476 LIFE-HISTORIES 



develop at eertain seasons in the stored water-supply of 

 many cities, often rendering it unfit for use. Through experi- 

 ments recentl}' performed on a large scale by the United 

 States Department of Agriculture, it has been found that 

 an exceedingly minute percentage of copper sulphate added 

 to the water will kill them and other harmful plants without 

 leaving any traces of itself which are either perceptible or 

 harmful to man or beast in the slightest degree. 



174. The green algae (Class Chlorophyceae) include many 

 familiar water plants. 



They are characterized bj' havit^g the chlorophyll ordinarily un- 

 masked bij any other pigment, their structure and their metiwds of 

 reproduction incliuling widely varied types. 



A very simple form, common throughout the world on 

 rocks or tree-trunks which are often wet by rain, is the wall- 

 stain alga (Pleurococcus) shown in Fig. 307. Except for 

 the absence of blue pigment it is much like a tint-ball alga. 

 The cell-wall of this alga is ordinary firm cellulose, the nu- 

 cleus is more distinct than in the tint-balls, and the proto- 

 plasm shows further differentiation in the presence of special- 

 ized bodies of varied form called chromatophores,^ to which 

 the chlorophyll is confined. Reproduction, as ordinarily 

 observed, is by fission in three directions; and, since the 

 cells often remain attached, more or less globular colonies 

 result. Further development under certain conditions has 

 been reported; but however that may be, the life-history of 

 the plant as commonly seen consists simply of fissions re- 

 peated indefinitely. 



Somewhat higher in organization are the 3-ellowish green, uni- 

 cellular, fresh water algse known as desmids, of which Cosmariinn 

 (Figs. 308-310) is a typical genus. Desmids are of varied and often 

 strikingl}' beautiful forms, the firm, cellulose wall being sometimes 

 curiously sculptured and frequently developing sharp projections, 

 while the chromatophores take the form of disks, plates or bands 

 symmetrically arranged. Within the chromatophores may be seen 

 transparent spots called pyrenoids - in which starch is formed. 

 Many of the genera have the plant-body constricted in the middle, 

 as in Cosmarium, forming thus two semicells; and in all cases tlie 

 halves are symmetrical. This peculiarity modifies in an odd way 



' Chro'mat-o-phore < Gr. chroma, color; phoros, bearing. 



- Py-re'noid < Gr. pyren, the stone of a fruit; eidos, resemblance. 



