THE ALG^ 223 



207. Reproduction of Pleurococcus. New Pleurococcus 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 

 Pleurococcus is a very simple plant in its structure, nutrition, 

 and reproduction. 



208. Spirogyra. One of the most abundant of the common 

 floating pond scums is the green alga, Spirogyra. 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 algae. 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 photosynthetio 

 work, than can Pleurococcus. 



