2JO 



Introduction to Botany. 



use of the carbon dioxide of the air, or of the water in 

 which they reside, in the same manner as do the more 

 complex green plants. Having need of the sunlight, they 

 usually float near the surface if their habitat is water ; but 

 if they live on the land, they are adapted only to moist and 

 shady situations. They have no waterproof protective 



covering such as is furnished 

 by the cuticle of the higher land 

 plants, and accordingly quickly 

 dry up if exposed to a dry at- 

 mosphere. 



163. Pleurococcus Viridis. 

 Pleurococcus viridis, which we 

 found as a mealy green growth 

 on the north side of trees, is one 

 of the simplest of the Algae. As 

 has been seen, an individual con- 

 sists of a single globular cell, 

 having an outer transparent wall 

 of cellulose, a nucleus which is 

 difficult to see, and a few, rela- 

 tively large chloroplasts occupying the bulk of the cell (Fig. 

 136). This single, minute individual is equipped to sustain 

 an independent existence, being able to perform within 

 its small compass the necessary nutritive and reproductive 

 functions. But since it consists of a single cell, it is 

 unable to construct special systems for the separate func- 

 tions ; and it is therefore restricted to habitats which by 

 their nature protect it against too great transpiration, the 

 beating of storms, and other sources of mechanical in- 

 juries. The thin and delicate wall necessary to allow the 

 ingress of its raw food materials will not permit it to exist in 

 places which are dry or fully exposed to the sun. We may 



FIG. 136. 



Pleurococcus viridis. a, a single 

 individual ; b, c, and d, various 

 stages in cell division. The 

 chloroplasts are so crowded that 

 no attempt has been made to 

 distinguish them in the figure. 



