same
time,
he
could
utilise
here
all
his
previous
excellent
experience.
In
Hieracium,
however,
"due
to
the
minuteness
of
the
flowers
and
their
peculiar
structure"
(llieraciunt
paper,
p.
26),
successfull
crosses
were
very
difficult,
and
Mendel
himself
speculated
about
possible
improvements
and
simplifications.
His
Cirsium
crosses
represented
his
technical
preparation
for
the
Hieraciunt
experi-
ments
as
given
in
part
03.03.
Several
possibilities
were
attempted
in
Cirsium
in
1866,
among
others
the
pollination
of
physiologically
pollen
-sterile
late
flowers
without
emasculation.
This
technique
was
corresponding
to
that
recommended
by
Nageli
in
the
letter
from
February
1876
(cf.
Hoppe,
1971,
p.
136):
"It
would
be
useful
to
have
plants
with
aborted
Pollen
(what
sometimes
occurs)
or
to
induce
this
failing
artificially".
It
is
clear
that
Mendel
investigated
this
technique
already
earlier.
Another
method
tested
in
Cirsium
was
successfully
applied
by
Mendel
in
his
combination
[6]
where
he
pollinated
young
prematurely
uncovered
stigmas
without
emasculation.
Further
improvement
recommeded
by
Niigeli
in
1871
and
1874,
respectively
(cf.
Hoppe,
1971,
p.
125),
came
too
late
as
no
Mendel's
Hieracium
cross
was
produced
after
1870.
Nevertheless,
in
his
Hieraciunt
paper
(p.
26-27),
Mendel
took
pattern
by
classical
methods
with
a
pollination
after
a
previous
emasculation,
without
men-.
tioning
any
improvements.
It
seems
that
in
the
period
immediately
before
1869,
Mendel
relied
solely
upon
the
careful
application
of
the
classical
method
of
crossing
while
he
became
mistrustful
against
any
simplification.
According
to
his
eighth
letter,
he
even
brought
an
occular
disorder
upon
himself
using
a
simpl
e
illumination
apparatus.
His
reliance
upon
the
very
laborious
classical
method
was
not
a
symptom
of
his
conservatism.
He
advocated
this
procedure
for
the
reason
that
no
other
sort
of
mistake
could
arise
besides
that
which
would
be
a
consequence
of
a
premature
self
-fertilisation
inside
the
young
yet
unopened
flower.
The
occurence
of
purely
maternal
types
among
supposed
hybrids
was
considered
by
Mendel
an
experimental
error.
The
phenomenon
of
apogamy
remained
unknown
to
him
in
that
time.
04.06
NAGELI'S
INFLUENCE
In
analysing
Nageli's
share
in
opening
and
continuing
Mendel's
Hieracium
experiments,
Olby
(1966,
p.
118)
came,
most
probably,
nearest
the
truth:
"Nfigeli
encouraged
Mendel
to
concentrate
his
attention
on
Hieraciunt".
28
Only
a
part
of
Niigeli's
answer
from
February
25,
1867
was
preserved.
It
seems
that
this
fragment
includes
all
what
Nageli
wrote
in
this
letter
about
Ilieracia.
Nageli
recommended
to
Mendel
to
continue
and
elaborate
his
Pisum
experiments,
and,
at
the
same
tune,
he
approved
his
choice
of
.11ieracium
as
of
a
new
object
(cf.
Hoppe,
1971,
p.
136):
"It
would
be
exceedingly
desirable
if
you
succeeded
in
carrying
out
hybrid
fertilisations
in
Ilieracium
as
this
genus
might
be,
in
a
brief
space
of
time,
the
best
known
one
as
regards
the
transitional
forms".
Here
Niigeli
might
have
attempted
to
concentrate
Mendel's
attention
to
Ilieracia.
Undoubtedy,
Mendel
held
this
passage
for
Nageli's
approval
of
his
decision
to
hybridise
Ilieracia
in
extended
experimental
series.
Along
with
the
letter
from
February
1867,
Niigeli
sent
Mendel
reprints
of
his
papers
which
were
mentioned
by
Correns
(1905,
p.
198).
The
shipment
contained
twelve
different
reprints.
