CHAPTER VII 



PHOTOSYNTHESIS 



In this chapter we come to the most important effect, from 

 both the practical and scientific points of view, of Ught on 

 vegetation, namely, photosynthesis or synthesis by light. 

 Briefly, the process is that, through the absorption of luminous 

 energy, green plants are capable of combining water and 

 carbon dioxide and, with the liberation of oxygen, of 

 producing carbohydrates such as starch. 



This synthesis is only one example of the numerous 

 syntheses in the interior of the plant which are promoted 

 or conditioned by Hght, but it is the oldest known and its 

 importance, both by the quantity of the products thus created 

 and by their value, has caused it to be regarded as the supreme 

 example of photosynthesis. 



General Characteristics of Photosynthesis 



It is also called assimilation through chlorophyll; in fact, 

 the assimilation of carbon from the air, of which green plants 

 have almost the exclusive monopoly, can be effected only 

 through the intermediary of chlorophyll. 



But, although chlorophyll is indispensable, it is only an 

 agent which is able to capture and transform luminous energy 

 and to fix it in the form of chemical energy in endothermic 

 compounds; it is therefore powerless in the dark. 



Moreover, its function can be fulfilled only in the living 

 plant. Chlorophyll can be extracted and isolated, but when 

 it is exposed to light in vitro it decomposes and is useless for 

 photosynthesis. 



Aquatic plants find carbon dioxide in solution in the 

 water, but plants growing on land find it in the air, which 



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