BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 105 



of the disk as foci; while the lines of force along which an elec- 

 trified particle would move to the disk lie in the surfaces of 

 hyperboloids which cut the ellipsoids at right angles and have 

 their foci in the edges of the disk. 



Consider the electrified disk to be replaced by a liquid film 

 which absorbs carbon-dioxide perfectly and which is surrounded 

 by still air. The ellipsoidal shells now become surfaces repre- 

 senting uniform concentrations of carbon-dioxide, and the 

 hyperboloidal shells represent surfaces along which the carbon- 

 dioxide molecules are streaming to the disk. It is evident that 

 the rate of diffusion through the absorbing surface is far greater 

 than for an equal area in free space; and if the electrostatic 

 analogy holds, the diffusion in unit time would be proportional 

 to the diameter of the disk, which is in accord with Brown and 

 Escombe's results. 



We pass now to the consideration of multi-perforate septa. 

 It is evident that if the small openings through the septum were 

 sufficiently close together, one aperture would modify the stream 

 lines through another, and the diameter law would not hold. 

 This matter was experimentally investigated by Brown and 

 Escombe, using septa perforated with holes 0.38 nun. in diameter. 

 They concluded that when the distance between the apertures 

 is not less than ten times the diameter of an aperture, there is 

 no interference in the diffusion through neighboring holes. 



It remains to be seen how closely the rate of diffusion of car- 

 bon-dioxide into the leaf of a plant may be calculated from the 

 diameter law, when the size and number of the stomatal openings 

 are known. Such calculations as a matter of fact lead to values 

 very much in excess of the actual quantity of carbon-dioxide 

 taken in by a leaf, even when it is assimilating most actively. 

 The maximum assimilation of carbon-dioxide by sunflower so 

 far recorded amounts to only 5 to 6 per cent of the possible as- 

 similation according to theory. This lack of agreement is 

 ascribed by Brown and Escombe to the fact that the cells in the 

 stomatal cavity are not perfect absorbers of carbon-dioxide ; in 

 other words that the partial pressure of carbon-dioxide at the 

 surface of these cells is not reduced to zero as assumed in the 



