ELEMENTARY BIOLOGY 



species are the green slime (pleurococcus] that we find grow- 

 ing on the bark of trees, on the shingles of houses, and on 

 damp rocks, and the pond scum, or " frog spit " (spirogyrd), 

 that we find floating on the surface of ponds. 



110. Green slime. The green-slime cell is a spherical cell 

 (Fig. 25) consisting of a mass of protoplasm with its nucleus, 

 a cell wall, and a quantity of chlorophyl. On the moist sur- 

 face of the tree it is in a position to absorb water, carbon 



dioxid, and various salts that 

 either are washed down by the 

 rains from the dust that strikes 

 the tree or are dissolved from 

 the bark. Supplied with these 

 materials, and containing chlo- 

 rophyl, the plant is able to 

 manufacture, during the day- 

 time, first carbohydrates and 

 FIG. 25. Green slime then proteins. The cell wall 



This plant consists of a single cell. When also permits the diffusion of 

 the cell divides into two, the daughter cells OX yg en from thf 



may cling together or they may be sepa- 

 rated. Sometimes a cluster, or " colony," 

 is formed, containing many cells ; in such 

 a cluster each cell is independent of the 

 others, since each is capable of making its protoplasm, releasing energy 



and thus the plant breathes. 

 ta l n ] are : n f u p 



" and producing carbon dioxid. 

 This gas is excreted by osmosis 



through the cell wall. It is possible that some of this is used, 

 in the daytime, in the process of photosynthesis. 



The food manufactured by the cell is used almost immedi- 

 ately in the construction of more protoplasm, and thus the cell 

 grows. When a certain size is reached, the nucleus divides, 

 and presently there are two cells in place of one. These two 

 cells may remain adhering to each other or they may soon 

 separate. In any case they are quite independent of each other 

 in every way that is to say, each cell can keep on taking in 

 substances from the outside, manufacturing food, growing, 



