THE SIMPLER ORGAXISMS 59 



the nucleus. About the new end a new cell wall is formed, 

 and in the protoplasm of that end new chlorophyl develops- 

 It is for the sake of the protoplasm that these other parts 

 exist. The normal structure is regained, during the period 

 of growth which ensues. Little is directly observable except 

 the increase in size of the plant. 



The two processes of growth and reproduction so simply 

 shown in Closterium, are characteristic of all living organ- 

 isms, and are their most distinctive phenomena. 



Many algae consist, like Closterium, of cells existing 

 singly, while many others consist of numbers of cells ag- 

 gregated together to form a more complicated plant body. 

 But whether the plant cell exist alone and apart, or whether 

 it live in contact or in combination with other cells, its 

 parts are usually those seen in the cell of Closterium: 



1. Protoplasm J cytoplasm 1 usually 

 "the physical basis of life" (and nucleus J inclosed by, 



2. The cell wall, an investing capsule of transparent cel- 

 lulose which envelops, besides the protoplasm, certain 

 diverse substances of greater or less importance that may 

 collectively be designated as: 



3. Inclusions, the more important of which are 



a) the cell sap; a watery fluid which fills all the 

 spaces (vacuoles) unoccupied by the more solid parts 

 and is the medium of exchange of food and waste 

 materials. 



b) chlorophyl, the greenish substance above noted, in 

 the presence of which occur carbon reduction, and 

 the storage of energy of the sun's rays (to be dis- 

 cussed under a subsequent heading), and 



c) secretions, excretions, reserve stores of starch and 

 other food materials, precipitations of mineral crys- 

 tals, (such as oxalate of lime), from the saturated 

 solutions of the cell sap, etc. 



