20 



PROTOPLASM AND THE CELL 



simply to call attention to the fact that this process is 

 necessary in order to keep the cells of their proper size 

 to maintain the structure of the organism, and that it is 

 an automatic, self-regulated process which is entirely 

 characteristic of living matter. 



The Structure of a Typical Cell. — It is not pos- 

 sible to choose any single cell which will show all the 

 characteristics of cells. In order to exhibit the main 

 features of simple cells it will be necessary to choose two 

 or more illustrations. 



If a cell be chosen from the growing portion of a young 

 root (Fig. 1) or stem of a plant, it will be found to con- 





Fio. 1. — A Typical Plant CVll, from the root tip of the Spiderwort 

 (TradoHcantia virKinica). c.w., cell wall; ryt, cytoplasm; n, nucleus; 

 chr, chromatin; ;«.«., nuclear sap; n', nucleolus; r, vacuole filled with 

 water and dissolved sukstancea. 



tain or to consist of the following parts: around the out- 

 side there is a delicate wall composed of a non-living 

 substance, cellulose; within this wall there is a solid mass 

 of protoplasm. In the center of the protoplasm there 

 is a rounded body, more or less spherical, called the 

 nucleus. Practically all cells contain protoplasm which 

 is differentiated into these two parts, for convenience of 

 reference called nucleus and cytoplasm. In addition to 

 these two structures, many cells possess other organized 



