If two plants which differ constantly in one or several characters
be crossed, numerous experiments have demonstrated that the common
characters are transmitted unchanged to the hybrids and their progeny;
but each pair of differentiating characters, on the other hand,
unite in the hybrid to form a new character, which in the progeny of
the hybrid is usually variable. The object of the experiment was to
observe these variations in the case of each pair of differentiating
characters, and to deduce the law according to which they appear in
the successive generations. The experiment resolves itself therefore
into just as many separate experiments as there are constantly
differentiating characters presented in the experimental plants.
The various forms of Peas selected for crossing showed differences in
the length and colour of the stem; in the size and form of the leaves;
in the position, colour, and size of the flowers; in the length of
the flower stalk; in the colour, form, and size of the pods; in the
form and size of the seeds; and in the colour of the seed-coats and
the albumen [cotyledons]. Some of the characters noted do not permit
of a sharp and certain separation, since the difference is of a “more
or less” nature, which is often difficult to define. Such characters
could not be utilised for the separate experiments; these could only be
confined to characters which stand out clearly and definitely in the
plants. Lastly, the result must show whether they, in their entirety,
observe a regular behaviour in their hybrid unions, and whether from
these facts any conclusion can be come to regarding those characters
which possess a subordinate significance in the type.
The characters which were selected for experiment relate:
1. To the _difference in the form of the ripe seeds_. These are either
round or roundish, the wrinkling, when such occurs on the surface,
being always only shallow; or they are irregularly angular and deeply
wrinkled (_P. quadratum_).
2. To the _difference in the colour of the seed albumen_
(endosperm)[25]. The albumen of the ripe seeds is either pale yellow,
bright yellow and orange coloured, or it possesses a more or less
intense green tint. This difference of colour is easily seen in the
seeds as their coats are transparent.
[25] [Mendel uses the terms “albumen” and “endosperm” somewhat
loosely to denote the cotyledons, containing food-material, within
the seed.]
3. To the _difference in the colour of the seed-coat_. This is either
white, with which character white flowers are constantly correlated; or
it is grey, grey-brown, leather-brown, with or without violet spotting,
in which case the colour of the standards is violet, that of the wings
purple, and the stem in the axils of the leaves is of a reddish tint.
The grey seed-coats become dark brown in boiling water.
4. To the _difference in the form of the ripe pods_. These are
either simply inflated, never contracted in places; or they are
deeply constricted between the seeds and more or less wrinkled (_P.
saccharatum_).
5. To the _difference in the colour of the unripe pods_. They are
either light to dark green, or vividly yellow, in which colouring the
stalks, leaf-veins, and calyx participate[26].
[26] One species possesses a beautifully brownish-red coloured pod,
which when ripening turns to violet and blue. Trials with this
character were only begun last year. [Of these further experiments it
seems no account was published. Correns has since worked with such a
variety.]
6. To the _difference in the position of the flowers_. They are either
axial, that is, distributed along the main stem; or they are terminal,
that is, bunched at the top of the stem and arranged almost in a false
umbel; in this case the upper part of the stem is more or less widened
in section (_P. umbellatum_)[27].
[27] [This is often called the Mummy Pea. It shows slight fasciation.
The form I know has white standard and salmon-red wings.]
7. To the _difference in the length of the stem_. The length of the
stem[28] is very various in some forms; it is, however, a constant
character for each, in so far that healthy plants, grown in the same
soil, are only subject to unimportant variations in this character.
[28] [In my account of these experiments (_R.H.S. Journal_, vol. XXV.
p. 54) I misunderstood this paragraph and took “axis” to mean the
_floral_ axis, instead of the main axis of the plant. The unit
of measurement, being indicated in the original by a dash (′), I
carelessly took to have been an _inch_, but the translation here
given is evidently correct.]
In experiments with this character, in order to be able to discriminate
with certainty, the long axis of 6–7 ft. was always crossed with the
short one of 3/4 ft. to 1-1/2 ft.
Each two of the differentiating characters enumerated above were united
by cross-fertilisation. There were made for the
1st trial 60 fertilisations on 15 plants.
2nd " 58 " " 10 "
3rd " 35 " " 10 "
4th " 40 " " 10 "
5th " 23 " " 5 "
6th " 34 " " 10 "
7th " 37 " " 10 "
From a larger number of plants of the same variety only the most
vigorous were chosen for fertilisation. Weakly plants always afford
uncertain results, because even in the first generation of hybrids,
and still more so in the subsequent ones, many of the offspring either
entirely fail to flower or only form a few and inferior seeds.
Furthermore, in all the experiments reciprocal crossings were effected
in such a way that each of the two varieties which in one set of
fertilisations served as seed-bearers in the other set were used as
pollen plants.
The plants were grown in garden beds, a few also in pots, and were
maintained in their naturally upright position by means of sticks,
branches of trees, and strings stretched between. For each experiment
a number of pot plants were placed during the blooming period in a
greenhouse, to serve as control plants for the main experiment in the
open as regards possible disturbance by insects. Among the insects[29]
which visit Peas the beetle _Bruchus pisi_ might be detrimental to the
experiments should it appear in numbers. The female of this species is
known to lay the eggs in the flower, and in so doing opens the keel;
upon the tarsi of one specimen, which was caught in a flower, some
pollen grains could clearly be seen under a lens. Mention must also be
made of a circumstance which possibly might lead to the introduction
of foreign pollen. It occurs, for instance, in some rare cases that
certain parts of an otherwise quite normally developed flower wither,
resulting in a partial exposure of the fertilising organs. A defective
development of the keel has also been observed, owing to which the
stigma and anthers remained partially uncovered[30]. It also sometimes
happens that the pollen does not reach full perfection. In this event
there occurs a gradual lengthening of the pistil during the blooming
period, until the stigmatic tip protrudes at the point of the keel.
This remarkable appearance has also been observed in hybrids of
_Phaseolus_ and _Lathyrus_.
[29] [It is somewhat surprising that no mention is made of Thrips,
which swarm in Pea flowers. I had come to the conclusion that this is
a real source of error and I see Laxton held the same opinion.]
[30] [This also happens in Sweet Peas.]
The risk of false impregnation by foreign pollen is, however, a very
slight one with _Pisum_, and is quite incapable of disturbing the
general result. Among more than 10,000 plants which were carefully
examined there were only a very few cases where an indubitable false
impregnation had occurred. Since in the greenhouse such a case was
never remarked, it may well be supposed that _Bruchus pisi_, and
possibly also the described abnormalities in the floral structure, were
to blame.