TRANSPIRATION AND TEMPERATURES 43 



while the writers have observed numbers of bodies of plants in which 

 the midday temperature reached 115° and 116° F. without injury, and 

 from their location must have undergone similar conditions repeatedly. 

 This gives rise to two suggestions. The protoplasm of plants which 

 have become adapted to this region must have undergone certain vari- 

 ations in composition, as its maximum point of activity is beyond that 

 of other plants, and these forms must also be furnished with specially 

 adapted chloroplasts by which the chlorophyl would be kept from in- 

 jury at temperatures beyond the critical point at which it suffers damage 

 in other plants. The existence and growth of certain algit in the waters 

 of hot springs lends favor to this supposition. It is however probable 

 that the death of plants which have found foothold in such regions may 

 result from the intense insolation quite as much as from a lack of a proper 

 supply of water. 



In connection with the above observations it was deemed important 

 to make some comparative observations upon the climatic features 

 encountered by alpine plants, and Mr. MacDougal in company with 

 Mr. A. E. Douglass and his assistants made an ascent of San Francisco 

 mountain north of Flagstaff, and about fifteen miles due west from 

 the station at which the observations on desert plants were made but 

 at an elevation about a mile higher. Camp was made and maintained 

 during the first week in August, 1898, at an elevation of 1 1 ,500 feet and 

 in addition to the astronomical equipment a battery of instruments 

 including a thermograph, a hygrograph, and mercurial thermometers 

 were allowed to remain on the summit at 12,500 feet for fifteen days 

 (Plate XXIX) . The records obtained of the temperature and relative 

 humidity of the air are given in the accompanying diagram (Fig. 4). 



It is to be seen that the temperature of the air does not undergo such 

 great diurnal variation as that at lower altitudes. A greater difference 

 is to be found between the temperature of the soil and air, however, 

 than at lower elevations. At an elevation of 3,000 feet the mean 

 temperature of the soil in humid localities is 2.7° F. greater than 

 that of the air; at 4,000 feet it is 3° F. ; at 5,000 feet, 4.3° F. ; at 

 6,000 feet it is 5.4° F. and at 7,000 feet it is 6.5° F. In an observa- 

 tion made at 4 P. M. August 8, on the western slope of the peak the 

 soil stood at 71.6° F. and the air at 57.6" F. ; and at 7 A. M. the next 

 morning the Uiinimum of 21.2° F. was obtained for the air and 48.2° F. 

 for the soil. This increase in the difference between the temperature 

 of the soil and the air is due to the increase in intensity of the sun's 

 rays and the attenuation of the atmosphere at such altitudes. It is esti- 

 mated that the intensity of the sun's rays at an altitude of 9,000 feet is 