The
last
three
reprints
(Nageli,
1866
abc)
were
surely
most
welcome
to
Mendel.
They
discussed
transitional
forms
of
Ilieracium
as
a
specific
phenomenon
in
this
genus,
and
this
matter
was
the
most
attractive
to
Mendel.
Mendel
suggested
already
in
the
first
letter
that
his
first
five
Ilieracium
crosses
were
unsuccessfull;
according
to
the
appearance
of
seedlings,
self
-fertilisation
oecured.
Under
these
circumstances,
a
danger
of
Mendel's
resignation
was
imminent
in
the
winter
period
1866-1867.
Fortunately,
a
plant
fro►n
the
cross
[6]
of
1866
was
found
to
be
doubtless
of
hybrid
origin.
Already
in
this
connection,
Mendel
anticipated
the
fact
which
was
formulated
by
him
in
extenso
as
late
as
in
-
1869
that
some
species,
as
his
II.
pilosella
or
H.nutrorum
used
in
the
co►nbinations
[1]
to
[5],
represented
"bad"
mothers
while
his
IL
praealtum
var.
obscurum
used
in
the
suecessfull
cross
[6]
was
"good".
Thus
among
six
further
crosses
of
1867,
three
of
Mendel's
forms
of
IL
praealtum
participated
as
mother
partners.
On
April
18,
1867,
Mendel
wrote
to
Niigeli
for
tl►e
second
time.
He
waited
for
Niigeli's
answer
in
vain,
and
more
than
after
six
months,
on
November
6,
1867,
he
wrote
to
Nageli
again.
Filially,
after
three
additional
months,
he
wrote
(in
February
9,
1868)
for
the
fourth
time
without
obtaining
answer
to
his
third
letter.
Niigeli's
long
silence
might
have
been
peculiar
to
Mendel.
At
that
time,
Nageli
was
surely
absorbed
in
his
own
Ilieracium
studies,
and
he
could
not
sup-
pose
from
Mendel's
second
letter
that
his
interest
in
Ilieracium
was
serious,
especially
if
he
himself
did
not
believe
in
the
feasibility
of
artificial
crosses
with
Ilieracium.
On
the
other
hand,
it
might
be
expected
that
Niigeli
really
intended
to
answer
Mendel's
third
letter
where
much
interesting
data
were
given about
Mendel's
first
successful]
Ilieracium
cross.
Complications
arose
from
Niigeli's
eye
injury,
mentioned
by
Correns
(1905,
p.
219)
and
also
in
Mendel's
fifth
letter.
In
his
fourth
letter
of
February
1868,
Mendel
asked
Niigeli
for
sending
the
experimental
Ilieracium
material.
Mendel
could
not
wait
for
Niigeli's
answer
on
his
third
letter
if
he
did
not
want
to
miss
the
period
of
early
spring
sowing
when
flowering
plants
might
be
obtained
in
the
same
year.
This
time,
as
soon
Niigeli
was
explicitely
asked
'by
Mendel
to
send
material
for
further
crosses,
and
it
was
for
the
first
time
during
their
correspondence,
Nageli
did
not
refuse
his
aid,
and
on
April
28,
1868,
he
complied
with
Mendel's
wishes
and
sent
to
hint
ten
samples
of
the
seeds
from
the
subgenus
Enhieraciu/n.
No
peculiar
conclusions
can
be
drawn
from
this
irregularity
of
correspondence
between
Mendel
and
Niigeli.
Even
if
some
psychological
factors
suggested
by
Iltis
(1924,
p.
123)
must
be
considered
the
contents
of
the
relevant
documents
29
seem
to
testify
to
a
rather
normal
development
of
contacts
between
the
two
investigators,
as
it
was
newly
suggested
also
by
I
-
Ioppe
(1971).
In
his
fourth
letter,
Mendel
noted
that
he
resolved
to
intercross
only
the
main
species.
He
formulated,
thereby,
his
aims
of
his
Hieracium
crosses
for
the
next
time
after
his
first
letter.
An
idea
crystallised
in
his
mind
to
investigate
whether
the
constant
intermediate
or
transitional
forms
could
originate
from
crossing
bet-
ween
main
species.
