SCIENCE IN THE PLANT WORLD 
Plants have paid particular attention to the 
manipulation of gases. They maintain an in- 
ternal atmosphere of their own composed of 
oxygen, nitrogen and carbon dioxide in propor- 
tions varying greatly from those of the outside 
air. If the stem of a Water Lily be broken be- 
low the surface of a pond, gas bubbles will often 
be observed to issue from the wound, indicating 
that the internal gas pressure of this particular 
plant is greater than that of the external air. In 
other cases, the reverse is true and we find par- 
tial vacuums within the bodies of plants. 
Man long ago found it impossible to “live 
on air” but the plants have solved the difficulty 
of aerial existence and have become creatures 
of the air rather than the earth, so far as their 
food is concerned. The great bulk of the largest 
tree is preponderantly composed of carbon, 
which has been slowly and labouriously ex- 
tracted from the air. The mineral salts and water 
which have been filtered out of the ground by 
the roots are essential but are present in a much 
lesser quantity. 
It is well known that plants breathe in carbon 
dioxide and breathe out oxygen. This can be 
[133] 
