CHAPTER IX. HYBRIDISM.
5. RECIPROCAL DIMORPHISM AND TRIMORPHISM.
This subject may be here briefly discussed, and will be found to throw some
light on hybridism. Several plants belonging to distinct orders present
two forms, which exist in about equal numbers and which differ in no
respect except in their reproductive organs; one form having a long pistil
with short stamens, the other a short pistil with long stamens; the two
having differently sized pollen-grains. With trimorphic plants there are
three forms likewise differing in the lengths of their pistils and stamens,
in the size and colour of the pollen-grains, and in some other respects;
and as in each of the three forms there are two sets of stamens, the three
forms possess altogether six sets of stamens and three kinds of pistils.
These organs are so proportioned in length to each other that half the
stamens in two of the forms stand on a level with the stigma of the third
form. Now I have shown, and the result has been confirmed by other
observers, that in order to obtain full fertility with these plants, it is
necessary that the stigma of the one form should be fertilised by pollen
taken from the stamens of corresponding height in another form. So that
with dimorphic species two unions, which may be called legitimate, are
fully fertile; and two, which may be called illegitimate, are more or less
infertile. With trimorphic species six unions are legitimate, or fully
fertile, and twelve are illegitimate, or more or less infertile.
The infertility which may be observed in various dimorphic and trimorphic
plants, when they are illegitimately fertilised, that is by pollen taken
from stamens not corresponding in height with the pistil, differs much in
degree, up to absolute and utter sterility; just in the same manner as
occurs in crossing distinct species. As the degree of sterility in the
latter case depends in an eminent degree on the conditions of life being
more or less favourable, so I have found it with illegitimate unions. It
is well known that if pollen of a distinct species be placed on the stigma
of a flower, and its own pollen be afterwards, even after a considerable
interval of time, placed on the same stigma, its action is so strongly
prepotent that it generally annihilates the effect of the foreign pollen;
so it is with the pollen of the several forms of the same species, for
legitimate pollen is strongly prepotent over illegitimate pollen, when both
are placed on the same stigma. I ascertained this by fertilising several
flowers, first illegitimately, and twenty-four hours afterwards
legitimately, with pollen taken from a peculiarly coloured variety, and all
the seedlings were similarly coloured; this shows that the legitimate
pollen, though applied twenty-four hours subsequently, had wholly destroyed
or prevented the action of the previously applied illegitimate pollen.
Again, as in making reciprocal crosses between the same two species, there
is occasionally a great difference in the result, so the same thing occurs
with trimorphic plants; for instance, the mid-styled form of Lythrum
salicaria was illegitimately fertilised with the greatest ease by pollen
from the longer stamens of the short-styled form, and yielded many seeds;
but the latter form did not yield a single seed when fertilised by the
longer stamens of the mid-styled form.