912 CONCENTRATION FACTORS CHAP. 27 



area is less than 1 mm.^ and the concentration drop is not more (and often 

 less) than 1 X 10~'^ mole/1, (which is the normal C(\ concentration in the 

 open air)? The second question was : Granted a remarkably low diffusion 

 resistance of the stomata, is this resistance nevertheless an important 

 "limiting factor" in photosynthesis of higher plants, particularly at low 

 carbon dioxide concentrations? 



To understand why the first question had to be asked, suffice it to recall 

 the experiment of Brown and Escombe (1900), who showed that a leaf 

 takes up carbon dioxide from quiet air almost as rapidly as an eciually large 

 surface of an alkali solution! It Avas soon found that this unexpectedly 

 high rate of diffusion has nothing to do with the physiological properties of 

 the leaf but is a general property of multiperf orate septa, i.e., barriers con- 

 taining many small openings. Model experiments on transpiration showed 

 {cf. Sierp and Seybold 1929, 1930) that the rate of evaporation from a ves- 



Fig. 27.7. Stoma of Hellehorus sp. in transverse section. Darker lines show shape 

 assumed by guard cells when stoma is open; lighter lines when stoma is closed (from 

 Strassburger et al, after Schwendener). In closed state, vacuole (shaded area) con- 

 tracts because of water loss caused by decreased turgor (produced by polymerization 

 of sugars). 



sel, covered with a septum, can be as high as three fourths of that from an 

 equally large open vessel— even if the aggregate area of the holes is less 

 than 1% of the total surface of the liquid! The theoretical solution of this 

 apparent paradox was given (for the case of evaporation) as early as 1881 

 by the Austrian physicist Stefan. He used, for this purpose, the formal 

 similarity of the equations describing the diffusion flow of matter from 

 an extended surface and from a point source, with the equations de- 

 scribing the hues of force in the electrostatic field in front of an extended 

 conducting surface, and around a small conductor. In this formal 

 analogy, the diffusion flow corresponds to the electrostatic capacity of 

 the conductor; and it is known that the capacity of a large flat condenser 

 is determined by the area of its plates, while the capacity of a small spherical 

 conductor is determined by its radius. In the same way, the amount of 