This
clearly
determined
problem
laying
stress
on
taxonomical
and
evolutional
aspects
must
have
been
influenced
somehow
by
Nageli.
Mendel's
formulation
of
his
designs
with
Meracia
resulted
evidently
from
his
concentrated
study
of
Nageli's
recent
papers
and
from
his
own
subsequent
deliberation.
It
is
clear
from
available
documents
that
Nageli
continued
to
provide
Mendel
with
plant
material.
Nageli's
letters
were
thus
rather
brief
comments
to
his
shipments,
similarly
to
his
second
letter
of
April
28,
1867.
Also
his
third,
fourth,
and
sixth
letters
of
May
11,
1867,
Septembre
1867,
and
April
27,
1868,
respectively,
were
probably
of
the
same
character.
Undoubtedy,
the
choice
of
forms
consisted
mainly
in
Mendel's
initiative
(see
Mendel's
fourth
letter).
In
fragment
fraoi
cut
of
Niigeli's
sixth
letter,
lie
himself
called
Mendel
to
give
desirable
species.
On
the
other
hand,
Nageli
occasionally
sent
some
forms
of
his
own
accord
so
that
he
might
influence
the
direction
of
Menders
experiments
by
his
deliberate
choice
of
materials
with
which
he
entrusted
Mendel.
To
his
third
letter,
Nageli
added
also
two
schemes
demonstrating
the
occurence
of
transitional
forms
in
the
subgenera
.Pilosella
and
Euhieracium.
Correns
(1905,
p.
221)
reproduced
the
first
of
them.
Nageli
returned
to
this
matter
again
in
his
sixth
letter
(cf.
Hoppe,
1971,
p.
137-138).
It
seems
that
Mendel's
orientation
on
intererossings
with
the
main
species
was
acceptable
to
Nageli.
Nevertheless,
Nageli
never
believed
in
the
creative
role
of
hybridisation
in
the
evolution
of
Ilieracia.
Natural
hybridisation
seemed
to
him
a
rare
phenomenon
and
natural
hybrids
too
instable.
In
spite
of
it,
he
was
interested
in
obtaining
samples
of
Menders
hybrids,
especially
after
1873
(cf.
Hoppe,
1971,
p.
126).
It
is
not
easy
to
say
what
scientific
aims
were
pursued
by
Niigeli
when
he
supported
Mendel
in
his
Meracium
experiments.
At
the
same
time,
Nageli
did
not
impose
any
definite
tasks
upon
Mendel
in
order
to
utilise
the
results
in
his
own
reflections.
When
Nageli
wrote
in
his
letter
from
April
1870
that
he
,is
happy
he
found
in
Mendel
"a
handy
and
successfull
collaborator"
(Hoppe,
1971,
p.
138)
no
direct
evidence
can
be
drawn
from
it
that
he
exploited
the
work
of
his
younger
colleague.
Thus
Mendel
continued
his
Hieracium
experiments
as
independently
of
Nageli
as
he
did
it
before.
A
testimony
to
this
fact
can
be
found
in
one
passage
of
Mendel's
Hieracium
paper.
On
p.
27-28,
Mendel
clearly
exposed
the
problems
pursued
by
him
in
his
11
ieracium
crosses.
After
having
discussed
the
peculiar
taxonomic
situation
in
this
genus,
he
picked
out
the
question
"whether
and
to
what
extent
hybridisation
plays
a
part
in
the
production
of
this
wealth
of
forms"
(p.
27).
hi
this
context,
three
groups
of
authors
were
named
according
to
their
attitude
to
the
role
of
hybridisations
in
the
polymorphism
of
Hieracia.
Among
others,
an
intermediate
group
of
authors
which
did
not
accept
nor
neglect
this
role
was
characterised
by
their
assertion
that
the
Hieracium
hybrids
were
not
important
due
to
their
short
duration
in
the
consequence
of
the
restricted
fertility
or
even
sterility
as
well
as
of
the
inability
of
the
hybrid
pollen
to
bring
about
self
-fertilisation
in
the
presence
of
the
parent
pollen.
Thus
the
Hie
-
30
racium
hybrids
cannot
constitute
and
maintain
themselves
as
fully
fertile
and
constant
forms
when
growing
near
their
parents.
