SIGNIFICANCE OF THE SEXUAL PROCESS. 53 



gonium, Coleochcete and, as the researches of Oltmanns seem to 

 indicate (See Chapter IV), certain Rhodophycece. From the fecun- 

 dated egg of CEdogonium four swarm-spores are developed, while in 

 Coleochcete a multicellular body is developed, from the cells of which 

 asexual swarm-spores are formed. In both cases the swarm-spores 

 give rise to sexual plants, or the first generation. The product of the 

 fecundated egg in Coleochcete bears a striking resemblance to the 

 sporophyte of such liverworts as Riccia. The fundamental differ- 

 ences lie chiefly in the fact that the covering of the spoi'ophyte in 

 Coleochcete is derived from vegetative branches of the thallus, the 

 oogonium being unicellular, and that the asexual spores are motile, a 

 correlation with the aquatic habit of Coleochcete. In the Rhodo- 

 phycece the cystocarp or cystocarps are the product of the fecundated 

 egg, and the spores give rise to the first generation. This is made all 

 the more probable by the researches of Oltmanns, which go to show 

 that the fusion of the cells of the sporogenous filaments with auxiliary 

 cells is merely a nutritive process. It is of interest to note further 

 that a similar condition is preserved in certain Ascomycetes in which 

 Harper has proved that unquestioned sexuality exists. Such algse 

 as Coleochcete, therefore, seem to point out more or less clearly the 

 phylogenetic road along which the ancestors of the Archegoniates have 

 passed. 



Research upon the process of fecundation and indirect nuclear 

 division, especially in reproductive cells, during the past twenty years, 

 has given a new insight into the significance of sexuality and the alter- 

 nation of generations in plants. Our knowledge along this line was 

 very materially advanced by the discovery of Van Beneden ('83) that 

 the number of chromosomes is the same in both conjugating nuclei. 

 Further investigations have established the still more important fact 

 that, in both plants and animals, a reduction to one-half of the number 

 of chromosomes in the sexual nuclei preceded the sexual act, and that, 

 as a consequence of the fusion of the male and female nuclei, the 

 number of chromosomes in the fecundated egg is doubled. 



In all the higher plants it is a well-established fact that the numeri- 

 cal reduction of the chromosomes takes place in the spore mother-cell, 

 and that in the cells of the gametophyte arising from the spore the 

 reduced number persists. In cells of the sporophyte, resulting from 

 the fecundated egg, the increased number obtains until the differentia- 

 tion of the spore mother-cells. It will thus be seen that the funda- 

 mental characteristic of both sexual and asexual generations lies in the 

 number of the chromosomes, and upon this phenomenon rests the 

 sexual differentiation of cells. 



