PHYOOPHTTA. 141 



248. In their reproduction diatoms resemble the des- 

 mids, the only differences being those made necessary by 

 their rigid walls. 



249. 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 tised as microscopic tests. 



250. The Pond-scums {Zygnemacem). — The plants of 

 this family, which are all aquatic, are elongated un- 

 branched filaments, composed of cylindrical cells arranged 

 in single rows. The cells are all alike, and each one ap- 

 pears to be independent, or nearly so, of its associates. 

 The filament is thus, in one sense, rather a composite body 

 than an individual. The chlorophyll is generally arranged 

 in bands or plates. 



251. 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 elon- 

 gated. It is interesting to note that this increase of cells, 

 which here constitutes the growth of the plant-body, is 

 that which in simpler plants is called the asexual mode of 



