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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