EFFECT OF INVISIBLE RADIATION AND EMANATIONS 615 



we have seen, in the tissue, so long as the temperature is 

 below the optimum, to give rise later to an acceleration of 

 the rate of growth. Hence, frequent variations of tempera- 

 ture below the optimum will, by reason of these alternate 

 retardations and accelerations, produce little total effect on 

 the rate of growth ; but above the optimum, the stimu- 

 lating action of variation of temperature will retard growth, 

 and as there is here no latent factor, this will not be made 

 up by any subsequent acceleration (p. 461). Hence, the 

 total effect of such variation will be a retardation. This 

 consideration explains the different conclusions to which 

 observers have been led as to the effect of frequent variation 

 of temperature on growth. 



Demonstration of fundamental effect of thermal radia- 

 tion on growth. — In the usual experiments on the effect 

 of thermal radiation on the induction of growth-variation, 

 a difficulty arises in distinguishing between the effect of 

 thermal radiation and that of temperature. In order, then, 

 to determine the fundamental effect of thermal radiation, 

 the experiment must be so arranged that the radiation whose 

 effect is to be observed causes no change of temperature. 

 I have been able to accomplish this by mounting the growing 

 organ in a plant chamber surrounded by a wide heating coil 

 of platinum wire. Between the coil and the specimen there 

 is a cylinder of mica, which is opaque to thermal radiation. 

 By means of a string attached to it from above, this cylinder 

 may be alternately lifted and lowered. The plant is attached 

 to the Crescograph, and a balanced record is taken of its 

 growth when the shield is down, and when, by maintaining a 

 current through the heating coil, the chamber has already 

 been brought to a steady temperature of 34° C. As every- 

 thing inside the chamber has now attained a steady tempera- 

 ture, the movement of the shield up and down will produce 

 no change in this condition. By now raising the mica 

 shield, we can subject the specimen to the action of thermal 

 radiation, which proceeds from the heated spiral wire, with- 

 out producing any variation of temperature. When the 



