102 A TEXT-BOOK OF BOTANY 



shown by the cells of a colony to work together. Different 

 forms of cells are exhibited by Nostoc ; and this condition, 

 spoken of as the differentiation of cells, implies also a differ- 

 entiation of work. 



62. Presence in water reservoirs. Until recently the 

 Algse were thought to be of no importance to man; but it is 

 now known that the offensive odor and taste too often 

 observed in drinking water are due almost entirely to them, 

 and chief among the polluting forms are the blue-green 

 Algae. This pollution of water becomes very conspicuous 

 when it occurs in city reservoirs or in ponds, and various 

 methods of purification have been suggested. Of these 

 none had proved satisfactory, until in 1904 the Department 

 of Agriculture at Washington announced that an effective 

 method of destroying the Algae or preventing their appear- 

 ance had been discovered. It consists in introducing into 

 the water a solution of copper sulphate so dilute that it is 

 tasteless and harmless to man; but the warning is given that 

 each reservoir or pond must be studied before the proper 

 amount of the solution can be known. 



2. GREEN ALG^ (Chlorophycece) 



63. Pleurococcus. These plants are exceedingly com- 

 mon, occurring in masses, especially on the north side of 

 tree trunks, old fences, etc., and looking like a green stain. 

 After a few damp days the green of the masses becomes 

 more vivid and noticeable. These finely granular green 

 masses are found to consist of multitudes of spherical cells, 

 resembling those of Gloeocapsa, except that there is no blue 

 with the chlorophyll, and the cells are not embedded in a 

 jelly-like substance derived from the walls. 



The cells may be solitary, or they may cling together in 

 groups of various sizes (Fig. 95). Cells that have just 

 divided may be observed easily, the evidence being that the 



