82 LIGHT, VEGETATION AND CHLOROPHYLL 



what the most carefully made direct measurements have 

 shown. 



Ivanoff and Thielmann exposed plants to the filtered 

 radiation of an electric arc and ensured the equality of the 

 energy from the illuminations by means of a thermocouple. 

 They measured the variations, in the weight of water evap- 

 orated in a given time, with the composition of the visible 

 radiation to which the plant was submitted, and found that 

 the blue-violet radiation causes more rapid transpiration 

 than the yellow-red. 



These experiments were made before those of Sierp on 

 the opening of the stomata, but they agree well with his, in 

 spite of the different interpretation that Ivanoff and Thielmann 

 had originally given to their results. 



With equal energy, the blue stimulates a rate of trans- 

 piration exceeding by 20 per cent to 60 per cent that observed 

 in red hght. 



Thus, both the visible radiation and the infra-red, absorbed 

 by the leaf, provide it with a quantity of heat which causes 

 the evaporation of water, but what is pecuhar to the visible, 

 and more particularly to its blue radiations, is the faculty of 

 stimulating the opening of the stomata, by a process which is 

 totally unknown to us. 



This process cannot be purely physical and is not, for 

 example, simply one of heating, for that would not explain 

 the specific character of the action of the blue and, more 

 generally, of the visible radiation. It is probably, at least in 

 origin, photochemical; certain substances in the leaf, capable 

 of absorbing the active radiations, are subjected to a chemical 

 excitation which has the effect of modifying their composition, 

 or of putting them into a state in which they are ready to 

 react with other substances. In this way the opening of the 

 stomata would be produced by a chemical action. 



This is only an hypothesis, but it is supported by analogy 

 with other better known phenomena, also connected with an 

 action of light. It is very probable that the action of light, 

 apart from cases of simple heating, most frequently consists. 



