122 BOTANT. 



256. In their reproduction diatoms resemble the desmids, 

 the only differences being those made necessary by their 

 rigid walls. 



257. 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 shells. The varied and frequently very beautiful 

 markings 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. 



258. The Pond Scums {Zygnemaoem). The plants of this 

 order, which are all aquatic, are elongated unbranched fila- 

 ments, composed of cylindrical cells arranged in single 

 rows. The cells are all alike, and each one appears to be 

 independent, or nearly so, of its associates. The filament 

 is thus, in one sense, rather a composite body than an indi- 

 vidual. The chlorophyll is generally arranged in bands or 

 plates. 



259. The vegetative increase of the number of cells takes 

 place by the fission of the previously formed cells. The 

 protoplasm in a cell divides, and a plate of cellulose forms 

 in the plane of division. This is repeated again and again, 

 and by it the filament becomes greatly elongated. It is 

 interesting to note that this increase of cells, which here 

 constitutes the growth of the plant-body, is that which in 



