THE FERN PARADISE. 
nation of the oxygen of the air with the carbon 
of the blood, and is called carbonic acid gas. 
Were a process of accumulation of this deleterious 
gas to continue, the equilibrium which makes 
animal life possible would be destroyed, and that 
life would cease to exist. But the great plant 
kingdom requires for a chief part of its suste- 
nance the very gas which the animal world 
rejects. The primary food of plants is carbon, 
and the necessary supply is obtained almost 
wholly from the carbonic acid gas of the atmos- 
phere. When it is remembered that half of the 
weight of our forest trees, for instance, consists 
of carbon, and that the whole of their supply is 
derived from the carbonic acid gas of the air, it 
will be recognized to what a large extent the 
atmosphere is concerned in the manufacture of 
wood. It is not, however, trees alone, but shrubs, 
flowering plants, Ferns, mosses, grasses — indeed 
all vegetation, which relies upon the atmosphere, 
with its freight of carbonic acid gas, for its supply 
of carbon — the material which, furnishing the 
bulk of their substance to plants, gives to them 
their strength and stability. 
204 