This
characterisation
fits
in
with
Niigeli's
ideas
in
many
respects
(cf.,
e.g.,
Nageli
and
Peter,
1885,
p.
62f.).
It
is
now
clear
that
Mendel
gave
Nageli's
standpoint
along
with
others
which
should
not
have
been
proved
without
testing,
and
this
fact
demonstrates
that
he
did
not
accept
Nageli's
views
uncritically.
In
the
following
paragraph,
Niigeli
was
indirectly
named
as
a
famous
Hieracium
specialist
who,
in
the
spirit
of
the
Darwiinan
teaching,
defended
the
view
that
the
constant
intermediate
forms
arose
from
the
"transmutation"
of
extinct
or
existing
species.
The
whole
context
suggests
that
Mendel
(lid
not
agree
with
Nageli's
speculation
and
that
he
was
stressing
the
experimental
proof.
Two
levels
of
knowledge
were
given
by
Mendel
to
be
necessary
in
solving
the
problem:
In
the
first
place,
the
appearance
and
fertility
of
hybrids
and
the
behaviour
of
their
progeny
should
be
recognised.
Only
subsequently,
a
judgment
should
be
done
with
the
aid
of
knowledge
of
geobotanical
and
ecological
conditions
(=---
"Vegetationsverhaltnisse")
of
wild
forms.
Niigeli's
approach
consisting
mainly
in
the
investigation
of
"Vegetationsverhaltnisse"
was
given
here
clearly
insufficient.
Thus
Mendel's
designs,
even
if
they
referred
to
a
more
narrow
problem,
were
different
if
not
contradictory
compared
with
Niigeli's
attitude,
esepcially
regarding
the
significance
ascribed
to
the
experimental
solution.
During
the
period
from
1863
or
1864
when
Mendel
met
for
the
first
time
with
natural
Hieracium
hybrids
up
to
1869
when
he
exposed
his
definitive
experimental
programme
with
this
genus
he
had
brilliantly
mastered
the
contemporaneous
knowledge
of
problems;
lie
achieved
to
formulate
his
aims
with
a
precision
comparable
with
that
of
the
preface
to
his
Pisum
experiments.
Even
if
his
conception
was
still
deepened
in
the
next
years
after
1869,
the
preponderance
of
more
general,
i.e.,
taxonomical
and
evolutional
problems
over
those
dealing
purely
with
heredity
was
here
clear.
Nageli
and
Mendel
were
two
scientists
who
relatively
independently
endeavour-
ed
to
get
an
insight
into
similar
problems
of
the
genus
Hieracium.
Their
thought
developed
Iffirallelly
in
many
instances
but
not
identically;
each
of
them
strived
for
his
own
approach.
Mendel
was
able
to
take
over
from
Nageli
much
of
his
theoretical
experience
and
to
include
it
to
his
own
assests,
some
of
which
were
acquired
by
a
painstaking
pioneer's
effort.
lendel
made
full
use
of
Niigeli's
authority
not
only
in
the
classification
of
Hieracia
but
especially
in
collecting,
cultivating,
and
studying
natural
specimens.
In
all
other
respects,
Mendel
showed
a
large
degree
of
independence,
and
in
emphasising
the
necessarity
of
an
experimental
approach
in
the
study
of
taxonomical
and
evolutional
problems
of
hybridisation
in
Hieracium,
he
had
surely
priority.
04
.0
7
EVALUATION
OF
RESULTS
In
his
Hieracium
paper
Mendel
summarised
his
results
into
four
paragraphs
referring
to
the
appearance
of
hybrids;
their
fertility;
the
behaviour
of
the
progeny
of
hybrids;
and
the
role
of
the
hybrid
and
the
parent
pollen
in
their
fertilisation.
After
1869,
some
new
problems
appeared
and
some
conclusions
were
modified
in
agreement
with
recent
resultS.
31
According
to
Mendel's
plans,
all
possible
crosses
among
main
species
in
Pilosella
and
in
Euhieracium
were
designed
but
only
a
small
part
of
intended
crosses
appeared
to
be
realisable.
Fortunately,
Mendel
recognised
relatively
soon
that
the
success
of
a
cross
was
not
determined
by
the
two
parents,
but
exclusively
by
the
maternal.
