12 THE NATURE-STUDY OF PLANTS 
II. NutTRITION 
The subject of vegetable nutrition is one of 
surpassing interest, but in this book it cannot be 
explained in detail or allowed more than an illustration 
or two. 
I may remind the reader that our own food, or, 
more accurately, the nutriment which it contains, 
consists of what are known as carbohydrates and 
proteids. The former, among which are sugar and 
starch, are built up of carbon, hydrogen, and oxygen, 
the last two elements being in the same proportion 
as in water—that is to say, there is twice as much 
hydrogen as oxygen. The proteids are very much 
more complex; the water relation between the 
hydrogen and oxygen that they contain is not respected, 
and in addition to carbon we find nitrogen, sulphur, 
and other elements necessary to adequate nutrition, 
such as phosphorus. 
Carbohydrates and proteids, as we might expect 
from their different composition, do not serve the 
same purpose in the economy of the human or animal 
body; the former are used up in respiration for 
keeping us warm and supplying us with energy, while 
the latter go to replace the wear and tear of life and 
its activities. It is by them that our life-substance 
is nourished and our bones, muscles, and brains 
formed and kept in good working order. Now, as a 
matter of fact, plant food consists, like our own, of 
carbohydrates and proteids, and they are utilized 
for the nutrition of the plant body in a similar way. 
How plants manage to get the food is a question 
of which I can only touch the fringe; but there is 
this great difference between them and us—we eat 
