56 LIGHT, VEGETATION AND CHLOROPHYLL 



of ten to forty minutes, in full sunlight, the average trans- 

 mission factor can increase by 40 per cent; at the blue- 

 violet end of the spectrum it can increase threefold. This 

 seems to indicate that all the absorbing coloured pigments, 

 and not only chlorophyll, are involved in the movements 

 taking place inside the tissues of the leaf under the influence 

 of Ught. 



The starch produced by photosynthesis and accumulating 

 in the tissues can also reduce the transmission factor. 



All these variations, the diversity of species and the 

 different structures of the same plant when raised in shade or 

 in sunlight make it very difficult to estimate the average 

 factors for reffection, transmission and absorption. 



Thus Schanderl and Kaempfert studied the transmission 

 factor of the epidermis detached from the upper face of 

 various leaves. The differences between shade plants, growing 

 in poor light, and desert or mountain plants, growing in 

 intense light rich in ultra-violet, are enormous and show how 

 the plants are adapted to the conditions of illumination. The 

 transmission factor of the epidermis in the first case may 

 reach 98 per cent, while the thick epidermis of the plants in 

 the second category can transmit only 15 per cent to 20 per 

 cent of the incident flux. Often the colouring matter and 

 resinous material of the epidermis greatly reduce the part of 

 the spectrum of short wave-length, thus protecting the internal 

 fragile tissues against the injurious ultra-violet. 



Utilization of the Light Absorbed 



Some scientists estimate that an average of 80 per cent of 

 the light is absorbed by the leaf, but if we take into account 

 the solar infra-red it appears that this estimate is exaggerated 

 and that only 70 per cent is really absorbed, the remaining 

 30 per cent being transmitted or reflected. 



In any case, the greater part of the direct or diffused solar 

 radiation received by the leaf is absorbed by it and trans- 

 formed into energy of a different kind. What are these trans- 

 formations and for what purpose is the absorbed energy 



