236 



ESSENTIALS OF BOTANY 



of desmids three hundred. The number of individuals is 

 inconceivably great. For example, there are in various 



EiG. 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 magniiied, showing the aidherUUa, or sperm- 

 producing organs, a; C, a. minute portion of a female plant (A), showing 

 clustered carpospores enclosed in an urn-shaped enveloiic. 



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 



