178 THE MECHANISM OF GASEOUS EXCHANGE 



The same general principles are of importance in gaseous exchange as 

 in the exchange of other substances, and hence attention has already been 

 incidentally paid to gases as well as to solids and liquids (Chap. IV). It 

 has moreover been shown that carbon dioxide and oxygen, as well as nitrogen 

 and other gases, can pass with relative ease through cell-walls saturated 

 with water, and can also diosmose through the protoplast within. 



The readiness with which air currents are induced usually ensures 

 a sufficiently rapid removal of gaseous products and a continual presentation 

 of fresh supplies, so that an assimilating leaf may in a relatively short 

 period of time obtain possession of all the carbon present as carbon dioxide 

 in a large volume of air (Sect. 57). No doubt similar streaming and ad- 

 mixing currents tend to maintain uniformity of composition in the air of 

 the aeriferous system, and thus enable localized differences to be evenly 

 distributed and continually readjusted. The special peculiarities developed 

 in connexion with the power of gaseous exchange cannot be mentioned in 

 detail, but a general idea of the activity of individual organs in gaseous 

 interchanges may be obtained by the consideration of the general factors 

 regulating exchange of all kinds (Sects. 25-27), allowing for the fact that 

 gases are especially favoured in so far as they can be absorbed not only 

 by sub-aerial organs, but also by those immersed in water. As a matter of 

 fact, an aquatic plant may be able to obtain all the gases it requires when 

 it is in a moist atmosphere, and many amphibious aquatic plants are 

 compelled, according to circumstances, to live at one time submerged in 

 water, at another time exposed to air. 



The amount of transpiration possible affords on the whole a correct 

 indication of the rapidity with which gaseous exchanges can take place, for 

 both gases and water vapour are largely dependent for entry and exit upon 

 the presence of open channels, as well as upon the nature of the epidermal 

 and other limiting membranes. The permeability of the cell-walls to gases 

 and water-vapour usually rises and falls concomitantly with the permeability 

 to water as such, and hence the cuticles of submerged plants and of moss- 

 leaves allow carbon dioxide, oxygen, &c. to pass through with ease, whereas 

 in terrestrial plants the impregnation of the cuticular membrane with wax 

 produces a marked diminution of permeability to these and other gases. 

 It is, however, an important physical fact that cuticularization appears to 

 render the walls more impermeable to water and water-vapour than to 

 carbon dioxide, oxygen, &c. (Sects. 21 and 30). 



All that has been said is directly applicable to cell-walls infiltrated 

 with water, with which we are mainly concerned in turgid plants. As the 

 cell-wall dries, its permeability to gases decreases, and to a greater relative 

 extent in unaltered cellulose walls saturated with water of imbibition than 

 in strongly cuticularized ones (Sect. 30). Just as is also the case with soft 

 gelatine, no pores filled with air appear in the substance of the cell-wall 



