CHAPTER VII. 



FOLIAGE. 



Guarda il calor del sol che si fa vino 

 Giunto all 'umor che dalla vite cola ; 



Dante, Piirgatorio^ XXV. 'j'j. 



From the lines that are here quoted Dante appears to 

 have grasped more or less distinctly a very important 

 botanical fact. It is the sunlight, which affords not 

 merely the energy of wine, but the motive power which 

 carries on the life of plants, and of all the animal world. 

 The leaves are intended to absorb the light (or, strictly 

 speaking, certain rays, the red, orange, yellow, and part 

 of the blue rays of the spectrum) ; and, this absorbed 

 light is utilised by the plant, as a supply of energy with 

 which to build up the complex substances on which life 

 depends. 



It is necessary to begin this chapter by a short 

 explanation of the whole process ; because it is impos- 

 sible to appreciate the beautiful arrangements which 

 exist in every detail of leaf shape, without a knowledge 

 of the object for which all foliage exists. 



Leaves are intended to intercept as much sunlight as 

 possible, and also to obtain, from the air, the carbon 

 which is necessary for the plant food. By the aid of 

 this intercepted light, the leaf manufactures sugar, by 

 combining the carbonic acid gas of the atmosphere with 

 the water which is supplied to it from the root. Very little 



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