THE ALG.E 223 
207. Reproduction of Pleurococcus. New Plewrococcus plants 
are formed by the division of the old plants. Thus, the grow- 
ing, or vegetative, plant body (one cell) divides, and each 
division is a new plant. Under favorable growing conditions 
reproduction goes on rapidly. The divisions follow one an- 
other in such a way that whole colonies, the descendants of one 
individual, are often grouped together (fig. 174). Obviously 
Plevrococcus 1s a very simple plant in its structure, nutrition, 
and reproduction. 
208. Spirogyra. One of the most abundant of the common 
floating pond secums is the green alga, Spiroyyra. The plant 
is sometimes called “brook silk” because of its soft, silken 
texture, by means of which it may generally be distinguished 
‘from other fresh-water alge. It is usually very bright green, 
except when it gathers in dense masses at the surface of the 
water, when it may be dark yellowish green. Spirogyra is a 
many-celled plant, with cylindrical cells arranged end to end, 
and all held in a common sheathing plant wall. Also, each 
cell contains one or more peculiar spirally arranged chloro- 
plasts, each of which extends almost or quite the entire length 
of the cell (fig. 175). There are different numbers of chloro- 
plasts in the cells in different species of Spirogyra. <A layer 
of cytoplasm usually lies just within the cell wall, and strands 
of cytoplasm run to the nucleus, which is suspended in the 
central part of the cell (fig. 175). 
209. The nutrition and growth of Spirogyra. In the water 
in which Spirogyra lives there are dissolved the carbon dioxide 
and other inorganic materials from which foods are made. In- 
deed, there is much water within the plant itself, as may be 
demonstrated by careful drying, by which it is sometimes 
found that as much as 98 per cent of this plant is water. 
That photosynthesis is carried on is often made evident by 
the oxygen bubbles which arise from the plants that are 
active. It is obvious that this plant can expose more chloro- 
phyll to the light, and hence can do more photosynthetic 
work, than can Pleurococcus. 
