74 CARNEGIE INSTITUTION OF WASHINGTON. 



attain to the zone of soil which is moistened by capillarity from the 

 water-table. This type of root-system, also that which includes super- 

 ficial roots as a prominent feature, is probably to be considered char- 

 acteristic of trees growing in regions where the water-table is deep 

 and the rainfall relatively small. The open character of the forests 

 in the California valleys, by which the individuals are usually 

 widely spaced, is directly related to the far-reaching superficial roots. 

 This condition is comparable to the relation between root character 

 and species distribution to be seen in many desert perennials. 



The Transpiration Behavior of Rain-forest Plants, by Forrest Shreve. 



The elaboration for publication of the transpiration data secured 

 in the tropical montane rain-forest at Cinchona, Jamaica, has yielded 

 conclusions regarding the influence of high humidities on transpira- 

 tion, the relative amounts of stomatal and cuticular transpiration 

 in rain-forest plants, and the influence of stomatal overture on trans- 

 piration, and has made possible a direct comparison of rain-forest 

 and desert transpiration activities, through the use of the method 

 of securing relative transpiration data. 



Although high humidities (90 to 95 per cent) have been found to 

 reduce the absolute rate of transpiration below its amount at rela- 

 tively low humidities (55 to 71 per cent), as is to be expected, the 

 rate of relative transpiration continues to be of the same general 

 order of magnitude at all humidities which are well above the mini- 

 mal point for rain-forest plants. This is equivalent to saying that 

 high humidities cause no physiological response affecting transpira- 

 tion, but reduce its amount only through their reduction of the 

 evaporating power of the atmosphere. 



In plants with hypostomatal leaves the simultaneous measure- 

 ment of transpiration in individuals with leaves coated above, leaves 

 coated below, and leaves not coated has confirmed the early work 

 of Comes to the effect that either surface transpiring alone does so 

 more actively than when transpiring with the other surface. Taking 

 this increase into account, calculations have been made from series 

 of readings such as to give values for the true stomatal transpira- 

 tion and for that of the cuticular surface of both the top and bottom 

 of the leaf. The true stomatal transpiration is thus found to be 

 from 42 to 48 per cent of the total water-loss of the leaf. The close 

 relation of transpirational behavior to evaporation is thus shown to 

 have its basis in the fact that rather more than half of the water-loss 

 of the plant goes on through the epidermal surfaces, through which 

 no marked physiological control of water-loss is possible under ordi- 

 nary conditions of humidity and water-supply. 



The amplitude of stomatal movement in rain-forest plants under 

 shade conditions has been found to be relativel}" small, the ratio of 



