160 THE FAIRY-LAND OF SCIENCE. 
then, that from the whitest suostance \\\ plants we 
can get this black carbon ; and in truth, one-half of 
the dry part of every plant is composed of it. 
Now look at my plant again, and tell me if we 
have not already found a curious history ? Fancy 
that you see the water creeping in at the roots, oozing 
up from cell to cell till it reaches the leaves, and there 
meeting the carbon which has just come out of the 
air, and being vorked up with it by the sun-waves 
into starch, or sugar, or oils. 
But meanwhile, how is new protoplasm to be 
formed ? for without this active substance none of the 
work can go on. Here comes into use a lazy gas 
we spoke of in Lecture III. There we thought that 
nitrogen was of no use except to float oxygen in the 
air, but here we shall find it very useful. So far 
as we know, plants cannot take up nitrogen out of 
the air, but they can get it out of the ammonia which 
the water brings in at their roots.* 
Ammonia, you will remember, is a strong-smelling 
gas, made of hydrogen and nitrogen, and which is 
often almost stifling near a manure-heap. When you 
manure a plant you help it to get this ammonia, but 
at any time it gets some from the soil and also from 
the rain-drops which bring it down in the air. Out 
of this ammonia the plant takes the nitrogen and 
works it up with the three elements, carbon, oxygen, 
and hydrogen, to make the substances called albumi- 
noids, which form a large part of the food of the 
plant, and it is these albuminoids which go to make 
protoplasm. You will notice that while the starch 
and other substances are only made of three elements. 
