Great diversity in the size of two plants, one being woody and the other herbaceous, one being evergreen and the other deciduous, and adaptation to widely different climates, does not always prevent the two grafting together.

As in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft together trees 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.