The
difference
between
"good"
and
"bad"
mothers
might
have
been
known
to
him
as
early
in
1866-1867
as
it
was
shown
in
part
04.06.
A
definitive
solution
appeared
in
1869-1870
when
a
long
series
of
forms
used
as
mothers
could
have
been
compared
(see
Mendel's
eighth
letter).
When
the
species
were
ordered
on
the
basis
of
the
success
of
crossing
from
the
"completely
reliable"
to
the
"worse",
the
sequence
was
following:
II.
Auricula,
H.
XII
[=--
H.
cymigerurn]
>
H.
pracalturn
>
>
II.
Pilosella,
H.
cynzosurn
>
H.
aurantiacurn.
In
general,
also
nearly
all
Euhieracia
showed
"bad"
mothers.
This
finding
was,
of
course,
purely
empirical,
and
Mendel
did
not
give
any
explanation.
We
will
see
later
that
he
met
here,
for
the
first
time,
with
the
consequence
of
apogamy
in
Hieracium.
After
1869,
Mendel
did
not
really
use
other
mother
forms
than
the
first
three
most
successfull.
And
this
intentional
Choice
of
mother
species
enabled
him
to
obtain
hybrids
not
by
one
or
two
as
yet
but
by
tens
or
even
hundreds
so
that
"mass"
experiments
were
possible.
At
the
same
time,
Mendel
did
not
believe
that
the
success
of
a
cross
might
have
been
determined
by
the
taxonomical
relationship
of
-partners
as
it
was
assumed
by
Nageli.
In
combination
[9],
the
parents
were
closely
related
in
comparison
with
combination
[12],
nevertheless,
no
difference
in
the
crossability
occured.
Mendel
carefully
analysed
different
traits
regarding
the
appearance
of
the
hybrids
compared
with
their
parents.
Both
intermedierity
and
dominance
were
found
in
individual
trait
pairs.
In
1868,
an
unexpected
phenomenon
was
disclosed.
In
the
combination
[7]
and
[12],
two
diffeirent
hybrid
foms
appeared,
in
spite
of
Mendel's
conviction
that
the
F
1
plants
of
pure
parents
should
have
been
uniform.
In
1869,
even
three
such
different
forms
appeared
in
the
com-
bination
[14].
The
explanation
that
"we
have
here
only
single
terms
in
yet
unknown
series"
(Hieracium
paper,
p.
29)
seemed
evidently
unsatisfactory
to
him,
and
that
is
why
he
strived
for
a
more
detailed
study
of
this
F1
non
-uniformity.
"Mass"
hybridisations
in
combinations
[12,
second
cross]
and
[16]
revealed
an
extraordinary
variability
among
the
hybrids.
All
possible
transitions
from
one
parent
to
another
appeared.
Mendel
estimated
that
"many
hundreds"
combina-
tions
should
arise
in
the
F
1
in
[12].
That
was
why
he
supposed
that
a
more
easy
analysis
of
this
phenomenon
could
have
been
done
if
the
parents
were
not
so
different.
Unexpectedly,
if
more
closely
related
forms
were
intercrossed,
a
uni-
formity
appeared
among
25
to
84
hybrids
obtained
in
combinations
[9,
third
cross],
[18],
and
[19]
in
1871.
Thus
in
solving
the
problem.
of
uniformity
vs.
non
-uniformity
of
his
hybrids,
Mendel
obtained
different
results
in
different
periods
of
his
Hieracium
experiments,
and
before
closing
his
correspondence
with
Nageli,
he
attempted
no
more
to
draw
any
conclusions
in
this
respect.
Correns
(1905,
p.
251-252),
relying
upon
Nageli's
determinations,
threw
some
doubts
on
the
extremely
high
variability
of
some
of
Mendel's
hybrids
in
the
F
1
.
It
is
true
that
Mendel
obtained
altogether
174
hybrids
in
combination
[12,
32
second
and
third
cross]
while
Nageli
recognised
only
8
different
forms
among
them.
Among
only
29
hybrids
from
combination
[16],
however,
Nageli
found
even
9
different
forms.
