184 THE MECHANISM OF GASEOUS EXCHANGE 



fatty material that the permeability of the cork and cuticle decreases to 

 a relatively less extent on drying, but it has not as yet been determined 

 what are the means by which lignified walls attain similar properties. 



Except in the layers of bark, dry cell-walls hardly ever occur in 

 the living plant. Cell-walls are, however, not always in a condition of 

 maximal saturation, but at the commencement of withering, the amount 

 of water present even in the walls bounding the aeriferous canals is not 

 sufficiently reduced to perceptibly affect the diosmotic gaseous exchange 

 (cf. Sect. 12 and Chap. VI). It is also doubtful whether the already small 

 amount of water which the cuticular layers of the living epidermis contain, 

 is temporarily still further reduced to any great extent when transpiration is 

 very active. In any case, such changes are more likely to be concerned 

 in the regulation of transpiration than of gaseous exchange, and the same 

 importance probably attaches to the drying of gelatinous investments. 

 Gases and water-vapour pass but slowly through the bark, provided no 

 open pores are present, while as shown by mosses, if no protecting corky 

 or cuticular layers are present the influence of the decreased amount of 

 water held by the walls does not suffice under normal conditions to prevent 

 further and complete drying, although the last traces of water are held 

 with great tenacity. Walls saturated with water, which alone need now be 

 considered, behave just as a moist gelatine layer or fixed film of water 

 would do. This is also the case when the water is partly or entirely 

 replaced by fatty substances, though here, as in the case of an india-rubber 

 film, only gases soluble in the water of imbibition or in the substance of 

 the wall can pass through it. The main principles which regulate the 

 diosmosis of dissolved substances in general apply also to this diosmotic 

 transference, and determine the positive or negative osmotic pressures due 

 to the presence or absence of particular gases. An increased partial 

 pressure causes an increased absorption by the saturated walls, and hence 

 acts similarly to an increase in the concentration of a watery solution. The 

 same change occurs when the partial pressure is raised by mechanical com- 

 pression of the external air, except that the increased external mechanical 

 pressure must be taken into consideration, as is also necessary in dealing 

 with turgid pressures. It is clear, however, that no additional channels 

 for gaseous absorption are opened by such mechanical pressure, nor are 

 any open passages formed by driving out the fluid filling the micellar 

 interstices of the cell-wall. Hence a diosmosing gas will be transferred in 

 a given direction only when its density or partial pressure is greater on one 

 side of a membrane than on the other, no matter whether the difference 

 is due to the presence of a higher percentage of the gas on that side or 

 to mechanical compression l . When the passage of the gas is sufficiently 



1 YViesner apparently (I.e.) supposes a difference to exist between an increased partial pressure 

 produced by a different admixture of the component gases and that caused by compression. This is 



