PLANTS, Sex in, Origin, Evolution and Determination of. For the origin of sex and the early stages in its evolution, one must turn to the algae and fungi, since all plants from the liverworts to the sunflowers and orchids have already reached that degree of development which is characterized by eggs and sperms. In lower members of the algal., reproduction by swimming spores is common. Repeated divi sions within a cell give rise to several nuclei, each surrounded by a mass of protoplasm. (See CELL; CHROMOSOME and PROTOPLASM ) . Each macs of protoplasm, with its nucleus, becomes transformed into a swimming spore, which owes its motility to thread-like cilia. The spore comes to rest, elongates and then divides trans versely. By repeated divisions, a filament like the parent plant is built up (Fig. 1, A) . In Ulothrsx, which is commonly used for illustrat ing the origin of sex, the zoospores are formed, 1, 2, 4, 8 or 16 in a cell. If one, two or three more divisions take place — the cells divide simultaneously — so that 32, 64 or 128 cells are produced, the cells develop into the motile con dition but seem too small and deficient in energy to divide. However, they unite in pairs — for which reason they are called gametes— their protoplasm fuses into one mass and the two nuclei blend into one. The result of the union is a vigorous cell which may at once de velop into a filament like that produced by the zoospore, or may produce a few zoospores, which then behave like those formed in the cells of a filament (Fig. 1, B). Thus the gamete habit originated from the zoospore habit, and sexuality, characterized by the fusion of two gametes, represents an advance beyond the zoospore condition. The zoospore is the an cestor of the gamete. The gamete habit arose from the zoospore habit independently in vari ous alga: and fungi. In the illustration just given, the two gametes are of the same size and shape; but, after the gamete habit became established, the gametes began to differ, one remaining small and active, and the other grow ing larger and less active and finally losing the swimming habit. When the two gametes have begun to differ in size, the terms, male and female, may be used, the smaller being the male and the larger the female; also, the male gamete may properly be called a sperm and the female an egg. Stages in the evolution from similar gametes to highly differentiated eggs and sperms may be illustrated by Cutleria, one of the brown alga:, in which the egg, although motile, is much larger than the sperm. In Oedogoniutn, a very common green alga, the egg is immensely larger than the sperm and has not only lost all motility, but remains in the cell (Fig. 2, A and B). From this stage in the
evolution of sex, the egg is always retained in the organ which produced it, further de velopment being more conspicuous in the organs which produce and nourish the eggs and sperms than in the eggs and sperms themselves. In liverworts, mosses, ferns and their allies, and in most Gymnosperms, the organ which pro duces the egg is called an archeganium and that which produces sperms is called an antheridium, although in the Gymnosperms the antheridium character is not so obvious to the layman.
In the Gymnosperms the eggs arise in structures concealed in large cones and the sperms are finally developed from spores which were pro duced in smaller cones. We have no hesitation in calling these larger cones female cones and the smaller ones male. In the same way we should call the ovaries of the Angiosperms female organs and the anthers, male. When ovaries and anthers are formed on separate plants, as in the willows and many others, we believe it is perfectly correct to speak of male and female trees. Some botanists object to using the terms male and female, except as ap plied to the gametophyte generation. (See ALTERNATION OF GENERATIONS). The determina tion of sex is a problem which is receiving increasing attention from zoologists, especially it has been determined that, in at least some cases, the sexes are characterized by differences in the number or sizes of chromosomes, e.g., in one of the worms, two of the four sperms produced from a mother cell has five chromo somes while the other two have six. All of the eggs have six chromosomes. An egg fertilized by a sperm with five chromosomes produces a male individual, while fertilization by a sperm with six chromosomes results in a female in dividual. In plants, investigations have not ad vanced to any such extent but it is known that, in some cases, the two sexes are differentiated during the reduction of chromosomes (see CHROMOSOME), while in others the differentia tion takes place at various stages in the life history. In both animals and plants, efforts have been made to control, artificially, the sex of offspring, chiefly by nutrition. If the num ber or size of the chromosomes should be the determining factor, no experiments in nutrition could affect the result, unless such experiments could affect the chromosomes. Consult Coulter, J. M., 'The Evolution of Sex in Plante ; Morgan, T. H., 'Heredity and Sex' ; Walter, H. E.,