164 
THE FAIRY-LAND OF SCIENCE. 
ing the carbon which has just come out of the air, and 
being worked 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, the 
active protoplasm is made of these three added to a 
fourth, nitrogen, and it also contains phosphorus and 
sulphur. 
And so hour after hour and day after day our prim- 
rose goes on pumping up water and ammonia from 
its roots to its leaves, drinking in carbonic acid from 
the air, and using the sun-waves to work them all up 
