66 LIGHT, VEGETATION AND CHLOROPHYLL 



Arthur and Stewart also found that the transpiration 

 was still more intense when the same incident energy com- 

 prised not only infra-red but also visible radiation. 



The heat used in vaporization therefore accounts for 

 nearly half of the incident energy. This increase may be due 

 to a stronger absorption of the visible than of the infra-red. 

 It should probably be related to another phenomenon: the 

 opening of the stomata which facilitates the interchange of 

 gases between the tissues of the plant and the atmosphere. 

 The stomata open in visible rays, but they remain closed 

 when the incident radiation comprises only infra-red. It is 

 rather surprising that the opening of the stomata increases 

 the transpiration by only 14 per cent. 



In open-air cultivation in natural daylight, when the 

 visible always accompanies the infra-red, the opening of the 

 stomata coincides with the arrival of the whole range of 

 radiation and the beginning of the phenomenon of trans- 

 piration. It has, in fact, long been known that transpiration 

 in natural conditions is very small at night; it occurs, and 

 passes through a maximum, during the day. 



We may conclude that the preponderant action of infra- 

 red radiation (and the same could be said of the visible) is 

 to provide heat inside the leaf and thus to stimulate the 

 evaporation of water. 



Is this transpiration beneficial? What is its use and in 

 what circumstances can it be harmful? The quantity of water 

 lost by plants in transpiration is very large; an annual plant 

 evaporates several hundreds of times its weight in water. 



This current of liquid is indispensable to good growth. 

 Thick-leaved desert plants, offering a small surface to radiation 

 and adapted to a severe economy of water, show only very 

 poor growth, undoubtedly because photosynthesis is checked 

 through the lack of absorbed energy, and perhaps also 

 because they cannot pump water from the soil in sufficient 

 quantity. 



The simple physical fact that a small, but not negligible, 

 quantity of elements such as calcium, potassium, sulphur, etc., 



