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 a fourth species, when thus grafted were rendered barren.

On the other hand, certain species of Sorbus, when grafted on other species, yielded twice as much fruit as when on their own roots.

We are reminded by this latter fact of the extraordinary cases of Hippeastrum, Passiflora, etc.