CHAPTER IV. FECUNDATION; HETEROGAMETES. 



In the preceding chapters we have considered sexual reproduction 

 in certain of those Thallophyta in which no very marked differentia- 

 tion of the gametes has been attained, although in Rctocarpus espe- 

 cially, and even in Spirogyra and Basidiobolus, a tendency toward 

 a differentiation into male and female cells is manifested. Nor have 

 we found any modification of the cells bearing the gametes into dif- 

 ferentiated sexual organs, unless the gametangia of such forms as 

 Ectocarpus be so considered, and even then there is no apparent 

 difference between male and female gametangia. As already men- 

 tioned in the introductory chapter, the terms male and female sexual 

 cells are essentially the expression of a certain fundamental kind of 

 division of labor, and in the developmental history of sexuality in 

 plants we find this division of labor manifested in the gametes them- 

 selves before a corresponding differentiation is apparent in the organs 

 bearing them. 



SPH^ROPLEA. 



Among the algae one of the best known and most interesting exam- 

 ples of this fact is illustrated in Sphceroplea annulina. To Ferdinand 

 Cohn ('55) is due the credit of having established the fact of sexual 

 reproduction in this genus, a phenomenon among the algae little known 

 at the time. Later Sphceroplea was studied by Heinricher ('83), 

 Rauwenhoff ('88), Kny ('84) and more recently by Klebahn ('99). 

 Although both Heinricher and Rauwenhoff followed the behavior of the 

 nucleus during certain stages in the development of the sexual cells and 

 in fecundation, yet in many respects their work was incomplete. For 

 a more thorough investigation of this process, however, we are indebted 

 to the researches of Klebahn, who studied the two varieties of the 

 species, ,5. annulina var. braunii (Keutz) Kirchner and 5. annulina 

 var. crassisepta Heinricher. The chief interest in the sexual repro- 

 duction of this plant centers upon the fact that in var. braunii several 

 nuclei are usually present in the egg-cell. 



The contents of the multinucleate cells of Sphceroplea present the 

 well-known and characteristic arrangement : In typical cases the cen- 

 tral cavity of each cell is traversed by a row of large vacuoles inter- 

 spersed by smaller ones of varying size. The protoplasm, which forms 

 only a thin layer between the larger vacuoles and the cell-wall, is 

 collected into dense ring-like or band-shaped masses between the 



