202 FACTORS AFFECTING TRANSPIRATION 



Atmospheric Pressure. — It has been demonstrated experimentally that 

 a reduction in atmospheric pressure results in an increase in the rate of 

 transpiration (Sampson and Allen, 1909). This result would be predicted 

 theoretically since reduction in the density of the atmosphere would be ex- 

 pected to permit diffusion of w^ater-vapor to occur into it more rapidly. In 

 any given locality variations in atmospheric pressure are too slight to have 

 any significant effect upon the rates of transpiration. Plants growing at 

 high altitudes are subjected to distinctly lower atmospheric pressures than 

 the plants of lowlands, and in comparative evaluations of the transpiration 

 rates of species growing in these two types of habitats the influence of this 

 factor must be considered. 



Structural Features of Plants which Influence the Rate of Transpira- 

 tion. — A number of diverse species of plants growing under identical envi- 

 ronmental conditions usually differ very greatly in transpiration rates. Physio- 

 logical factors, such as differences in stomatal behavior, cell sap concentration, 

 colloid content of the cells, etc. are undoubtedly partially responsible for the 

 fact that all plants do not lose water-vapor at the same rate even under the 

 same environmental conditions. Structural differences, particularly in the 

 leaves, also account in part for unlike rates of transpiration of different 

 species growing in the same environment. Many of the supposed effects 

 of structural differences in plants upon their transpiration rates are, however, 

 based solely on inferences, and not upon experimentally determined facts. 

 Many species which on the basis of their anatomy have been judged to have 

 a low transpiration rate later were actually found to transpire very rapidly 

 when environmental conditions were suitable. The following discussion of 

 the anatomical features known or generally believed to influence the rate of 

 transpiration will therefore be very brief. 



1. Thickness of the Cutin. — It can be easily demonstrated that water 

 evaporates more readily from uncutinized epidermal cell walls than from those 

 which are coated with a layer of cutin. It is sometimes assumed, therefore, 

 that the thicker the layer of cutin on the outer walls of the epidermis of a 

 leaf, the slower the rate of cuticular transpiration from that leaf. Actually, 

 however, it seems obvious that any increased thickness in the cutin layer be- 

 yond that at which cuticular transpiration becomes negligible will have no 

 appreciable influence on the rate of evaporation from the epidermal cell walls. 



2. Epidermal Hairs. — Hairs are prominent features of one or both of the 

 surfaces of the leaves of many species. Living hairs probably increase the 

 rate of cuticular transpiration by increasing the exposed surface of the leaf. 

 Many botanists have assumed that dead hairs reduce the rate of stomatal 

 transpiration, particularly under conditions of intense sunlight or strong 



