THE CELL 



11 



The complicated structure of protoplasm, and the fact that it is 

 constantly in a state of flux and change, prevent its existence in large 

 masses It must be intimately bathed with the fluids that feed and 

 oleansa it. Hsnce the cellular structure, and the evolution of circu- 

 latory mechanisms in the higher multicelluJar animals. Protoplasm, 

 in order to live, must protect itself from extremes of temperature, 

 and from other active physical or chemical changes which split up its 



FIG. 8. To SHOW THE MIGRATION OF THE NUCLEUS TO THE POINT OF GROWTH IN 

 PLANTS. (Redrawn after Haberlandt, from Wilson's " The Cell," etc.) 



A, Young epidermal cell of Luzula, with central nucleus before thickening of the 

 membrane ; B, three epidermal cells of Monstera during thickening of outer 

 wall ; G, cell from seed coat of Scopulina during thickening of the inner wall; 

 D,-E, position of the nuclei during the formation of branches in the root-hairs 

 of the pea. 



molecules into simpler (dead) compounds. It must have the food 

 necessary to keep up its cycls of change served to it in proper form. 

 Protoplasm, therefore, moves, not only to find food, but to avoid 

 injurious influences. Protoplasm moves by effecting a redistribution 

 of its substance, and certain parts are especially set apart, so situated 

 as to produce definite changes in the shape of the living organism, and 

 so differentiated in structure as to perform rapid movements cilia, 

 muscles. It is well to remember that movement in response to ex- 



