52 



MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



air pressure cannot be neglected. At sea level the barometer reading 

 is about 760 mm, while at 1,000 m it is 600 mm, and at 6,000 m only 

 300 mm. The evaporation rate (i. e,, from atmometers) at 1,000 m is 

 about 25 percent more than at sea level and at 6,000 m about two and 

 one-half times as much. It is, therefore, obvious that at higher eleva- 

 tions, other things being equal, transpiration will be increased. It is, 

 however, very difficult to separate the altitude effect from the other 

 factors concerned, because at these different altitudes there are always 

 differences in relative humidity, temperature, light intensity, etc. 

 Renner (175), however, carried on some experimental work to show 

 the effect on transpiration of the decreased atmospheric pressure with 

 the results shown in table 24. 



Table 24. — Effect on transpiration of decreased atmospheric pressure 



Air pressure (millimeters of mercury) 



Relative 



increase in 



transpiration 



Relative 



decrease in 



pressure 



734 .... 



1 



1.34 



1.39 



1 



508 



1 44 



381.. 



1.99 







Shreve (194) determined, by means of the cobalt paper method, the 

 relative transpiring power of 30 perennials of various biological types, 

 including Prosopis julifiora veluiina Sarg., at different altitudes in the 

 Santa Catalina Mountains of Arizona. He found the maximum tran- 

 spiring power was later in the day as the elevation increased. In the 

 case of the mesquite (Prosopis) at 2,400 feet, the relative transpiration 

 was greatest at 8 a. m., i. e., the leaf behaved most like a free water 

 surface at this time and differed least from the atmometer. At 4,400 

 feet the maximum transpiring power was at 10 a. m., while at 5,000 

 feet, it occurred at 11 a. m. 



ETHER AND VOLATILE OILS 



Jumelle (117) showed that leaves of Quercus, Ostrya, and Fogus 

 always transpired more in an atmosphere containing ether when in the 

 light, but in the dark the transpiration of the anesthetized leaves was 

 always less than that of untreated ones. Other workers, however, 

 have found that ether always increases transpiration. This would 

 seem to be more reasonable, since it is known that ether increases the 

 permeability of cells, and materials which increase cell permeability 

 favor transpiration. 



The effect of volatile oils is more important from a silvicultural 

 point of view, inasmuch as such substances occur in nature. Many 

 plants exude volatile oils, which increase the total gas pressure and 

 decrease the conductivity of the air to radiation. This had, therefore, 

 appeared to earlier authors as an adaptation to dry environments, 

 the best known examples among trees being Eucalyptus species ; later 

 researches, however, have shown that plants which give off volatile 

 oils are not characteristic of any particular ecological environment 

 but are found as frequently in moist places as in dry ones. Marloth 

 (147) mentions that the Mediterranean region and the deserts of 

 north Africa, Asia, and California contain many plants which are oil- 

 bearing; but he also calls attention to the fact that many other 



