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 through diaphragms pierced 



FIG. 83. TRANSVERSE SECTION OF THE BLADE OP A LEAP, SHOWING THE 

 DIFFERENT ARRANGEMENT OF THE MESOPHYLL ON THE Two SIDES, x 100. 



by minute apertures, they found that, if the latter be suffi- 

 ciently small, diffusion takes place through them as rapidly 

 as 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. 



