ri. THE NATURE OF THE CELLS OF WHICH 

 ANIMALS AND PLANTS ARE COMPOSED 



A cell. — Most of us, no doubt, think of a cell as a space 

 inclosed by walls, for example, a crayon box. In zoology, 

 however, we must learn to thmk of a cell as something 

 quite different from the above. In fact, a living plant or 

 animal cell, instead of being simply a walled-in cavity, 

 is a minute p(jrtion of semifluid substance known as pro- 

 toplasm, which may or may not be inclosetl by a wall. 



A plant cell. — Since animal and plant cells are so much 

 alike, and smce it is easier to see plant ceUs, we shall 

 examine these first. A melon vme or squash vine is covered 

 with a multitude of small white hairs. Each of the hairs 

 (Fig. .3, .1), when examined under the microscope, is found 

 to be a slender, cylindrical filament, cut up by cross walls 

 into several distinct divisions. Each of these divisions 

 is a ceU, and each cell is bounded by a thin, transparent 

 wall. The wall, therefore, forms a delicate sac within 

 which is an almost colorless, granular liquid called proto- 

 plasm. The granules may be seen to flow along definite 

 threads or streams (Fig. .3, B) in the cell by which we con- 

 clude that the protoplasm has a streaming movement. 

 Protoplasm, in its simplest form, appears to be of the 

 same structure throughout and somewhat resembles the 

 white of an egg. Actually, it is a very complex substance 

 incapable (in the living state) of chemical analysis. Usu- 

 ally, as seen in animals and plants, it is granulai'. In 



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