236 ESSENTIALS OF BOTANY 
of desmids three hundred. The number of individuals is 
inconceivably great. For example, there are in various 
Fic. 167. Polysiphonia, a Red Alga. 
A, a considerable portion of a female plant about natural size; B, a small 
bit of a male plant much magnified, showing the antheridia, or sperm- 
producing organs, a; C, a minute portion of a female plant (4), showing 
clustered carpospores enclosed in an urn-shaped envelope. 
parts of the United States, in Africa, in Bohemia, and else- 
where extensive beds of siliceous shells or cases of fossil 
diatoms, often from thirty to fifty feet thick, and sometimes 
