78 LIGHT, VEGETATION AND CHLOROPHYLL 



plant life. In the near ultra-violet, we should expect an increase 

 in the chemical activity of photons of more and more con- 

 centrated energy, but the plant seems to defend itself from 

 this action by confronting them with an opaque epidermis. 

 In the more extreme ultra-violet, the chemical activity becomes 

 so great that the photons destroy the molecules of the 

 epidermal cells and cause injury to the plant. 



If we compare these actions, and their character, which 

 may be beneficial to a greater or less extent to the health 

 of the plant, with the composition of sunhght in the corre- 

 sponding spectral regions, we cannot but admire the perfect 

 adaptation of the plant to the radiation which is offered to it. 

 The essential visible is the most abundant in solar radiation, 

 which has its maximum energy in that region; the infra-red, 

 which ensures transpiration, the circulation of water and 

 the provision of indispensable mineral salts, is also quite 

 abundant; the near ultra-violet, whose use is doubtful, is 

 present in a proportion of about only 1 per cent; the extreme 

 ultra-violet, which is mortally injurious, is eliminated exactly 

 at the limit imposed by the atmospheric ozone. And this limit 

 is such that it still permits the passage of certain radiations 

 of the near ultra-violet which, if they are perhaps useless 

 to the plant, are vitally necessary for men and animals. 



The part played by visible Hght, of wave-lengths between 

 4,000 A and 7,500 A, is certainly more complex than that of 

 all the other radiations. It has also been the most studied, if 

 only because the visible quahty of this fight makes it more 

 interesting and easier to use for experiment. 



Three phenomena, in which fight plays a particularly 

 active part, are so important that special chapters have been 

 devoted to them. They are: 



1. Photosynthesis, or assimilation through chlorophyfi. 

 ChlorophyU, the substance in plants which gives them their 

 green colour, has the property of absorbing light and trans- 

 forming the radiant energy thus captured into chemical 

 energy. This energy is used to reduce carbon dioxide and 

 water, fiberating a part of the oxygen and making the carbon 



