CHAPTER XVIII 



DIFFUSION OF GASES 



They have their exits and their entrances. 



— As You Like It. 



We have seen that, of the various materials which the plant 

 needs to carry on efficiently its various activities, two — oxygen and 

 carbon dioxide — come as gases from the air. The oxygen is neces- 

 sary in the release of energy in the process of respiration, while the 

 carbon dioxide, as has been emphasized already many times, is 

 combined with water to form the carbohydrates, which, in turn, 

 form the basis of the other products synthesized by the plant. 

 During photosynthesis, the oxygen given off is so much greater 



a 



b 



Fig. 9.— Cross section of a leaf (schematic), a, upper epidermis; b, palisade 

 tissue; c, vein; d, spongy parenchyma; e, lower epidermis with stomata. 



than that required for respiration that probably no oxygen enters 

 the cells containing chlorophyll at this time. The other cells 

 must have oxygen continuously and, during periods of darkness, 

 even the chlorophyll-bearing cells must, of course, take in oxygen 

 from the outside. 



Structure of the Leaf .—Plants are built to permit an exchange 

 of gases and to facilitate the free circulation of air with its contained 

 oxygen and carbon dioxide. If we refer to the drawing of the cross 

 section of a leaf (Fig. 9), it will be seen that the cells are in contact 

 with air spaces, which connect with each other and with the stom- 



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