CHAPTER IV 



EFFECT OF VARIATION OF TEMPERATURE ON GROWTH 



Accurate determination of the effect of temperature on 

 growth presents many difficulties due to the presence of the 

 numerous comphcating factors. In order to reduce these 

 to a minimum, it is advisable to carry out the experiment 

 with an identical plant, observing its rate of growth at 

 different temperatures. The period of the experiment should, 

 moreover, be as short as possible, so as to eliminate the 

 complication which arises from, the diurnal variation of 

 the rate of growth. A quick change of the temperature 

 of the plant-chamber is therefore necessary, which un- 

 fortunately introduces unexpected difficulties ; for a rapid 

 change of temperature acts as an excitatory shock, inducing 

 contraction of the growing organ. This drawback can be 

 obviated to a great extent by securing a gradual instead 

 of an abrupt variation of temperature. The temperature 

 of the plant-chamber has, therefore, to be gradually raised 

 or lowered. This has been secured in the following manner : 

 a cylinder of thin metal with its base closed serves as the 

 plant-chamber P, kept in a humid condition by a piece of 

 moist sponge (fig. 14). The cylinder P is enclosed in 

 a larger vessel T.R., fiUed with water and serving as the 

 thermal regulator. There are two reservoirs, H and C, 

 containing hot and ice-cold water respectively. Opening 

 of the stopcocks Si and S3 allows hot water to stream 

 through the thermal regulator, the excess escaping through 

 the outflow pipe O. The opening of So and S3 allows, on 

 the other hand, a stream of cold water to flow through the 



