i 4 TRANSPIRATION AND ASCENT OF SAP ch. 



will diffuse into the intercellular spaces more quickly than 

 the heavy gases, and so come into contact with evapora- 

 ting cells more quickly. Besides this, it seems probable 

 that the poisoning and anaesthetic effects of one may be 

 more rapid than those of another. The observations, on 

 which the numbers given above are based, were com- 

 menced in each case after the branch had been surrounded 

 by the gas for five minutes, and were discontinued before 

 any lethal effects could be observed in the leaves ; for such, if 

 arising, would cause the osmotic pressures obtaining in the 

 leaf-cells to become diminished, by rendering the proto- 

 plasmic membranes permeable. These effects were usually 

 visible within 45 minutes after starting the experiment. 

 It is possible that the denser vapour could not, within this 

 time, diffuse into all the intercellular spaces of the leaves. 



An error arising from this possibility is most unsatis- 

 factory, as it seems extremely difficult to make proper 

 allowance for it. It seems impossible, at present, to 

 decide how soon the surrounding gas will come into con- 

 tact with the evaporating cells, and, also, when the 

 anaesthetising or stimulating action will cease, and the 

 lethal effects will begin, if, indeed, there is any sharp line 

 of distinction. 



Next we come to an error which can, in some degree, be 

 eliminated. It is known that the rate of diffusion of a gas 

 will be influenced by the nature of the gas occupying the 

 space into which it is diffusing. Thus water-vapour will 

 diffuse more slowly into carbon dioxide gas than into 

 oxygen. This difference depends on the relative sizes of 

 the molecules of the gases into which the water-vapour 

 has to diffuse. For the same pressure and temperature, 

 there will be the same number of molecules of these gases 

 in the surrounding space ; but if their sizes are different, 

 it is plain that the water-molecules will less readily diffuse 

 into the space occupied by the gas composed of the larger 

 molecules. 



