144 VEGETABLE PHYSIOLOGY 
exists in the atmosphere in very small amount, not quite 
four parts in ten thousand being normally present. The 
very large green surface which an ordinary terrestrial plant 
possesses renders, however, a considerable amount of 
absorption possible. Brown and Escombe showed in 1900 
that it is possible for carbon dioxide, though present in the 
air in such minute quantities, to enter a leaf through such 
small apertures as stomata in quantities so great that a 
sunflower is able to manufacture 1:8 grain of carbohydrate 
per square metre per hour. In a series of experiments on 
the passage of carbon dioxide anes diaphragms pierced 
<2 o = 2 eae: 
BARRE 5 
ob 7 
5 4A8 fo AgAvsh 
on 3 r ie, 
gee o 2 
° 
Fic. 83.—TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING THE 
DIFFERENT ARRANGEMENT OF THE MESOPHYLL ON THE Two SipEs. x 100. 
by minute apertures, they found that, if the latter be suffi- 
ciently small, diffusion takes place through them as rapidly 
ag if there were no separating partition at all. If the general 
conditions are favourable, the absorption is continuous, 
for carbon dioxide is at once decomposed or made to enter 
into some form of combination in the cells of the green 
tissues, and so a stream is always entering. 
Both nitrogen and oxygen are soluble in water, though 
to a different extent. It has already been stated that 
the nitrogen so taken in is not used in the constructive 
processes, and accordingly a mere trace is absorbed in this 
way. A larger amount of oxygen enters, but experiments 
have proved that it is not used for the manufacture of 
nutritive substances, being applied to other purposes. 
