CHAPTER XVIII 



EFFECT OF HEAT ON GROWTH 



As in other eases, so in describing the effect of heat we shall 

 consider separately its effect on the rate and on the direction 

 of growth.* 



§ 1. Effect of Heat on the Rate of Growth 



The descriptive fact that the rate of growth — of develop- 

 ment in general — in both animals and plants is dependent 

 upon temperature, has long been known. The work upon 

 plants was begun earlier than that upon animals, and has been 

 carried further. "We may consider, first, the results gained in 

 that group. 



1. Plants. — As an introduction to the study of the effect 

 of heat upon growing plants, we may consider the results of 

 measurements made upon phanerogams, showing, for various 

 temperatures, the increase in length of the plant after 48 

 hours : — 



* The methods to be employed in subjecting organisms to heat have already 

 been discussed on pp. 219-222. The constant-temperature oven, of the kind 

 employed in bacteriological laboratories, may be used for rearing growing plants 

 or aquatic animals at a high temperature. An ordinary refrigerator will serve 

 for temperatures from near 0° to 8° or 10° C. The methods employed in thermo- 

 tropic experimentation are referred to on p. 464. 



To test the question of the dependence of the optimum for growth upon the 

 temperature to which the organism is normally subjected it would be necessary 

 to rear a species attuned to a warm climate at a constantly low temperature for 

 •several generations, or the reverse, and then compare the optimum of the normal 

 race with that subjected to the new conditions. To test the question of the 

 dependence of the range of the growing temperatures upon the range of tem- 

 peratures in the environment, one lot of individuals of a species should be 

 reared in a constant-temperature room, and another in a very fluctuating 

 temperature. 



450 



