222 



INTRODUCTION TO BOTANY 



plants, placed side by side, to make a row that would reach 

 across the unsharpened end of an ordinary lead pencil. How 

 does Pleurococcus compare in size with average bacteria ? 



The spherical plants consist each of a small mass of living 

 material, or protoplasm, surrounded by a cell wall. The pro- 

 toplasm is so thoroughly colored by chlorophyll that usually it 

 is not easy to see the centrally placed nucleus (fig. 174). Some- 

 times in these plants special parts 

 of the protoplasm hold the chloro- 

 phyll, and each of these is known 

 as a chloroplast, which means 

 "a green body," or a chlorophyll- 

 containing body. Parts of the pro- 

 toplasm that surrounds the nucleus 

 and the chloroplasts are granular 

 and are called cytoplasm. Some of 

 the internal space of this one-celled 

 plant may be occupied by one or 

 more vacuoles, which are regions 

 surrounded by cytoplasm and filled 

 with air or water. 



206. The food of Pleurococcus, 

 The bark or other substance upon 

 which Pleurococcus grows is often 

 sufficiently moist to provide it with water. Rains and dew sup- 

 ply water intermittently. When dry, the plants remain dormant 

 until conditions again become favorable. Carbon dioxide (and 

 possibly some moisture) may be absorbed directly from the air, 

 and with carbon dioxide and water, and favorable temperature 

 and light, Pleurococcus plants may carry on photosynthesis, 

 thus making their own foods. Heat, cold, and extreme drought 

 are some of the severe conditions which this plant must be 

 able to resist in order to live. When plants from any of these 

 extreme conditions are placed in favorable moisture, tempera- 

 ture, and light, they become bright green within a few hours, 

 showing that they are manufacturing foods. 



FIG. 174. Green slime 

 (Pleurococcus) 



a, single plants showing cell wall, 

 granular cytoplasm, and nucleus ; 



b, plants in process of reproduc- 

 tion by division, or fission ; c and 

 (I, further divisions sometimes re- 

 sulting in formation of colonies of 



plants. Greatly enlarged 



