UNICELLULAR ORGANISMS 



77 



ters that have caused biologists, in the past, to group them with 

 animals. Modern scientists, however, are unanimous in opinion, 

 grouping the following organisms among the plants. 



PLEUROCOCCUS 



Pleurococcus appears like a green stain, growing in abundance 

 upon damp tree trunks, fence posts, or even damp rocks. Upon 

 scraping off some of the material 

 and examining it with a microscope 

 it is found to consist of a great num- 

 ber of small green cells. These 

 (Fig. 30) are spherical, and contain 

 no visible internal organs except a 

 nucleus. The cells are found massed 

 together into irregular bunches, 

 but are not really attached together. 

 As they grow in size they divide by 

 fission in two parts, each of which 

 divides subsequently, the new in- 

 dividuals sometimes remaining at- 

 tached, to form irregular masses 

 which are easily shaken apart. No 

 other method of reproduction is 

 known. It is possible that this little 

 plant is really a stage in the life of 

 some higher plant whose develop- 

 ment is not yet known, since it 



has been shown that some of the more complex plants have a 

 stage in which they are simple green cells like Pleurococcus. 

 Concerning this organism, however, nothing is known positively 

 except that it occurs abundantly in damp places and, so far 

 as known, has no other phase of its life than that already 

 noticed. 



FIG. 30. PLEUROCOCCUS 



a, a single cell; b, one showing 

 division by fission; c, a later stage 

 of division. The plant in its grow- 

 ing condition is bright green. 



