THE GREEN ALG.E (CHLOROPHYCE^) 205 



of calcareous material, which, as the plant dies, falls to the 

 bottom of the pond or stream. Chara grows in such luxuri- 

 ance that its deposits eventually form deep layers of this cal- 

 careous material, or marl as it is called. Marl has been found 

 of great value in the manufacture of cement, and not a few 

 of the lakes in which Chara grows 

 are being dredged to secure the 

 marl deposits for this important 

 manufacture. 



188. Algae and water supply. 

 Many of our large cities have 

 found it advisable to construct 

 reservoirs for water. These are 

 open pools, lakes, tanks, etc., 

 and they are intended to hold 

 water enough so that in times 

 of scarcity there will be at hand 

 a sufficient supply. Such reser- 

 voirs have proved so admirable 

 as growing places for algse that 

 these plants have often become 

 a nuisance. Their presence in 

 water for domestic use is not at- 

 tractive, and, besides, they may 

 stop up the water pipes ; but far 

 more serious than these objec- 

 tions is the actual pollution of 

 the water because of their pres- 

 ence. When they die they be- 

 come the food for decay -produc- 

 ing organisms (Sect. 154), and often positively injurious sub- 

 stances may thus be generated. It has been found that by tow- 

 ing about in such reservoirs a quantity of copper sulphate, 

 inclosed in coarse sacking, minute quantities of the salt be- 

 come dissolved and the algse are thus killed. The solution is 

 not strong enough to render the water unwholesome for use. 



FIG. 168. Desmids 



A,Micrasterias; B, B', Staurastrum 



(two views) ; C, Closteriwn. After 



West 



