80 CARNEGIE INSTITUTION OF WASHINGTON. 



(7) The acidity changes in these plants at high temperatures make 

 necessary an investigation of the absorbing capacity of tissues which 

 have been kept under certain controlled conditions, before any conclu- 

 sions can be drawn concerning the connection between acidity and the 

 transpiration or root-absorption behavior. 



The Relation of Altitude and Habitat to the Transpiring Power of Plants, 



by Forrest Shreve. 



During the month of June 1915 work was begun upon the relative 

 transpiring power of some 30 species of perennial plants of the encinal 

 region of the Santa Catahna Mountains. The method of standardized 

 hj^grometric paper, recently elaborated by Livingston, was employed, 

 making it possible to secure readings in the field from plants growing 

 in their normal environment. The species used in this work were 

 selected with a view to representing all of the physiological types in 

 this richly diversified vegetation, and also with a view to being able to 

 compare the transpiring power of the same plant at different elevations. 

 In order to make adequate comparisons between different plants it 

 was found necessary to take an hourly series of readings throughout 

 the day on each of the individuals investigated. The hygrometric- 

 paper method was found inadequate for the measurement of the low 

 transpiration-rates of Agave, Yucca, and Opuntia, although a very thin 

 grade of paper was used. It was also impossible by this method to 

 measure the water-loss from the upper sides of the leaves of several 

 sclerophyllous trees. In these cases comparisons were secured for the 

 lower surfaces alone, while in the majority of cases the behavior of 

 upper and lower surfaces was averaged. 



Differently situated individuals of the same species, growing at the 

 same altitude, were found to exhibit differences of transpiring power — - 

 the plant in situations of highest soil-moisture content having the 

 highest coefficients. Species characteristic of the streamways, and 

 confined to them, were found to have much higher coefficients than the 

 species characteristic of the adjacent upland and slopes. Among the 

 different tj^pes of plants investigated at 5,000 feet, marked differences 

 were found in the coefficients of transpiring power and slight differences 

 in the daily march of the coefficients. When plants of the same species 

 were compared at different elevations, but in similar topographic sites, 

 there were found to be differences in the coefficients and also in the daily 

 march. In a comparison of the behavior of the mesquite (Prosopis 

 veluiina) the maximum is found to occur earliest at 2,400 feet (8 a. m.), 

 later at 4,400 feet (10 a. m.), and still later at 5,000 feet (11 a. m.). 

 The behavior of Calliandra eriophylla is similar in having an earlj- maxi- 

 mum at 2,400 feet, but some individuals at 5,000 feet exhibit a late 

 maximum coincident with that of evaporation. A comparison of the 

 bellota {Quercus emoryi) at 5,000 and at 6,000 feet showed the highest 



