S2 A MONTANE RAIN-FOREST. 



curves under discussion show a slighl genera] tendency toward a fall 



of relative rate with rising evaporation, but they fail to show a decided 

 break in the relative Pate, unless the abrupt rise and fall of Pcperomin 

 may be so interpreted. The evidence of these averaged curves is quite 

 different from that of single curves and has the effect of swamping the 

 possible differences of behavior in different individuals. The actual 

 break in the rise of the relative rate may best be sought on the individual 

 curves and is very conclusively shown in the several cases to which 

 attention has already been called. The rarity with which the hourly 

 evaporation rate for Cinchona rises above 16 mg. per hour may be 

 inferred from the small number of readings in table 30 above that 

 amount. The curves (in fig. 12) afford some evidence that the physio- 

 logical controls which are operative in lowering the relative rate may 

 have operated during the rise of the evaporation from about 5 mg. 

 per hour to 16 mg., w r hile the further rises in relative rate shown for 

 Pilea under 23 mg. of evaporation and for Diplazium under 20, 23, 

 and 25 mg. doubtless represent the ratios derived from a water loss 

 which is due in large measure to cuticular transpiration, and is bej^ond 

 the retaining power of any of the normal controls of the plant. 



COMPARISON OF RELATIVE TRANSPIRATION RATES IN RAIN-FOREST 



AND DESERT PLANTS. 



Livingston 1 has determined the rates of relative transpiration for 

 several desert annual plants, at the Desert Laboratory at Tucson, 

 Arizona, and Mrs. Edith B. Shreve 2 has secured, at the same place, 

 readings for Parkinsonia microphylla, a typical desert perennial, in 

 plants of several ages and seasonal conditions. The possession of these 

 readings, made by the same methods used in my own w r ork, makes 

 possible a comparison of the amounts and limits of relative transpira- 

 tion in plants of tw r o most w T idely unlike regions. The species used by 

 Livingston are ephemerals, which complete their life cycle during the 

 summer rainy period, and are typical desert plants in no respect except- 

 ing the rapidity with which they grow and come to maturity. His 

 experiments w r ere all made in the sun, but many of his minimum rates 

 of relative transpiration were secured for nocturnal or partly nocturnal 

 intervals. Parkinsonia microphylla is a perennial microphyllous tree, 

 which passes a portion of the year in a leafless condition. The experi- 

 ments of Mrs. Shreve w r ere made on small plants without leaves, and 

 on the twigs of trees, both with and without leaves, as w r ell as on plants 

 grow r n from seed under hot-house conditions. Her experiments were 

 all performed in the sun with the one exception noted. It will be 

 recalled that all of my own work w r as carried on in the shade, with the 

 exception of that on Alchornea, Clethra, and Dodonaa. 



1 Livinston, B. E. The Relation of Desert Plants to Soil Moisture and to Evaporation. Carnegie 

 Inst. Wash. Pub. 50, pp. 45-65, 1906. 



2 Shreve, Edith B. The Daily March of Transpiration in a Desert Perennial. Carnegie Inst. 

 Wash. Pub. 194, 1914. 



