TRANSPIRATION. IOQ 



temperature of the air which surrounds the transpiring organs, 

 but also by the temperature of the medium in which the 

 absorbent organs are situated. Sachs observed that when the 

 soil around the roots of a plant is warmed, the transpiration 

 of the plant becomes more active. We have learnt in a 

 previous lecture (p. 52) that a slow rise in the temperature of 

 the soil causes an increased absorption of water by the roots : 

 we may therefore conclude that the more active transpiration 

 which is induced by a rise in the temperature of the soil is 

 connected with the increased absorbent activity of the roots. 



From the observations of Burgerstein on Taxus baccata it appears 

 that transpiration may take place to a slight extent even at a tempera- 

 ture of - 107 C. 



It has been found by a great number of observers that 

 transpiration is more active in the light than in darkness. 

 This is doubtless to be accounted for, to a certain extent at 

 any rate, by the fact that, as we have seen, the stomata are 

 open in the light and closed in the dark, and further, that 

 exposure to light usually involves a rise of temperature. 



Wiesner has endeavoured to shew by a series of experiments, con- 

 ducted in such a way that the temperature and the degree of saturation of 

 the air were maintained as nearly as possible constant, that light affects 

 transpiration independently of the stomata. Transpiration is very much 

 increased by exposure of the plant to light, and the rays which are 

 especially active appear to be those which correspond to the absorption- 

 bands of the chlorophyll-spectrum (see plate). 



The following are some of Wiesner's figures : 



Light (Gas) Darkness 



Hartwegia coinosa 



gave off during ist hour 59 mgr. water 31 mgr. 



2nd 48 30 



3rd 44 29 



4th 42 29 



In both cases the loss of water remained constant to the end of the 

 experiment. 



The following table gives Wiesner's results for a number of the plants, 

 calculated per hour for 100 sq. centim. of leaf-surface. 



