106 THE LIFE OF A TREE. 
all, the most important food of plants. Some 
pages back, in our home analysis of a seed, we 
saw that it contained a sticky substance, which 
we could wash out of a piece of dough by means 
of water; this substance was termed gluten. It 
contains a large quantity of the element which 
chemists call nitrogen. Now where did this ni- 
trogen come from ? and how did the plant obtain 
it? The answer will come very quickly, perhaps, 
into some minds. Why, the air is composed of 
oxygen and nitrogen, therefore it was obtained 
from the nitrogen of air. This ingenious reply, 
however, will not avail us; for it has been posi- 
tively ascertained that it does not come from the 
nitrogen composing the air. 
Perhaps there may be a little difficulty in 
comprehending the exact nature of the correct 
answer; but we shall endeavour to render it as 
intelligible as possible. Beneath the spout in 
yonder farm-yard is a wooden butt, into which 
the rain, collected from the roof of the farm- 
house, is conveyed, and is there kept for different 
purposes. Let us remember, all the water this 
butt contains comes from the clouds,—and as it 
dropped through the air, naturally would dissolve 
