Page:Popular Science Monthly Volume 74.djvu/553

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549
DETERMINATION AND INHERITANCE OF SEX

entirely different matter from the position that Correns takes that sex is fixed at the time of fertilization and determined by the spermatozoa. Correns made his experiments with two species of Cucurbitaceæ, the family to which our pumpkin and squash belong, growing wild in central Europe. These two species are Bryonia alba and Bryonia dioica. The former is the plant which supplies the root employed in pharmacology for the treatment of dropsy. The especial value of these species for experiments relative to the problem of sex, lies in the fact that the former is hermaphroditic or monoecious, i. e., mature male and female flowers appear on the same plant; and the latter is diecious, i. e., male and female flowers appear on different plants. These two species can be crossed in either direction. Correns made three sets of experiments, the results of which have led him to a unique and simple explanation of the cause of sex, in the light of which former theories must undoubtedly be more or less extensively modified. Correns first crossed Bryonia dioica with Bryonia alba, using the eggs of the former and the pollen grains of the latter. Sterile hybrid-offspring appeared from the cross and they were all females. Correns explains that since germ-cells of the hermaphrodite forms carry only hermaphrodite determinants, the male gametes (pollen grains) can have had no influence on the sex of the offspring of this cross. In other words, hermaphroditism is recessive to dieciousness. And since the offspring were all female, all the eggs of Bryonia dioica must have had female tendencies.

Correns next pollinated the pistils of Bryonia dioica with the pollen of the same species. Forty-one pure dioica offspring were obtained from this cross, twenty-one of which were female and twenty male, or in round numbers each sex appeared in 50 per cent, of the individuals. Since the eggs were all female, as shown by the first experiment, this result must mean that the pollen grains determine the sex and that they must be of two kinds, male and female in tendency, and approximately equal in number. It must also mean that the male tendency dominated over the female, else no males could have appeared.

In a third set of experiments the pistils of Bryonia alba were pollinated by Bryonia dioica. The offspring in this case were sterile hybrids, and of the seventy-six which came to maturity thirty-eight were male and thirty-eight female. This again shows that hermaphroditism is recessive to dieciousness since no hermaphrodites appeared. It shows also again that there must be two kinds of male gametes with male and female tendencies, respectively, of equal number, and that they determine the sex. It shows, moreover, that sex is determined at the time of fertilization, for before that instant all the female gametes (ovules) had the tendency to develop into hermaphrodite forms. It appears also in the light of this experiment that hermaphroditism and