SECT. I 



CRYPTOGAMS 



353 



called luarennin. This pigment is the cause of a green coloration of the oysters 

 in which the niarennin undergoes alteration and is accumulated (-■'). 



Order 2. Diatomeae Centricae. — In these the valves are symmetrical about 



-/>^^- 



•v^: 



Fig. 2GT. — PlanktonieHa sol. Atlantic Ocean. A disc- 

 .sliaped plankton diatom with a hollow floating wing 

 arising from the girdle side. The protoplast con- 

 tains a nucleus and numerous chromatophores. 

 (X 322. After G. Kakstkn.) 



Fig. 26S. — Corethron raldiviae. A 

 plankton diatom of the Antarctic 

 Ocean with bristle hairs and pre- 

 hensile bristles, (x 100. After 

 G. Karstex.) 



a centre, and have the sculpturing radially or concentrically arranged. The great 

 majority of the forms of this order are marine, and play a large part in the com- 

 position of the plankton ("*). The plankton diatoms are provided with special 



vA^. 









c . 



Fig. 209. — Biddulphia moHliensis. A, View from the girdle side; B, Auxospore forn 

 cell divided into two sporangia preparatory to the formation of microspores ; D, spore for 

 the sporangia ; E, swimming microspore. (A-D x 228, E x 570. After P. Bergen.) 



D 



lation ; ( 

 mation i 



arrangements for floating, horn-like projections or wings of the cell wall such as 

 are seen in Figs. 267 and 268. 



The auxospore formation in the Centricae does not take place by the conjugation 

 of two gametes but by the ])rotop''asmic body of a cell becoming free from the cell 

 walls and increasing iu size ; the enlarged cell is first surrounded by a weakly 



2a 



