CHAPTER 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 con- 

 tribute 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, sup- 

 ply 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 them- 

 selves 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 nour- 

 ishing than straw; but the reason why it is more nourish- 

 ing is that it contains more nitrogen, in the form of 

 nitrogenous or albuminous compounds, commonly called 

 albuminoids, because they resemble the albumin, or white, 



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