34 THE PHYSIOLOGY OF STOMATA. 



The diffusion capacity of the stomata considered as openings with no depth 

 varies directly with the linear dimensions* which are derived, for purposes of 

 computation, by comparing the stomata with circles of equal area. The 

 actual rate of diffusion through stomata will depend upon the length of the 

 tube, the gradation of density of the water-vapor between the surface of the 

 cells of chlorenchyma, and the outer air, modified by air-currents. f 



It seems clear from Brown & Escombe's experiments (c/., p. 259, fig. 6) that 

 there exists such a gradation of density, from which it follows that under 

 conditions of rapid transpiration the vapor-pressure within the leaf -cavities 

 is less than when a low rate occurs, assuming the evaporation capacity of the 

 cells to be constant. As in the case of C0 2 , the pressure of the water-vapor 

 within the leaf, though much greater, must vary with the size of the stomatal 

 pores, but here the relative humidity without is a very variable factor and 

 will therefore modify the rate of transpiration independently of the pores. 



It is also to be noted that the evaporating capacity of the surface of the 

 chlorenchyma cells of the leaf is less than that of the pure water, so that 

 the cell- walls tend to hold back the water. This, combined with a high 

 relative humidity, would produce a high vapor-pressure within the leaf and a 

 low rate of transpiration, even though the stomata are widely open. A low 

 vapor-pressure within ths leaf would follow on a low relative humidity, so 

 that with open or partially open! stomata the rate of transpiration may 

 be high, but because of the incapacity of the cells to give up water-vapor, the 

 capacity of the stomata to allow its escape may not be made full use of. If 

 this argument be sound we should expect that the rate of transpiration will 

 vary independently of the size of the stomatal openings. 



This view is borne out by the facts obtained during the progress of this 

 investigation. As an illustration experiment 54 will serve, the full data of 

 which are given beyond. In this the transpiration rate was found to be, at 

 9 h i5 m a.m., 216 grams, and at n h i5 m a.m. 221 grams per hour per square 

 meter of leaf. At 9 h i5 m a.m., the stomata were open, and measurements 

 showed that the average width to be 6.5 micra and the average length 14 

 micra. The average area, by computation, was 71.66 sq. micra, and the 



*This Brown & Escombe inferred from experiments with openings with diameters 40 to 

 2000 times the diameter of the stomata. 



f Increase beyond a slight movement of air will not make the increase of outward diffusion 

 of water-vapor greater, because this slight movement will be sufficient to remove the density- 

 shells. The ratio between diffusion in still and moving air in a Helianthus leaf is 1 to 1.23 

 (Brown & Escombe, 1905. p. 65). In 1904 experiments were conducted to determine 

 the effect of wind upon the transpiration in ocotillo, with a result which practically sub- 

 stantiates Brown & Escombe's figures. There are no such great differences as are com- 

 monly supposed to occur. (For ratios of diffusive capacity in s+ill and moving air see 

 table 18.) 



JThe diameters of stomata might be reduced to from one-fifteenth to one-twentieth of 

 their length, and still allow a sufficient amount of CO2 to pass for maximum assimilation, 

 provided the absorption was perfect [italicizing mine]. (Brown & Escombe, be. cit.) 



