122 BOTANY. 
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 (Zygnemacee). 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 
