r^ 



Volume 1 



Volume 2 



Surface 1 



Surface 1.92 



w ^ <4\ » « «^ 



f;«^j/\>^ ^-.- v/ 



^n^l.#W\l 



.=*==si 



LIMITATION ON GROWTH 



A cube twice as large as another has 8 times as much material in it, but only 4 

 times as much surface. But it would be difficult to make sure that any particular cell 

 actually stopped growing because its volume had become too large in proportion to 

 its surface. This purely mathematical idea is supported, however, by the fact that 

 thread-shaped cells or series of cells, like those of certain algae and fungi, appear 

 to grow indefinitely in length. In the growth of such structures the volume increases 

 very little faster than the surface 



larger than some of its hungry companions, but it will not make a mouse 

 grow to the size of a rat. Conversely, insufficiency of food, though it may 

 not kill the organism, may stunt its growth. 



Size, like any other characteristic of a living thing, is influenced by the 

 surrounding conditions. The pine tree, for example, attains its size and 

 shape influenced in part by soil and weather; that is, it grows better in some 

 locations or in some climates than in others, growing faster when it is 

 warmer. The squirrel in the tree's branches is also influenced in its growth 

 by the food it can get, by weather conditions, and perhaps by enemies. In 

 each case, however, the organism reaches a size that is fairly characteristic 

 of the species; that is, how fast an organism grows and how long it con- 

 tinues to grow are determined in part by the kjnd of protoplastn of which it 

 consists (see illustrations, pp. 346 and 561). 



Moreover, as a baby or any other living thing grows, it is constantly 

 changing in shape, as well as in size. That means, of course, that some parts 

 are growing faster than others, or that some parts slow up or even stop 

 growing, while other parts keep right on. In the body of any particular 

 individual, each cell stops growing when it reaches a certain size; and the 

 cells of a particular part will stop dividing when the structure or organ 

 reaches a certain size (see illustration, p. 347). As a consequence, our own 

 bodies, for example, contain many different kinds of cells, of many dif- 

 ferent sizes, and in various proportions. 



345 