On
the
other
hand,
Nageli
found
different
forms
also
among
F
1
plants
of
combinations
and
crosses
held
by
Mendel
for
uniform.
For
example,
in
combination
[9,
third
cross]
and
[18],
two
different
forms
were
determined
by
Nageli
in
each
while
in
[19],
the
uniformity
was
really
proved
by
recognising
only
one
form
here.
It
seems,
nevertheless,
improbable
that
Mendel
would
exaggerate
the
extreme
variability
of
some
hybrids.
It
is
not
known
whether
he
sent
all
obtained
hybrid
specimens
to
Nageli
or
whether
he
submitted
to
him
only
a
more
of
less
representative
sample.
Mendel's
results
concerning
the
fertility
of
his
hybrids
were
clearer.
In
his
paper,
he
divided
the
hybrids
into
five
groups:
fully
fertile
(combination
[8]);
fertile
(combinaiton
[6]);
partially
fertile.(combinations
[7]
and
[14]);
slightly
fertile
(combination
[9]);
and
infertile
(combination
[12]).
Thus
always
a
certain
grade
of
fertility
was
reached
with
one
exception.
Just
in
[12],
however,
the
later
two
crosses
gave
a
better
result:
about
25
per
cent
of
F
1
plants
were
completely
fertile,
about
50
per
cent
fertile,
and
about
25
per
cent
sterile.
Thus
not
only
the
appearance
of
the
hybrids
but
also
their
fertility
were
different
within
the
same
combination.
The
degree
of
fertility
was',
however,
independent
of
the
type
of
the
hybrid.
It
seems
that
Mendel
was
surprised
by
this
generally
high
hybrid
fertility.
In
each
case,
his
crosses
were
more
successfull
from
the
viewpoint
of
fertility
than
those
of
his
predecessor
F.
Schultz
(1854-1855;
1861).
The
fertility
of
hybrids
was
an
important
precondition
to
study
the
behaviour
of
their
progeny.
Just
in
this
respect,
Mendel
awaited
the
results
with
greatest
thrill.
This
time,
his
conclusion
was
unambiguous:
In
all
five
combinations
([6],
[7], [8],
[9],
and
[12])
where
the
experiments
were
performed
at
least
till
the
F2,
the
progenies
of
individual
F
1
plants
did
not
show
any
variability,
and
agreed
with
the
hybrid
mother
plant
from
which
they
were
derived.
This
fact
did
not
change
even
in
higher
generations
(in
the
F3
and
F4
in
[6],
and
in
the
F3
in
[7]
and
[8]).
No
doubt
arose
in
Mendel's
mind
about
the
correctness
of
this
result
from
the
methodical
point
of
view.
All
F2
progenies
said
to
be
"constant"
were
derived
from
different
F2
plants,
and
also
the
number
of
plants
in
individual
progenies
seemed
him
to
be
sufficient.
In
the
first
four
F
2
progenies,
from
14
to
112
plants
were
grown
per
one
F
1
plant.
In
accord
with
his
Getun
experiment,
Mendel
was
convinced
that
the
"variability"
should
have
been
proved
when
a
recessive
at
least
in
one
trait
would
occur
so
that
it
would
been
practically
impossible
to
omit
such
an
elementary
manifestation
of
the
hybrid
"variability",
even
among
the
minimum
numerous
progeny
with
14
plants.
On
the
other
hand,
Mendel
himself
was
aware
of
the
incompleteness
of
his
results
on
higher
levels.
Only
few
F2
progenies
from
few
combinations
were
taken
into
experiments.
it
seems,
however,
that
he
held
this
fact
for
not
very
important
as
any
"variable"
hybrid
should
have
shown
differences
among
his
progenies.
In
combinations
[6],
[7],
[8],
and
[9],
only
one
progeny
was
studied
in
each
from
this
point
of
view,
and
in
[12],
only
three
progenies.
According
to
his
tenth
letter,
materials
were
pre-
pared
in
1871
and
1872
for
investigating
the
behaviour
of
higher
generations
of
hybrids
obtained
in
the
"mass"
hybridisations
since
1869,
but
these
experimets
were
never
realised.
The
starting
point
to
the
relatively
independent
experiments
with
the
role
of
33