ABSORPTION OF FOOD MATERIALS 145 
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 
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. 
We have seen that most of the water absorbed by the 
roots is conveyed regularly through the axis of the plant 
until it reaches the leaves, in which, after traversing the 
cells of the mesophyll, it is evaporated into the intercellular 
spaces. Into these cells of the interior of the leaf, all the 
materials used in the construction of food are thus at once 
transported, both those entering the tissues from the soil,. 
and those absorbed from the air. These mesophyll cells 
have generally a different arrangement on the two sides of 
the leaf (fig. 83), but they all agree in containing chloro- 
plasts. In them takes place the work of construction 
of organic nutritive substance, such as the plant can live 
upon—work which is carried out mainly through the 
instrumentality of the chloroplasts. 
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