NATURE OF GASEOUS INTERCHANGE 433 



system may thus be said to owe its origin to the influence of the 

 fundamental principle of maximum exposure of surface. 



The manner in which a living cell absorbs or liberates gaseous 

 substances is always the same, whether the cell in question is situated 

 in the interior of the plant body in contact with an air-space, or 

 whether it lies at the surface, and is consequently directly exposed to 

 the external atmosphere. In either case, the gaseous substances have to 

 pass through membranes saturated with water of imbibition. In other 

 words, gaseous interchange is essentially an osmotic process. In view 

 of the fact, however, that we are regarding the ventilating system as 

 an integral part of the plant- body, it will be necessary to consider 

 gaseous interchange as a function of the whole plant rather than of the 

 individual cell. 



From this point of view we may where the ventilating system is 

 provided with special outlets or pneumathodes distinguish between 

 osmotic gas-exchange on the one hand, and direct intermingling of gases 

 on the other. 207 The latter process generally depends wholly upon 

 diffusion, as, for example, in the case of the gas-exchange attendant 

 upon respiration and photosynthesis. It may, however, in part consist 

 of a movement en masse, as, for instance, when variations of temperature 

 or pressure occur in the outer atmosphere, or when the intercellular 

 spaces expand or contract suddenly as the plant sways in the wind. 

 These statements apply equally to movements of gases from one part 

 of the ventilating system to another. 



It was at one time generally assumed, in view of experimental 

 results obtained by Boussingault, Mangin and others, that the gaseous 

 interchange of leaves and other aerial organs, was not carried on solely 

 through the intercellular spaces and pneumathodes, but that cutinised 

 cell-walls were pervious to carbon dioxide and oxygen, and conse- 

 quently that diffusion of these gases through the epidermal walls 

 played an essential part in the normal processes of gaseous exchange. 

 More recent experiments, performed by Blackman and by Stahl, have 

 shown that, under natural conditions, this cuticular diffusion is practi- 

 cally negligible, so far as photosynthetic gas-exchange is concerned and 

 the same statement probably applies to respiration in comparison with 

 the stomatic diffusion which goes on through the pneumathodes. The 

 normal condition in this respect is, in fact, closely comparable to the 

 relation between cuticular and stomatic transpiration, which has 

 already been discussed at some length (p. 105). The modern view thus 

 attributes a greater physiological importance to the ventilating system, 

 and to the pneumathodes associated therewith, than the early physi- 

 ologists were disposed to concede, at any rate, in the case of aerial 

 organs. 



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