78 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



to pass through by diffusion alone. The authors showed that no 

 theory of streaming currents into the leaf or from it was needed. 

 Lloyd (1908) apparently was the first to point out that since this 

 rate of diffusion is much greater than the rate of transpiration, the 

 gas inside the leaf can not be fully saturated: 



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

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

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

 It seems clear from Brown and Escombe's experiment 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 con- 

 stant. As in the case of CO2, 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." 



In this Lloyd summed up the entire problem of the water -loss 

 from a plant-leaf. 



The problem of stomatal regulation is therefore complicated by 

 several conditions, the chief of which is the changing vapor-pressure 

 of the air-spaces under different conditions of stomatal opening, 

 relative humidity, wind, radiant energy, and air-temperature. Tre- 

 lease and Livingston (1916) believe that changes in the chloren- 

 chyma reduce transpiration before stomatal closure occurs. As this 

 belief is based upon the "relative transpiration" calculated from the 

 evaporation-rate of the porous-cup atmometer, it has not been 

 demonstrated conclusively. Nevertheless, it is probable that this 

 condition also enters into the problem and adds its complications to 

 it. The problem of stomatal regulation of transpiration, therefore, 

 can not be confined to the thesis that small changes of apertures 

 produce like changes in the absolute rate of transpiration. Stomata 

 must be considered definitely regulatory, if changes in their openings 

 produce similar changes in the effectiveness upon the leaf of the 

 evaporating power of the air. 



The problem was attacked from another angle by Muenscher 

 (1915), who compared the water-loss from 9 species of plants with 

 the size and number of their stomata. While the investigation 

 apparently showed no relationship between transpiration and stoma- 

 tal opening, the results are vitiated by the failure to take into account 

 the length of time the stomata of each plant were open. The follow- 

 ing table is taken from his paper. 



TABLE 3. Relation between amount of transpiration and stomatal aperture (from Muenscher). 



