ABSORPTION OF FOOD MATERIALS 



139 



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. 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 



FIG. 83. TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING THE 

 DIFFERENT ARRANGEMENT OF THE MESOPHYLL ON THE TWO SIDES. X 100. 



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. 



The absorption of carbon dioxide takes place usually 

 at the ordinary atmospheric pressure. In some parts of 

 the internal reservoirs it exists at a slightly higher pressure, 

 in consequence of a local production in the tissues. Plants 

 can, however, absorb this gas when it is present in much 

 larger quantities than it is in air. Too much, however, is 

 possible, and then the cells are unable to take it in at all. 



The continuous absorption of carbon dioxide is possible 

 only under certain conditions ; the cells which contain 

 chloroplasts are the only ones which can take it in any 



