SECT. I 



CRYPTOGAMS 



347 



of conjugation, which is the one exhibited by most species, is described as scalari- 

 form (Fig. 273 A), as distinct from the lateral conjugation of some species, in 

 which two adjacent cells of the same filament conjugate by the development of 

 coalescing processes, which are formed near their transverse wall (Fig. 273 ). In 

 some genera the zygote is formed midway in the conjugation tube. 



Fio. 273. A, Conjugation of Spirogyra qulnina (y. 240). B, Spirogyra longata(x 150); *, 



C, Cell of Spirogyra jugalis ; k, nucleus ; ch, chromatophores ; p, pyrenoid. (x 250.) 



On germination the zygote gives rise to only one plant ; its contents grow 

 out as a cylindrical cell which on division forms a filament. According to 

 CHIMIELEWSKY the nucleus of the zygote of Spirogyra first divides amitotically into 

 four, two of which disintegrate while the remaining two again fuse to form the 

 single nucleus of the young plant ( 16 ). 



CLASS VII 

 Diatomeae (Diatoms) (" 1T ) 



The Diatomeae (Bacillariaceae) constitute a very large class of 

 unicellular Algae. They occur usually associated together in large 

 numbers, in both fresh and salt water, and also on damp soil. 



The cells are either solitary or form colonies ; they are free-swim- 

 ming, or are attached by means of gelatinous stalks, excreted by 

 the cells themselves (Fig. 274). Sometimes the cells remain con- 

 nected and form bands or zigzag chains, or, on the other hand, they 

 are attached and enclosed in gelatinous tubes, while in the case of 



