s which, either normally or abnormally,
require to be fertilised by pollen from a distinct individual or species._

The facts now to be given differ from those hitherto detailed, as the
self-sterility does not here result from long-continued, {132} close
interbreeding. These facts are, however, connected with our present
subject, because a cross with a distinct individual is shown to be either
necessary or advantageous. Dimorphic and trimorphic plants, though they are
hermaphrodites, must be reciprocally crossed, one set of forms by the
other, in order to be fully fertile, and in some cases to be fertile in any
degree. But I should not have noticed these plants, had it not been for the
following cases given by Dr. Hildebrand:[303]--

_Primula sinensis_ is a reciprocally dimorphic species: Dr. Hildebrand
fertilised twenty-eight flowers of both forms, each by pollen of the
other form, and obtained the full number of capsules containing on an
average 42.7 seed per capsule; here we have complete and normal
fertility. He then fertilised forty-two flowers of both forms with
pollen of the same form, but taken from a distinct plant, and all
produced capsules containing on an average only 19.6 seed. Lastly, and
here we come to our more immediate point, he fertilised forty-eight
flowers of both forms with pollen of the same form, taken from the same
flower, and now he obtained only thirty-two capsules, and these
contained on an average 18.6 seed, or one less per capsule than in the
former case. So that, with these illegitimate unions, the act of
impregnation is less assured, and the fertility slightly less, when the
pollen and ovules belong to the same flower, than when belonging to two
distinct individuals of the same form. Dr. Hildebrand has recently made
analogous experiments on the long-styled form of _Oxalis rosea_, with
the same result.[304]

It has recently been discovered that certain plants, whilst growing in
their native country under natural conditions, cannot be fertilised with
pollen from the same plant. They are sometimes so utterly self-impotent,
that, though they can readily be fertilised by the pollen of a distinct
species or even distinct genus, yet, wonderful as the fact is, they never
produce a single seed by their own pollen. In some cases, moreover, the
plant's own pollen and stigma mutually act on each other in a deleterious
manner. Most o