i6o Leaves and their Work 



in quality must be caused by some slight difference in 

 the soil, which is so slight as to be undiscoverable. 



If, therefore, so slight a difference in the vine's food 

 can make so large a difference in its produce, it seems 

 altogether rash to conclude that the cheese of beans is as 

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

 difference whether one dines on cauliflower or beef. 



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

 genous compounds ; and even the same plant contains 

 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 most 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 con- 

 tain 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, com- 

 pounds. As for potatoes, 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 



