128 



SYSTEMATIC BOTANY. 



generally more or less con- 

 stricted in the middle, and 

 divided into two symmetri- 

 cal half-cells. A sexual re- 

 production takes place by 

 the elongation and fission 

 of the neck, uniting the two 

 halves of the cells. The two 

 small half-cells grow until 

 239 240 they are as large as the 



original ones, when they separate. Sexual reproduction 

 results in the formation of a zygospore ; thus, elongations, 

 or conjugating tubes, of two adjacent cells grow out till 

 they meet, their protoplasm fuses upon the absorption of 

 the partition between them, and this becomes covered with 

 a thick wall, or exospore. 



162. The Diatoms (JDiatomaeem), as are also the Desmids, 

 are microscopic, unicellular Algse (Fig. 241), with a multi- 

 tude of ornamental forms. They differ, however, in having 

 silicious cells, and their chlorophyll concealed by a brown- 

 ish or yellowish coloring matter, called phycoxanthine. The 

 cell (called a frustule) consists of two portions, 

 called valves, one of which is slightly larger and 

 fits over the other like the lid of a pill-box. Ee- 

 production takes place very similar to that in the 

 Desmids, the protoplasm divides in a plane parallel 

 to the valves, each portion secretes a shell or 

 valve, which, in every case, is slightly smaller and 

 fits into the old or outer valve. It is evident, 

 therefore, that as multiplication by fission con- 

 Fig. 239. Micrasterias /areata, a Desmld. Fig. 240. Cosmaritim parimla, i. 

 Desmid. Fig. 241. Navicula vtridU, a Diatom. 



