CHAPTER VI 



THE ROLE OF VISIBLE LIGHT 



The Particular Importance of the Visible 



It is in the region of the visible radiations that hght has the 

 most important effects on plants, the chief being the stimu- 

 lation of photosynthesis. By this process the energy of the 

 light is "eaten" by the plant to promote its own growth and 

 to maintain its Ufe and that of every Uving thing. Light also 

 has many other effects on growth, flowering, tuberization, 

 hibernation and the opening of the stomata. 



All these phenomena, which are certainly not due to 

 light alone although it is often of fundamental importance, 

 undoubtedly begin by a photochemical reaction, i.e., a 

 chemical modification caused by hght. 



The most interesting characteristic of the effects of visible 

 radiations is therefore Unked with their power of chemical 

 activation. We know that this activation, which is an excitation 

 of the chemical molecule capable of absorbing light, is a 

 contribution of energy in quantities equal to the energy 

 quantum /zv, which is proportional to the frequency of the 

 luminous vibration. 



This quantum, or photon, transports an increasing amount 

 of energy as we go from the infra-red towards the ultra- 

 violet, passing through the visible. In the infra-red, the quanta 

 are individually weak but numerous ; they make the molecules 

 vibrate without chemically activating them, heat them up 

 and provoke the evaporation of water. In the visible, the 

 heating up and the evaporation are produced in the same 

 way, but in addition the quanta are now capable of chemical 

 actions, which become extremely irnportant; the visible 

 radiations are unique in being absolutely indispensable to 



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