ABSOEPTION OF FOOD MATEEIALS 



153 



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. 



The absorption of carbon dioxide takes place usually 

 at the ordinary atmospheric pressure. In some parts of 



FIG. 85. TRANSVERSE SECTION OF THE BLADE OF A LEAF, SHOWING THE 

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



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 in any 

 quantity, and they can only do so when they are exposed to 

 light, preferably that of bright sunshine, and when the plant 

 is maintained at an appropriate temperature. Its absorption- 

 is accompanied by the exhalation of a volume of oxygen 

 which is equal to the volume of the carbon dioxide absorbed, 

 and it is attended by a continuous increase in the weight 

 of the plant. 



