THE GREEN ALG^ (CHLOEOPHYCE^) 189 



the unsharpened end of an ordinary lead pencil, approximately 

 500 plants would be required to complete the row. How 

 many would be required to occupy one cubic inch ? 



Each plant is somewhat spherical and has a well-defined 

 cell wall. Within this wall is the protoplasm, which is colored 

 green by the chlorophyll. The chlorophyll is usually confined 

 to a special part of the protoplasm, this part being called the 

 chloroplast, which means " a body which bears chlorophyll." 

 Often it is not possible to distin- 

 guish the chloroplasts in Pleuro- 

 coccus, the chlorophyll appearing to 

 be evenly distributed throughout 

 the plant. The nucleus is another 

 special part of the protoplasm. It 

 is a mass of protoplasm denser 

 than the other portions, usually 

 lies near the center of the cell, and 

 is a structure of great importance 

 in the activities of the plant. 



When chloroplasts are evident 

 it is possible also to see about 

 them a thin, almost colorless, pro- 

 toplasmic substance, the cytoplasm. 

 A single Pleurococcus plant, there- 

 fore, consists of the cell wall and 

 the protoplasm that is contained by this wall. The protoplasm 

 may be divided into chloroplasts, cytoplasm, and nucleus. In 

 some cases, also, there may be seen vacuoles, which are regions 

 within the cell surrounded by cytoplasm but containing air 

 or cell sap. 



175. Pleurococcus: nutrition. The protoplasm of this plant 

 contains chlorophyll, and if carbon dioxide and water are ob- 

 tainable it can carry on photosynthesis. Carbon dioxide may 

 be secured from the air. The bark, or other support upon 

 which the plants grow, is often sufficiently moist 'to enable 

 Pleurococcus to obtain water from it. Rains and heavy dew 



FIG. 155. Green slime (Pleuro- 

 coccus) 



a, single plants showing cell wall, 

 granular cytoplasm, and nucleus ; 

 6, plants in process of reproduc- 

 tion by division or fission ; c and 

 d, further divisions sometimes re- 

 sulting in formation of colonies 

 - of plants. Greatly enlarged 



