CHAPTER XVI 



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EXTERNAL FACTORS THAT INFLUENCE GROWTH 



IN the last chapter we studied some of the characteristics of 

 normal growth. In this and in the following chapter the exter- 

 nal and internal conditions that modify growth will be considered. 



In anticipation of what is to follow I may state that while the 

 rate of growth may easily be accelerated or retarded, the char- 

 acter of the growth is more difficult to modify. The following 

 external agents have been found to affect the rate, and in 

 some cases the character of growth: (i) Food; (2) Stimuli; 

 (3) Salts; (4) Heat; (5) Light; (6) Gravity; (7) Electricity; 

 (8) Pressure -and Contact. 



Influence of Food 



To a certain extent the rate of growth depends on the amount 

 of food. It is clear that if less food is obtained than needed to 

 make new tissues, the rate of growth must slacken. On the 

 other hand, if more food is given than can be assimilated, the 

 rate of growth is not thereby increased. This is, however, 

 by no means the whole question; for animals show a curious 

 regulative power, and store up reserve materials usually in the 

 form of fats when an excess of food is taken which may be drawn 

 upon later if the food supply temporarily diminishes. Let us 

 take an imaginary case. Suppose an individual to be growing 

 at its normal rate with an abundance of food. If a certain small 

 amount of this food is taken away, it does not follow that the rate 

 of growth will correspondingly decrease. Take more away and 

 the rate will decrease somewhat, but again not in proportion to 

 the amount removed. The same fact is brought out in another 



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