180 PHYLUM III. ZYGOPHYCEAE 
257. The cellulose walls in most diatoms soon become 
more or less silicified and rigid, and incapable of further 
expansion. ‘This is probably a protective device, many 
diatoms living 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 
Le ts elongation, and the formation of two walls 
oetiion in the plane of the joint which become the 
ofadiatom. ends of the two new cells (“fission’’); (2) by 
the separation of the two halves of the cell allowing the 
escape of the protoplasm which then rapidly grows into a 
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