The Physiology of Self-sterility in Plants

Hermaphroditic animals and plants are known where fertilization of the eggs by the sperm or by the pollen produced by the same individual is very unusual. The extent of the phenomenon in the animal kingdom is unknown, Ciona intestinalis, an ascidian, having been the only species used in experimental work. In the angiosperms, the characteristic is widespread, and must be supposed to have originated many times. At least 50 families are represented, including both monocotyledons and dicotyledons. The problem which this condition presents is interesting from several angles. The evolutionist wishes to know how and why selfsterility originated, since it often arose in groups where various provisions for cross-fertilization already existed. The geneticist wishes to know whether it is hereditary, and if so what method of inheritance is followed. The physiologist desires an interpretation in physiological terms. And finally, since many apples, pears, peaches, plums, cherries, and other useful fruits are self-sterile, the horticulturist hopes to find a practical method of dealing with the condition in orchard work. Darwin (5) and his friends and correspondents, Miiller (12) and Hildebrand (9), made a number of rather unsystematic cross-pollinations with the plants of several self-sterile species and obtained fruit in each instance. These results led Darwin to conclude that the sexual organs of every such individual have been specialized with respect to every other individual in the same sense in which the external characters of every individual are so specialized that no two are alike. He assumed that differentiation of some sort in the sexual elements is essential to fertilization. After making some 40 or 50 cross-pollinations , he even went so far as to say, "we may therefore confidently assert that a self-sterile plant can be fertilised by the pollen