136 



THE INDIVIDUAL ORGANISM 



related to starch. The cell membrane is normally held in close contact 

 with the inner surface of the cell wall by osmotic pressure within the 

 cell, but the cell can be made to shrink away from the cell wall by immer- 

 sion in a concentrated salt or sugar solution. Many animal cells contain 

 liquid-filled vacuoles; in plant cells, the vacuoles are often so large as to 

 occupy most of the space within the cell, the living protoplasm lining the 

 cell walls or sometimes extending through the vacuoles as protoplasmic 

 strands. Plant cells divide by mitosis just as animal cells do, except that 



Fig. 9.1. Diagram of a typical plant cell. 



in most plant cells there are no centrosomes, and the division of the cyto- 

 plasm in the telophase is usually accompanied by the formation of a 

 cell-wall plate between the daughter cells. 



Cell differentiation and division of labor are as marked among plants 

 as among animals. The cells of the higher plants are of many types, 

 each specialized for the performance of a particular function. As in 

 animals, similar cells are grouped into tissues, and the tissues are, in 

 turn, built into organs and systems of organs adapted for carrying out 

 major functions. The organization of the plant body is less complex 

 than that of the higher animals, in conformity with the greater simplicity 



