128 St'ifi'- Diffusion of Gases am/ Li<jtiiils, &c., in 7V////A*. 



plate is not much reduced by cutting most of it away, so also is it 

 possible to block out a large portion of the cross-section of the diffusing 

 column without materially altering the general static conditions on 

 which the flow depends. 



The importance of these results in relation to diffusion through 

 porous septa is next considered, diffusion through a thin porous 

 septum being only an extreme case of free diffusion through a multi- 

 perforate diaphragm, whose apertures are so far reduced in size as to 

 materially interfere with the mass movement of the diffusing substance. 



A section of the paper is devoted to the application of these new 

 oteervations to the processes of gaseous and liquid diffusion in living 

 plants, and it is pointed out that the structure of a typical herbaceous 

 leaf illustrates in a striking manner all the physical properties of a 

 multiperforate septum. Regarded from this point of view it is shown 

 that the stomatic openings and their adjuncts constitute even a more 

 perfect piece of mechanism than is required for the supply of carbon 

 dioxide for the physiological needs of the plant, and instead of ex- 

 pressing surprise at the comparatively large amount of the gas which 

 .an assimilating leaf can take in from the air, we must in future rather 

 wonder that the intake is not greater than it actually is. 



From data afforded by actual measurements of the various parts of 

 the stomatal apparatus of the sunflower it is shown that an extremely 

 .small difference of tension of the carbon dioxide within the leaf, as 

 compared with that in the outer air, will produce a gradient sufficient 

 to account for the observed intake during the most active assimilation. 



It is also shown that the large amounts of water-vapour which pass 

 out of the leaf by transpiration are well within the limits of diffusion, 

 and that it is unnecessary to assume anything like mass movement in 

 the outcoming vapour. 



The translocation of solid material from cell to cell in the living 

 plant is next considered, especially with reference to this transference, 

 being, at any rate in part, brought about by means of the minute 

 openings in the cell-walls through which the connecting threads of 

 protoplasm pass. Notwithstanding the very small relative sectional 

 area of these perforations, they probably exercise an important function 

 in cell-to-cell diffusion, in virtue of their properties as multiperforate 

 septa. 



There are two appendices to the paper, one in which a full descrip- 

 tion is given of a series of experiments on the absorption of carbon 

 dioxide by solutions of caustic alkali from air in movement; the 

 second being devoted to a detailed description of the methods used for 

 accurately determining the carbon dioxide absorbed 



