In addition to producing new protoplasm, a growing organism produces 

 new cells through cell-division (see illustration below). Cell-division in- 

 creases the number of cells. Each new cell is, of course, smaller than the 

 mother-cell. In one-celled as well as in many-celled organisms, cell-division 

 increases the number of cells; assimilation increases the quantity of proto- 

 plasm. And in every case living is continuous', the new protoplasm formed 

 is like the old protoplasm, and the new cells are like the old cells. 



Sizes of Cells We have seen that as a body grows larger, the volume 

 increases more rapidly than the surface (see illustration, p. 117). In time, 

 therefore, the ratio of surface to volume may be too small to allow for the 

 absorption of surplus for new protoplasm. At that point, of course, there 

 can be no further growth (see illustration opposite). 



Conditions of Growth The most prominent single condition for the 

 growth of protoplasm is a supply of food, more specifically, protein. It does 

 not follow that a surplus of food will ensure greater growth. A Shetland 

 pony, for example, cannot grow to the size of a draft horse by merely in- 

 creasing its food intake. An excess of food may make a mouse grow to be 



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Region of rapid cell division Region of cell growth 



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GROWTH BY CELL DIVISION AND ASSIMILATION IN ONION ROOT 



Cell division increases the number of cells; the stained nuclei are close together. 

 Each new cell is, of course, smaller than the mother cell was; but it grows larger by 

 assimilating food, so that the entire structure increases in size 



344 



