446 



PLANT RESPONSE 



(2) Method of continuous observations. — The method 

 which I have just described gives us results which, though 

 obtained at closely consecutive temperatures, are nevertheless 

 discontinuous. There is, besides, some loss of time involved 

 during the variable period, in addition to which there is the 

 factor of transient stimulation during sudden changes of 

 temperature. For these reasons I was anxious to perfect 

 some method by which the curve of growth should afford a 

 continuous means of obtaining the rate of growth at all 

 temperatures. I also wished to eliminate from this record 

 the preliminary disturbance caused by sudden change of 

 temperature. 



I was enabled to do this in practice by bringing about a 

 gradual and continuous rise of temperature, instead of the 

 former sudden variations by steps. This was effected by 

 turning the handle of the electrolytic rheostat at a rate so 

 graduated that the rise of temperature within the chamber 

 was uniform. As it was necessary to complete the experiment 

 within not too long a period, I found that a rise of i°C. per 

 2*5 minutes was sufficient to meet the requirements of the case. 

 This means a rise of i° C. in fifteen seconds. An observer 

 watches a delicate thermometer, which is placed in the plant 

 chamber, with his hand on the handle of the rheostat. By 

 means of this, and a chronometer beating seconds, he is able 

 to regulate the uniform rise of temperature with the greatest 

 nicety. Should the rate be too quick, it may be reduced by 

 the slightest turn of the handle towards the increase of 



