PART III 

 THE CONTINUITY OF LIFE 



CHAPTER LII 



GROWTH AND REGENERATION 



318. How organisms grow. A living body consisting of 

 many cells increases in size (i) by the increase in the number 

 of cells through cell divisions, and (2) by the increase in the 

 sizes of cells through assimilation of nutrients. 



There are very many animals that keep on growing indefi- 

 nitely, as certain fishes ; and in plant species that have an 

 apparently definite limit of growth some parts may keep on 

 growing after the plant as a whole has reached its full height. 

 Most of the familiar animals reach a fairly definite limit of 

 growth, beyond which point they may continue to live for a 

 long time without growing any more. 



319. Limits of growth. What is it that stops the growth of 

 an organism without killing it ? 



The growth of a cell depends, for one thing, upon the intake 

 of suitable materials. In the presence of these the rate of 

 income will depend upon the amount of surface exposed to the 

 outside. The needs of the protoplasm, however, will depend 

 not upon the amount of surface exposed but upon the amount 

 of protoplasm that is, upon the bulk, or volume. As a cell 

 becomes larger and larger its volume increases with the cube 

 of the diameter, but the exposed surface increases only as the 

 square of the diameter (see Fig. 104). As a result, the cell 

 soon reaches a point at which the surface is no longer suffi- 

 cient to admit the necessary food, water, oxygen, etc., nor 



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