Leaves and Their Work 133 



nutritious as the cheese of milk; or that it makes no dif- 

 ference whether one dines on cauhfiower or beef. 



All plants do not contain an equal amount of the ni- 

 trogenous compounds; and even the same plant con- 

 tains very different quantities in different parts, and also 

 at different stages in its life. 



Leaves and stalks are less nutritious than seed, and 

 the seed itself is more nutritious when it is ripe, as it 

 is then that it contains the largest amount of nitrogen. 

 Ripe ears of maize, for instance, contain ten times as 

 much nitrogen as green ears; but even then they contain 

 less than either rye, oats, or wheat, and less than half the 

 amount contained in peas, beans, or lentils. Lentils, 

 indeed, are among the most valuable of the seeds used as 

 food, for nearly a fourth part of their substance consists 

 of albuminous, or nitrogenous, compounds. As for pota- 

 toes, they are very low down in the scale of food, for they 

 are chiefly water, and the amount of flesh-forming food 

 which they yield is only two parts in a hundred, less, that 

 is, than meadow-grass before it has blossomed. 



We must now look a little more closely at the work 

 done by the leaves, for it is they, as has been said, which 

 supply the plant with carbon. Carbon is wanted for the 

 nitrogenous compounds; carbon is wanted for the plant's 

 skeleton, and for its wood; carbon is wanted for the 

 manufacture also of starch, gum, sugar, oils, acids, and 

 the various aromatic compounds to which plants and flow- 

 ers owe their fragrance. 



And this carbon the leaves have no difficulty in provid- 

 ing, so long as the roots do their part; but if they fail, 

 the leaves must fail, too. For the plant is a whole, a 



