XII. 



LEAVES AND THEIR WORK 



All the plant-food which we have yet considered is 

 drawn from the soil, with the exception of- the minute 

 quantity of ammonia taken in by the leaves from the 

 air. But this latter is far from being all that the 

 leaves contribute to the food-supply. 



The roots furnish food from the soil — mineral and 

 organic matter ; and the leaves furnish food from the 

 air — the carbon, which makes up about half the dry 

 weight of a plant — half its weight, that is, when all the 

 water has been removed from it. Roots and leaves, 

 therefore, supply about an equal amount of food. 



But the leaves do more than merely supply food; 

 they prepare it for the whole plant, both that which 

 they themselves take up, and that which is procured 

 by the roots. Leaves are the food-manufacturers; 

 and it is they which combine the various materials, 

 and distribute food to the several parts. 



Nitrogen, the food derived from organic matter, 

 which we have last considered, is needed in some 

 combination or other by all parts of a plant, but 

 especially by the seed. No one needs to be told 

 that grain is more nourishing than straw; but the 



