THE CEL. 



n . 



FIG. 2. An Amwba. A cell 

 without a cell wall, n, nu- 

 cleus ; v, vacuoles, x 300. 



lining of the sac is a roundish body (n), which itself has a definite 

 membrane, and usually shows one or more roundish bodies within, 

 besides an indistinctly granular appearance. This body is called the 

 nucleus of the cell, and the small one within it, the nucleolus. 



The membrane surrounding the cell is known as the cell wall, and in 

 young plant cells is always composed of cellulose. 



The granular substance lining the cell wall (Fig. 1, pr.) is called "pro- 

 toplasm," and with the nucleus constitutes 

 the living part of the cell. If sufficiently 

 magnified, the granules within the proto- 

 plasm will be seen to be in active streaming 

 motion. This movement, which is very evi- 

 dent here, is not often so conspicuous, but 

 still may often be detected without difficulty. 



The cell may be regarded as the 

 unit of organic structure, and of 

 cells are built up all of the complicated structures of which 

 the bodies of the highest plants and animals are composed. 

 We shall find that the cells may become very much modified 

 for various purposes, but at 

 first they are almost identical 

 in structure, and essentially the 

 same as the one we have just 

 considered. 



Very many of the lower forms 

 of life consist of. but a single 

 cell which may occasionally be 

 destitute of a cell wall. Such 

 a form is shown in Figure 2. 

 Here we have a mass of proto- T 



FIG. 3. Hairs from the leaf stalk of 

 plasm with a nucleus (n) and 



cavities (vacuoles, v) filled with 

 cell sap, but no cell wall. The 

 protoplasm is in constant move- 

 ment, and by extensions of a 

 portion of the mass and contraction of other parts, the whole 

 creeps slowly along. Other naked cells (Fig. 12, B ; Fig. 16, <7) 



a wild geranium. A, single-celled 

 hair. B and (7, hairs consisting 

 of a row of cells. The terminal 

 rounded cell secretes a peculiar 

 scented oil that gives the plant its 

 characteristic odor. .#, x 50; C, 

 x 150. 



