CYTOLOGY OF REPRODUCTION IN PLANTS 



151 



fli^^sions in the si^orocytos proceed in the ordinary waA', one interesting; 

 feature being the regular arrangement of the proplastids in layers close to 

 the cell plate at each cytokinesis. The chromosome number tends to be 

 high, so that ferns generally are not favorable material for chromosomal 

 studies. 



Upon germination the spore produces a small, thin gametophyte, in 

 the large cells of which the chloroplasts appear with admirable clearness 

 (Fig. 18). Ferns in this stage have therefore been used in significant 

 investigations on the genetics of plastids and theii- mutability in response 

 to irradiation with X rays. The gametophytes develop sex organs 

 of two kinds. In the base of each archegonium one large egg is dif- 

 ferentiated. Each antheridium produces a considerable number of 

 spermatozoids, and the stages of this process are of much cytological 

 interest. It is to be remembered that no haplosis occurs in the formation 



ben 



Fig. 110. — Spermiogenesis in a fern. 1-3, stages in transfoiinatioii of the spermatid; 4, 

 spermatozoid; c, cytoplasm; b, blepharoplast; n, nucleus. (B<istd on drawings by A. Yuasa.) 



of these gametes, for the gametophj^te from which they arise has itself 

 developed with the gametic chromosome number. 



Spermiogenesis occurs essentially as follows (Fig. 110). The sperm- 

 atid, or cell that is to transform into a spermatozoid, consists of cyto- 

 plasm, a nucleus, a blepharoplast, small plastids, and probably other 

 inclusions. The blepharoplast is of special interest, for in several 

 pteridophytes it has been shown to be the centrosome that functioned 

 in the preceding mitosis. It is called a blepharoplast in the spermatid 

 because it bears the cilia of the motor apparatus, recalling thus the parallel 

 behavior of the centrioles in animal spermiogenesis. It apparently 

 differentiates into two longitudinal parts; then from one of these the 

 numerous cilia grow out as the whole structure elongates spirally together 

 with the nucleus, forming the coiled, compact body of the sperm. The 

 spermatid cytoplasm, often containing starch granules, is held mostly 

 as a vesicle in the large posterior coils. 



When water is present externally, the sperms are liberated from the 

 antheridium and swim about actively. If open archegonia with recently 



