114 



THE ALGAE 



of sexual differentiation. After the gametes have fused two of the 

 chloroplasts degenerate and the zygospore on germination divides 

 twice, during which meiosis takes place. Two daughter cells are 

 then formed, each containing one chloroplast and two nuclei, but 

 one of the latter subsequently degenerates. Under unfavourable 





a 



rv 



sc 



•r. 



-a. 



a 



:ty/- 



t: 



^ * » 



ts 



B 



g cl 



d I b 



Fig. 65 Closterium. Diagrams to explain cell division in species 

 of Closterium with (B) and without (A) girdle bands. The different 

 segments of the wall are indicated by shading, i, 2, 3, and A, B, 

 C =the successive generations. The individuals in i and A have each 

 arisen from a zygote and have not undergone division, a, b^ 

 J = semi-cells of various ages; c =the connecting band demarcated 

 by the two sutures 5, of the previous generation, and r, of the 

 present; g, /= girdle bands developed before {g) and after division; 

 s = suture between young and older semi-cells; r =the Hne of the 



next division. (After Fritsch.) 



conditions asexually produced chlamydospores have been ob- 

 served in C moniliferum. 



The genus is wholly fresh water and many species are plank- 

 tonic and possess modifications, e.g. spines, which may be re- 

 garded as adaptations to this mode of existence. The group is 

 extremely widespread but is scarce in waters containing much lime, 

 the individual species thriving best in soft or peaty waters. The 

 most favourable seasons for their development are the late spring 



