

( ,s 



Fig. 2-5. Stained bacteria at a magnification of 

 about 1000 diameters. (Copyright, General Biological 

 Supply House, Inc.) 



proportion of surface to volume steadily 

 diminishes. In a sphere, for example, the 

 surface increases merely as the square of the 

 diameter, whereas the volume increases in 

 proportion to the cube of the diameter. Ac- 

 cordingly, when a cell grows larger, it reaches 

 a natural limit when its surface area becomes 

 too small in proportion to its volume. This 

 limit varies according to each particular cell 

 shape and according to the intensity of me- 

 tabolism. But when the limit is reached, the 

 cell must either stop growing or it must 

 divide. 



The Form of Cells. Different cells assume 

 a great variety of shapes: spherical or oval; 

 fixed or changeable; flat or elongate; spindle- 

 shaped or lobose; polyhedral or cylindrical; 

 and so forth. But generally speaking, the 

 form of each cell bears a distinct relation to 

 its particular function. Nerve cells, for exam- 

 ple, are elongate and branched, a form that 



Protoplasm, the Cell, and the Organism - 1 9 



] enables the cells to conduct impulses from 

 one part of the body to another (Fig. 2-7); or 

 epithelial cells — which cover the surfaces of 

 the body — take the form of overlapping tiles, 

 as in the skin of many animals, or of vari- 

 ously shaped bricks, as in the lining of man's 

 digestive tract (Fig. 2-8). 



Likewise among plant cells form varies ac- 

 cording to function. Root hair cells (Fig. 

 2-9), for example, collectively constitute a 

 specialized surface layer, or epidermis, which 

 covers the root in the region where the plant 

 absorbs water and mineral salts from the 

 soil. The body of a root hair cell is bricklike, 

 and the many separate "bricks" fit together, 

 covering the surface of the root. But each root 

 hair cell also displays an elongate outgrowth 

 — the root hair proper. Each root hair ex- 

 tends out among the particles of soil, making 

 contact with the soil water. Thus the special 

 form of the root hair cells enables them to 

 perform a double function. Collectively they 

 provide an epidermis for the outer surface of 

 the root, and at the same time they absorb 

 substances from the soil water. In fact, with- 

 out root hairs the absorbing surface of the 

 root would be reduced by almost 90 percent. 



Regardless of their shape, all cells tend to 

 become rounded into droplike spheres, if 

 freed from various restraining factors. This 

 behavior results from the fact that proto- 

 plasm is essentially a liquid system. All small 

 liquid masses — such as droplets of mercury 

 or water — behave in this fashion. The round- 

 ing results from surface forces that act in the 

 boundary layers of liquids generally. The 

 surface forces in protoplasm are small, rela- 

 tive to those acting upon a drop of water ex- 

 posed to the air. But the typical cell is a 

 microscopic mass, and most cells do become 

 rounded like other liquids, unless there are 

 restraining factors. 



Many cells are rounded and droplike when 

 first they are formed by cell division, and 

 such cells must expend energy when they 

 distort themselves into another shape. But 

 after a cell has assumed its final form it may 

 retain this shape in a variety of ways. It may 



