1 66 Leaves and their Work 



selves and absorb it ; but, by so doing, they are con- 

 stantly diminishing the amount of the gas in the air 

 immediately surrounding them ; and as, according to 

 the law of their being, gases must mix equally one with 

 the other, more carbon-dioxide flows in to supply the 

 place of that which is absorbed. Streams of the gas 

 are therefore constantly flowing towards each leaf, 

 even when the air is still; when there is wind the 

 whole air is, of course, in motion. 



We have now to see what becomes of the carbon- 

 dioxide when the leaves have taken it up. As has 

 been said, in most plants nearly one-half the dry sub- 

 stance left when the water is removed consists of 

 carbon, of which charcoal is an impure form. Carbon 

 enters into the composition of every animal and vege- 

 table substance, no matter how minute. It is to be 

 found in every part of a plant from the root upwards, 

 but especially in the seed. In the grains which we use 

 as food the quantity of carbon amounts to some forty 

 or fifty per cent, of the whole ; and, though the car- 

 bon compounds are not, like the nitrogenous com- 

 pounds, flesh-formers, they are equally important as 

 fat-formers, and as supplyisg fuel to maintain the heat 

 of our bodies. The carbon of our food is oxidized, 

 burnt, by the oxygen of the air we breathe ; heat is 

 thus produced, and the greater part of the carbon is 

 given back to the air as carbon-dioxide. 



One pound of wheat-flour contains about nine and 

 a half ounces of starch, and starch is a compound of 

 carbon, oxygen and hydrogen ; but it also contains two 

 ounces of gluten — one of the nitrogenous compounds — 

 and half of this is carbon ; and besides these it con- 

 tains smaller quantities of sugar, gum and fat, and 



