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 
Alge 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 
Alge. 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 Alge 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 ALaa (Chlorophycee) 
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 Glwocapsa, 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 
