)
With dimorphic and trimorphic species, an illegitimate union between
plants of the same form presents the closest analogy with
self-fertilisation, whilst a legitimate union closely resembles
cross-fertilisation; and here again the lessened fertility or complete
sterility of an illegitimate union depends, at least in part, on the
incapacity for interaction between the pollen-grains and stigma. Thus
with Linum grandiflorum, as I have elsewhere shown, not more than two or
three out of hundreds of pollen-grains, either of the long-styled or
short-styled form, when placed on the stigma of their own form, emit
their tubes, and these do not penetrate deeply; nor does the stigma
itself change colour, as occurs when it is legitimately fertilised.
(9/15. 'Journal of the Linnean Society Botany' volume 7 1863 pages
73-75.)
On the other hand the difference in innate fertility, as well as in
growth between plants raised from crossed and self-fertilised seeds, and
the difference in fertility and growth between the legitimate and
illegitimate offspring of dimorphic and trimorphic plants, must depend
on some incompatibility between the sexual elements contained within the
pollen-grains and ovules, as it is through their union that new
organisms are developed.
If we now turn to the more immediate cause of self-sterility, we clearly
see that in most cases it is determined by the conditions to which the
plants have been subjected.
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