476 LIFE-HISTORIES 
develop at certain seasons in the stored water-supply’ 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 alge (Class Chlorophycez) include many 
familiar water plants. 
They are characterized by having the chlorophyll ordinarily un- 
masked by any other pigment, their structure and their methods of 
reproduction including 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 yellowish green, uni- 
cellular, fresh water alge known as desmids, of which Cosmarium 
(Figs. 308-310) is a typical genus. Desmids are of varied and often 
strikingly 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 the 
halves are symmetrical. This peculiarity modifies in an odd way 
1 Chro’ mat-o-phore <i Gr. chroma, color; phoros, bearing. 
2 Py-re’noid < Gr. pyrén, the stone of a fruit; eidos, resemblance. 
