6 PLANTS AND MAN 



hundreds of different shaped cells suffused with bright green 

 pigment, characteristic of the species belonging to the Green 

 Algae. 



Study of these unicellular organisms or of a single cell from 

 the body of a more complex plant or animal reveals the fact 

 that a cell is a highly organized mass of protoplasm, separated 

 from the environment by its protective cell wall (fig. 2). This wall 

 is a non-living structure secreted by the protoplasm; it is not 

 uniform in nature, the cell wall of plants having evolved along 



VACUOLE 



NUCLEUS 



MEMBRANE 



CELL WALL 



CYTOPLASM 



CHLOROPLAST 



Fig. 2. — A cell is a highly organized mass of protoplasm. 



slightly different lines from that of animals. The animal cell 

 wall is of protein nature and very pliable, while that of a plant 

 is of cellulose and often extremely rigid. Within the cell wall, the 

 protoplasm is often exceedingly complex, various portions taking 

 on special functions. Most important of these protoplasmic 

 structures are the nucleus, the cytoplasm and the various proto- 

 plasmic membranes. The nucleus is a rounded and dense mass 

 of protoplasm, often near the center of the cell, which exercises 

 a directive influence over the activities of the entire cell; celfs 

 deprived of their nuclei do not act normally and usually do not 

 live long. The nucleus is also important in heredity, for in it is 



