CHAPTER XVI 
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: (1) 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. Ifa 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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