Now, it is scarcely credible that a mere difference in
the length of the pistil can make a wide difference in its capacity for being
fertilised. We can believe this the less because with some plants, for instance,
Amsinckia spectabilis, the pistil varies greatly in length without affecting the
fertility of the individuals which are intercrossed. So again I observed that
the same plants of Primula veris and vulgaris differed to an extraordinary
degree in the length of their pistils during successive seasons; nevertheless
they yielded during these seasons exactly the same average number of seeds when
left to fertilise themselves spontaneously under a net.
We must therefore look to the appearance of inner or hidden constitutional
differences between the individuals of a varying species, of such a nature that
the male element of one set is enabled to act efficiently only on the female
element of another set. We need not doubt about the possibility of variations in
the constitution of the reproductive system of a plant, for we know that some
species vary so as to be completely self-sterile or completely self-fertile,
either in an apparently spontaneous manner or from slightly changed conditions
of life. Gartner also has shown that the individual plants of the same species
vary in their sexual powers in such a manner that one will unite with a distinct
species much more readily than another.
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