As in hybridisation, so with
grafting, the capacity is limited by systematic affinity, for no one
has been able to graft trees together belonging to quite distinct
families; and, on the other hand, closely allied species, and
varieties of the same species, can usually, but not invariably, be
grafted with ease. But this capacity, as in hybridisation, is by no
means absolutely governed by systematic affinity. Although many
distinct genera within the same family have been grafted together, in
other cases species of the same genus will not take on each other. The
pear can be grafted far more readily on the quince, which is ranked as
a distinct genus, than on the apple, which is a member of the same
genus. Even different varieties of the pear take with different
degrees of facility on the quince; so do different varieties of the
apricot and peach on certain varieties of the plum.
As Gartner found that there was sometimes an innate difference in
different INDIVIDUALS of the same two species in crossing; so Sagaret
believes this to be the case with different individuals of the same
two species in being grafted together. As in reciprocal crosses, the
facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, cannot
be grafted on the currant, whereas the currant will take, though with
difficulty, on the gooseberry.
We have seen that the sterility of hybrids, which have their
reproductive organs in an imperfect condition, is a very different
case from the difficulty of uniting two pure species, which have their
reproductive organs perfect; yet these two distinct cases run to a
certain extent parallel.
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