180 PHYLU:\r III. ZYGOPHYCEAE 



257. Tlic ccUulusc walls in most tliatoms soon 1)C('ome 

 more or less silicified and rigid, and incapable of further 

 expansion. This is proV)ably a protective device, many 

 diatoms Hving at or near the surface of the ocean waters 

 where softer walls would be likely to be crushed. This 

 rigidity of their walls has brought about some structural 

 details that are peculiar to this group of plants, and 

 which are quite puzzling to the beginner if not considered 

 in connection with the origin of diatoms and their rela- 

 tionship to the filamentous types. 



258. In order to understand the structure of any 

 diatom it is necessary to consider it as one cell of a 

 cylindrical, angled, or flattened filament. These cells 

 are usually short (measured along the axis of the fila- 

 ment), so that when separated from the other cells they 

 lie with one end up, and thus show a cross-section of the 

 filament. Compare this with the end view of the cells 

 in a filamentous plant like Ulothrix or Spirogyra. As in 

 Desmids, the cells of the Diatoms are transversely 

 jointed, allowing the two halves (really the two ends of 

 the cells) to move apart, and thus enlarge the cell cavity. 

 Each half of the silicified wall is shaped like a paper box 

 cover, the flat surface corresponding to the ''valve" and 

 the curving ring to the ''girdle." Sometimes there are 



additional rings known as " interzones, " giv- 

 ing a good deal of flexibility to the diatom 

 cell wall. 



259. Diatoms propagate (1) by the enlarge- 

 ment of the protoplasm of the cell resulting in 

 its elongation, and the formation of two walls 

 propagiui^ in the plane of the joint which become the 

 of a diatom. ^^^^ ^f ^j^^ ^^^.^ ^^^^ ^^^g (''fission"); (2) by 



the separation of the two halves of the cell allowing the 

 escape of the protoplasm which then rapidly grows into a 



