BIATOMACEJE!. 



239 



protoplasm of one of the small Diatoms leaving its silicious 

 shell (the latter falling apart), and then increasing by growth 

 until it reaches the normal size, when it forms a new coat 

 about itself. This is not unlike what has been called the 

 EejuYenescence of the cell. (See p. 42.) 



307. — The second mode of the formation of auxospores is 

 a sexual one, and is, in fact, the sexual mode of reproduc- 

 tion above referred to.* Two individuals come near each 

 other ; their valves separate, and the two protoplasm-masses 

 unite with each other into one mass, or in many cases two 

 masses (A, Pig. 155). These new masses' develop directly 

 into auxospores, the whole process 

 requiring from ten to fourteen 

 days {B, Fig. 155}. 



308. — Diatoms are exceedingly 

 abundant ; they occur in both 

 salt and fresh water, usually 

 forming a yellowish layer at the 

 bottom of the water, or they are 

 attached to the submerged parts of 

 other plants, and to sticks, stones, 

 and other objects ; they have been 

 dredged from the ocean at great 

 depths, and appear to exist there 

 in enormous quantities. They are 

 also found among mosses and other 

 plants on moist ground ; great 

 numbers occur as fossils, forming 

 in many instances vast beds composed of their empty 

 frustules. The varied and frequently very beautiful mark- 

 ings of their valves have long made Diatoms objects of 

 much interest to the microscopist. The great regularity 

 and the extreme fineness of the lines and points upon some, 

 have caused them to be used as microscopic tests. Tliti' 



Fig. ^55.— Navictila aaofomea, . 

 showing conjugation and forma- 

 tion of auxospores. A^ conjuga- 

 tion of two frustules ; £, two aux- 

 ospores, with the four valves of 

 the two parent frustules.— After ' 

 OBrsted, 



* This process takes place at certain seasons of the year for each 

 species ; according to Professor H. L. Smith, in Oomphonema olwaoeuni 

 it occurs ill February and March. 



