LIMITING FACTORS TO GROWTH MEASUREMENTS. 343 



was usually coincident with the time of exposure to full sun. 



This exposure to full sun was in fact often accompanied in 

 Eranthemum by a patent wilting of the leaves and shoot, 

 which showed under such circumstances a contraction in 

 length. Turgidity was restored when the plant became 

 shaded. This seems to point to the idea that the effect of the 

 intense full sunlight was to increase considerably transpiration 

 and loss of water from the leaves and shoot, and that the 

 shortening of the shoot was due to this loss. 



Experiments carried out with the aid of thermo-electric 

 junctions on the growing region of shoots of Capparis Roxbur- 

 ghii show that in full sun at about 3 p.m. the internal tem- 

 perature of the growing region rises 3° C. above the shade 

 temperature of the surrounding air. Radiation of sufficient 

 intensity to cause such a rise inside the shoot cannot be without 

 a considerable effect in the direction of loss of water from the 

 growing tissues. If this explanation of the facts be the true 

 one there is still no evidence in these cases for the direct re- 

 tarding effect of light upon growth as usually understood. 

 Changes in intensity of light, apart from exposure to full sun. 

 have not in these experiments ever been unaccompanied by 

 changes of humidity, whose effect would act in the same 

 direction, and the effect of each of the two factors has 

 therefore never been distinguishable in the results. 



Hanging roots of Vitis Lindeni. 



We now turn to a case of considerable interest, that of the 

 hanging roots of Vitis Lindeni. The plant is a straggling 

 climber whose thin stems sprawl over other vegetation often 

 to a considerable height and send down large numbers of long 

 thin roots which, on reaching the ground, take root and then 

 begin to increase in thickness, no doubt taking up the function 

 of conduction of water to the stems above. These roots are 

 positively geotropic and are quite sensitive to contact, but 

 exhibit no trace of heliotropism. If unimpeded their course 



