The facts by no means seem to me
to indicate that the greater or lesser difficulty of either grafting
or crossing together various species has been a special endowment;
although in the case of crossing, the difficulty is as important for
the endurance and stability of specific forms, as in the case of
grafting it is unimportant for their welfare.
CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.
We may now look a little closer at the probable causes of the
sterility of first crosses and of hybrids. These two cases are
fundamentally different, for, as just remarked, in the union of two
pure species the male and female sexual elements are perfect, whereas
in hybrids they are imperfect. Even in first crosses, the greater or
lesser difficulty in effecting a union apparently depends on several
distinct causes. There must sometimes be a physical impossibility in
the male element reaching the ovule, as would be the case with a plant
having a pistil too long for the pollen-tubes to reach the ovarium. It
has also been observed that when pollen of one species is placed on
the stigma of a distantly allied species, though the pollen-tubes
protrude, they do not penetrate the stigmatic surface. Again, the male
element may reach the female element, but be incapable of causing an
embryo to be developed, as seems to have been the case with some of
Thuret's experiments on Fuci. No explanation can be given of these
facts, any more than why certain trees cannot be grafted on others.
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