CELLS AND TISSUES 



2 9 



substances, while animals must get their food from organic 

 substances, either from plants or from other animals which 

 in turn have eaten plants. The plant builds up complex, 

 unstable substances by means of the sun's 

 light and heat (Fig. 24). The animal eats 

 the plant, and these substances are finally 

 reduced in the body to simple, stable sub- 

 stances, thus setting free the energy stored 

 up in the plant. (What is it, therefore, 

 that indirectly furnishes the energy that FIG. 25. Diagram of 



.. ,./ 5X the Parts of a Cell. 



sustains all life ? ) 



44. A Living Cell has two essential parts : a small mass 

 or globule of protoplasm, and a still 

 smaller body in its interior, also consist- 

 ing of protoplasm, called a nucleus (Fig. 

 25). There may be two other parts, for 

 often there is a still smaller body, a dot 

 in the nucleus, called a nucleolus, and 

 usually the protoplasm has a wall sur- 

 rounding it called the cell wall (Fig. 

 26). Cells are of various shapes and 



showing that protoplasm has constitute the substance of all the or- 



an intricate structure; in 



this case it appears some- gans. The cells are all of the human 



what like honeycomb; /,,.. ,-, -,. ,, -, 



protoplasm ; n, nucleus ; body that is really alive, the substances 

 in the body outside of cells not being 

 considered living matter. A cell may be defined as a tiny 

 particle of protoplasm containing a nucleus. 



FIG. 26. Diagram of 

 a Cell. 



FIG. 27. A Cell that is Slender and in the Form of a Fiber. 

 It is from the muscular coat of the intestines ; /, granular protoplasm around the nucleus, n. 



45. Fibers. In some of the tissues the cells are very 

 long and slender, and a single cell forms a fiber. A fiber is 

 stringlike in shape (Fig. 27). Sometimes the fibers consist 

 of rows of cells or of long, fine branches of cells. 



