Production of new cultivated varieties of fruit trees and shrubs from seed pp. 30-31 (1952)
Ivan Michurin

Easiest and most successful are crossings between two varieties belonging to one and the same species, e.g., two varieties of apple, or two varieties of pear, cherry, plum, raspberry, etc. It is far more difficult to cross different species, even if they are closely related—sour cherry with sweet cherry, raspberry with blackberry, etc. In such cases for greater success I would recommend, on the basis of my experience, to take at least as one of the components of the cross (preferably the maternal) a young plant, grown from seed, at the time of its first blossoming. And, strange as it may appear at first glance, incontestably greater success is obtained in this way. Further, in some cases, and especially when crossing plants whose flowers have compound pistils (multi-seeded), success in fertilization is greatly assisted by adding to the pollen of the plant of the other species a very small quantity of pollen from a variety closely akin to the maternal plant. Whether the chief factor here is that the stigmas of the compound pistil are more easily excited and activated by contact with the kindred pollen or whether the aroma of this pollen acts as a stimulating factor, I find it hard to say, but by using this method crossings succeed which, when only pollen of the other species is administered, are invariably abortive. I, for example, have managed in this way to secure hybrids of Amygdalus georgica and Prunus pumila L.; and there have been other cases as well. I have, moreover, observed that a large number of hybrids, especially at their first flowering, can be far more easily crossed with each other than plants of pure species.