48 



CHLOROPHYCEAE 



characters it overlaps Tribonema (cf. p. 117). For this reason it is 

 not impossible that the filamentous Xanthophyceae may be derived 

 via a form such as Microspora from an ulotrichaceous filament. On 

 this view Ulothrix and Tribonema cannot be regarded as end-phases 

 in separate lines of evolutionary development. Any cell of Micro- 

 spora can produce aplanospores instead of motile bodies, and 

 akinetes are also frequently formed, either singly or in long chains. 

 On germination these divide into four bodies, each of which gives 

 rise to a new filament. This genus, like Ulothrix, is also confined 

 to the winter or spring months. 



Cylindrocapsaceae : Cylindrocapsa (cylindro, cylinder; capsa, box). 

 Fig. 35- 

 The filaments, which are unbranched, are attached at the base by 

 means of a gelatinous holdfast, and when young each thread is com- 



Fig. 35. Cylindrocapsa. A, vegetative filament. B, thread with young antheridia 

 (n) and young oogonium (o). C, fusion of gametes, a = antherozoid, o = ovum. 

 D, old mature filament. (After Fritsch.) 



posed of a single row of elliptical cells with thick stratified walls, the 

 whole being enclosed in a tubular sheath. In older filaments, 

 however, the cells divide longitudinally, usually into pairs, and the 

 gelatinous nature of the threads suggests how the genus Mono- 

 stroma may have evolved, although Cylindrocapsa itself has de- 



