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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
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
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. |
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| As Gärtner found that there was sometimes an innate difference in different
individuals
the same two species in crossing; so
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. |
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| We have seen that the sterility of hybrids, which have their reproductive organs in an imperfect condition, is a
different case from the difficulty of uniting two pure species, which have their reproductive organs perfect; yet these two distinct
→classes of cases
run to a
extent parallel. Something analogous occurs in grafting; for Thouin found that three species of Robinia, which seeded freely on their own roots, and which could be grafted with no great difficulty on
species, when thus grafted were rendered barren. On the other hand, certain species of Sorbus, when grafted on other
yielded twice as much fruit as when on their own roots. We are reminded by this latter fact of the extraordinary
of Hippeastrum,
&c., which
much more freely when fertilised with the pollen of
species, than when
with
→pollen from the same plant.
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| We thus see,
although there is a clear and
difference between the mere adhesion of grafted stocks, and the union of the male and female elements in the act of reproduction, yet that there is a rude degree of parallelism in the results of grafting and of crossing distinct species. And as we must look at the curious and complex laws governing the facility with which trees can be grafted on each other as incidental on unknown differences in their vegetative systems, so I believe that the still more complex laws governing the facility of first
are incidental on unknown
in their reproductive systems. These
in both cases, follow to a certain extent, as might have been expected,
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