CYTOLOGY OF REPRODUCTION IN PLANTS 



161 



vegetative cells form a cystocarp. The carposporcs later develop into 

 diploid plants which bear monoploid tetraspores. These in turn develop 

 into monoploid male and female plants. Polysiphonia thus has a regular 

 alternation of sporophytic and heterothallic gametophytic generations 

 in the cycle, and in addition it has a diploid carposporic phase between 

 S3'ngamy and the initiation of sporophj'te development. This extra 

 phase is more prominent in certain other genera. 



Nemalion and Scinaia are haplonts. The only vegetative plants are 

 sexual, and after syngamy meiosis occurs in the zygote. Carpospores 

 soon formed by proliferation from the zygote are monoploid and produce 

 new sexual plants. 



Fig. 



5 6 7 8 



119. — Diagram of nuclear behavior during conjugation in a diatom (Rhopalodia) . 



Explanation in text. {Based on drawings by H. Klebahn.) 



Among the diatoms the cytology of reproduction is best known in the 

 bilaterally symmetrical Pennales. When ordinary vegetative multiplica- 

 tion by cell division occurs, the two-valved shell is pushed apart and each 

 of the two daughter cells secretes a new valve fitting inside the old one; 

 hence in successive asexual generations the average size of the cells tends 

 to diminish. At the time of sexual reproduction, e.g., in Rhoyalodia 

 (Fig. 119), two individuals conjugate in a common mass of secreted jelly. 

 In each individual the nucleus undergoes two successive divisions which 

 are meiotic in character. Two of the resulting nuclei become large and 

 two small. Cytokinesis occurs in a transverse plane, giving two cells 

 each with a large and a small nucleus. Then these cells fuse with 

 two similar ones formed in the same manner by the other individual. 

 The large nuclei unite, while the small ones disappear. The two zygotes 

 then grow and become large mixospores which secrete a pectic covering 

 and eventually the siliceous wall characteristic of the species. In this 



