III. THE EFFECT OF STOMATAL MOVEMENT UPON 



TRANSPIRATION. 



Indirect evidence that stomatal movement affects the water-loss 

 of plants had gradually accumulated during the course of the experi- 

 ments. This evidence, however, was not conclusive and in some 

 instances was capable of several interpretations. For this reason a 

 number of experiments were made to discover the relationship 

 between stomatal movement and transpiration, and to find some 

 reason for divergence of views concerning the effectiveness of the 

 stomatal regulation of water-loss. It seemed inconceivable that 

 complete closure of the stomata should fail to reduce the rate of 

 transpiration to a considerable extent, as Buscalioni and Polacci (1902) 

 have confirmed the earlier results as to the relative insignificance of 

 cuticular transpiration by showing that a collodion film placed on 

 a leaf clouded quickly just above the stomata only. In the course of 

 several series, plants were often found to wilt visibly during the early 

 forenoon, and the stomata to close, as a consequence of which they 

 recovered turgor by afternoon, when the stomata again opened. The 

 recovery of turgor during the part of the day when evaporation was 

 greatest must be ascribed to closure of the stomata. The commonly 

 accepted view that stomata are not regulatory was not in accord 

 with these facts, but further investigation was imperative to fully 

 explain the divergence of views. 



As a result of the researches of Mohl (1856), Merget (1873), Schwen- 

 dener (1881), Leitgeb (1888), Stahl (1894), Kohl (1895), Darwin 

 (1898), and many others, the mechanism of the stomata was demon- 

 strated, and they were shown to be the outlets for water-vapor 

 and hence of the greatest importance in transpiration. In conse- 

 quence, the view came to be held that the stomata by their move- 

 ments completely controlled the water-loss from the plant, except 

 for the inconsiderable amount evaporated from the cuticle. It was 

 apparently believed that the air-spaces of the leaf contained a 

 water-saturated gas with but little CO 2 and the vapor diffused out 

 through the stomata as the CO 2 passed in. Under such conditions 

 the evaporating power of the air, especially when moving, would 

 have little effect upon transpiration and the stomata would be wholly 

 regulatory. Brown and Escombe (1900), however, showed that the 

 theoretical diffusion of water-vapor from a leaf of Helianthus 

 annuus was about six times as great as that actually lost by the plant. 

 The full significance of this discovery was overlooked, however, in 

 view of the expressed doubt that the stomata were of sufficient size 

 and number to permit such large amounts of CO 2 and water-vapor 



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