XL] LEAVES, AND LEAF-DISEASES 249 



sunlight : they take up carbon-dioxide and water, and 

 traces of minerals, and by means of a molecular 

 mechanism which is as yet unexplained in detail, they 

 perform the astonishing feat — for it represents an 

 astonishing transformation when regarded chemically 

 and physically — of tearing asunder, by the aid of the 

 light, the carbon, hydrogen, and oxygen of the carbon- 

 dioxide and water, and rearranging these elements in 

 part so as to form a much more complex body — 

 starch, or an allied compound, oxygen being at the 

 same time set free. 



It is of course not part of my present task to trace 

 these physiological processes in detail, or to bring for- 

 ward the experimental evidence on which our know- 

 ledge of them is based. It must suffice to state that 

 these compounds, starch and allied substances, do not 

 remain in the chlorophyll-corpuscles, but become 

 dissolved and carried away through certain channels 

 in the phloem of the vascular bundles of the venation, 

 and thence pass to wherever they are to be employed 

 as food. The chemical form in which these substances 

 pass from one cell to another in solution is chiefly 

 that of grape-sugar, and it is a comparatively easy 

 observation to make that the cells so often referred to 

 contain such sugar in their sap. 



We are only concerned at present with the fate of 



