300 



PRACTICAL COURSE IN BOTANY 



over a sheet of white paper, and examine with a hand lens; 

 then place under the microscope. It will probably be found 

 to contain a number of minute organisms, but the pleurococci 

 can be recognized as small round bodies of a bright green 

 color, some of them separate, others adhering together in 

 groups of two, four, or more, with the sides that are in contact 

 slightly flattened. Each of these bodies is an individual 

 plant consisting of a single cell, whence they are said to be 

 unicellular. Draw one of the single cells and one of the 



groups, or colonies, as they appear 

 under the microscope. Try to make 

 out the cell wall and the nucleus, and 

 label all the parts (see 7). If you 

 have any difficulty in distinguishing 

 the cell wall, drop a little glycerine 

 or salt water on the slide. This will 

 cause the cell contents to shrink by 

 osmosis (56, 59). Can you make 

 out the structure of the cell colonies ? 



FIG. 429. Three stages in 



the division of a one-ceiled alga They have resulted from the peculiar 



(Glceocapsa polydermatica) : A, d of multiplication that prevails 



division of a cell just beginning ; r 



B, division further advanced ; among this claSS of plants. A Cell 



elongates, contracts in the middle, 

 and divides into two parts, each of 

 which becomes an independent plant like the mother cell. 

 See if you can find one in the process of division. The 

 daughter cells repeat the process, each one giving rise to two 

 new individuals, and so on indefinitely. The new cells do 

 not always separate immediately on their formation, but fre- 

 quently adhere together for a time, in colonies, before falling 

 away and beginning an independent existence. 



338. Reproduction by fission. - - This kind of reproduction 

 is called fission, or cell division, and marks a very primitive 

 stage of development. Under stress of adverse conditions 

 the cells formed by division may remain inactive for a time. 

 They are then called resting spores, and when more favorable 



C, four cells after division, re- 

 maining in contact. 



