262 



PLANT STUDIES 



The chief peculiarity is that the wall is composed of two 

 valves, one of which fits into the other like the two parts of 

 a pill box. This wall is so impregnated with silica that it 

 is practically indestructible, and siliceous skeletons of dia- 

 toms are preserved abundantly in certain rock deposits. 

 They multiply by cell division in a peculiar way, and some 

 of them have been observed to con- 

 jugate. 



They occur in such numbers in the 

 ocean that they form a large part of 

 the free-swimming forms on the sur- 

 face of the sea, and doubtless showers 

 of the siliceous skeletons are constant- 

 ly falling on the sea bottom. There 

 are certain deposits known as "si- 

 liceous earths," which are simply 

 masses of fossil diatoms. 



Diatoms have been variously placed 

 in schemes of classification. Some 

 have put them among the Brown 

 Algae because they contain a brown 

 coloring matter; others have placed 

 them in the Conjugate forms among 

 the Green Algae on account of the 

 occasional conjugation that has been 

 observed. They are so different from 

 other forms, however, that it seems 

 best to keep them separate from all 

 other Algae. 



176. Characeae. — These are common- 

 ly called " stoneworts," and are often 

 included as a group of Green Algae, 

 as they seem to be Thallophytes, and 

 have no other coloring matter than 

 chlorophyll. However, they are so peculiar that they are 

 better kept by themselves among the Algae. They are such 



Fig. 231. A common Chara, 

 showing tip of main axis. 

 —After Strasburger. 



