86 



A MONTANE RAIN-FOEEST. 



values for T at Cinchona if a general equality of the ratios exists for 

 the two regions, as has been shown. In other words, the existence of 

 a general equality of maximum relative transpiration between regions 

 of widely diverse climatic conditions, especially with respect to the 

 evaporating power of the air, indicates that there is a rough relation 

 of equality between the maximum transpiring power of the plants 

 native to these regions and the evaporation conditions by which the 

 regions are characterized. In a comparison, then, of the transpiration 

 capacities of plants found in regions with graduated differences of 

 evaporation conditions, it is possible that we may find the transpiration 

 capacities falling into a parallel series of proportional differences. 



These statements are not at all in harmony with the commonly 

 accepted view that the transpiration of desert plants is low as compared 

 with that of plants in moist regions. As a matter of fact it is the 

 transpiration of rain-forest plants which is low, and the transpiration 

 of desert plants which is high, in terms of unit areas, and (for Cin- 

 chona and Tucson) the rates are roughly proportional to the annual 

 evaporation of the two regions: as 1 is to 10. The question of the 



Table 32. — Influence on transpiration exerted by coating upper or lower leaf surfaces. 



Series run in laboratory, with three individuals of Pilea nigrescens, by weighing method. 

 First group of readings on uncoated plants, second on plants coated as indicated. Leaf areas 

 (top and bottom): A. 221.8 sq. cm.; B, 326.3 sq. cm.; C, 359.9 sq. cm. 



