476 LIFE-HISTOIUES 



develop at certain seasons in the stored water-supplj^ of 

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

 ments recently 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 

 famihar water plants. 



They are characterized by having the chlorophyll ordinarily un- 

 masked by any other pigment, their structure and their niethodK of 

 reproduction incluAing 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 l)lue pigment it is much like a tint-ball alga. 

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

 cleus is more di.stinct than in the tint-balls, and the proto- 

 plasm shows further differentiation in the presence of special- 

 ized bodies of varied form called chromaiophnres,^ 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 conchtions has 

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

 the jjlant as commonly seen consists simply of fissions re- 

 peated indefinitely. 



Somewhat higher in organization arc the j'ellowish green, uni- 

 cellular, fresh water alga? known as dcsmids, of which Cosniarium 

 (Figs. 308-310) is a ty])ical genus. Dcsmids are of varied and often 

 strikingly beautiful forms, the firm, cellulose wall being sometimes 

 curiously sculptured and freciuently developing sharp projections, 

 while the chromatophores tal-ce the form of disks, plates or bands 

 symmetrical!}' arranged. Within the chromatophores may be seen 

 transparent spots called pyrcnoids - in which starch is formed. 

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

 as in Cosmarium, forming thus two scmicells; and in all cases the 

 hah'es are symmetrical. This peculiarity modifies in an odd way 



' Chro'mat-o-)3hore < Or. chroma, cdlor; pharos, bearing. 



■' Py-rc'iioid < Gr. pyr'cn, the stone of a fruit ; cidos, resemblance. 



