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 
sare 
Fic, 83.—TRaNSVERSE SECTION ve aie BuAvDE 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 
