442 PLANT RESPONSE 



the growth of a particular plant, so that the varying factors 

 of age and constitution, or tonic condition, should remain 

 constant. And, further, it should be possible to carry out 

 this determination of growth at different temperatures in a 

 time so short that the spontaneous variations of the plant, 

 if any, would be insignificant. Only by such a method 

 could we hope to obtain results which would be reliable and 

 consistent. 



Keeping these considerations in view, I have been fortunate 

 enough to be able to devise four distinct methods of determin- 

 ing the effect of temperature on the rate of growth, the 

 perfection of which may be gauged from the fact that the 

 results of all agree with and corroborate each other within a 

 fraction of a degree. With some of these, the entire experi- 

 ment on the different rates of growth at various temperatures, 

 ranging from ordinary through optimum to maximum, can 

 be carried out within about half an hour. The determination 

 of any single cardinal point, such as the optimum, can always 

 again be made within five minutes. The investigation thus 

 gains by simplicity and experimental accuracy, and observa- 

 tions may be made on many different specimens within a 

 very short period. I shall now proceed to describe these 

 different methods. 



(i) The method of discontinuous observations. — We 

 shall first take the method by which rates of growth are 

 recorded at different temperatures, say one degree apart. 

 Some means of raising the temperature to exactly the 

 required point, and maintaining it unchanged during the 

 time of experiment, is an essential condition of all these 

 investigations. This I have been able to accomplish in the 

 following way. The electrical heating coil inside the plant 

 chamber is put in connection with an external battery. The 

 heat given out, and the consequent permanent rise of 

 temperature in the chamber, depend on the intensity of the 

 current that flows through the heating coil. This current 

 again may be progressively regulated by the interposition of 

 an electrolytic rheostat in the circuit, the resistance of which 



