TEMPERATURES OF PLANTS IN THE DESERT. 79 
The above data were obtained at Flagstaff, Arizona, at an altitude of 
about 7,000 feet, and some further measurements were made at the Desert 
Laboratory on April 13, 1907, on opuntias similar in form to the above 
and which were showing flower buds about to open. Fronds in a vertical 
position were selected which were facing toward the point on the horizon 
directly under the sun at 2 p. m., and into these were thrust the slender 
bulbs of thermometers the glass stems of which were shaded. Readings 
of 108° to 117° F. were obtained with the air at 92° F. 
In the thermometry of globular, decumbent, or cylindrical forms of 
fleshy plants, such as Cereus, temperatures of 113° to 115° F. were often 
found with the air at a temperature of 93° to 100° F. It is to be seen that 
plants in this region are subject to the action of a fierce insolation and to 
an atmosphere of low relative humidity. As a result of such insolation 
the body of the plant and the surface layers of the soil are raised to very 
high temperatures. The increase in temperature of the shoot aided by 
the direct action of the light upon the transpiratory mechanism would 
tend to increase the amount of water given off by the shoot. At the 
same time, however, the temperature of the soil undergoes a correspond- 
ing increase, thereby increasing the osmotic processes of absorption, so 
that the two processes, absorption and transpiration, automatically 
equalize each other, provided the maximum temperature of protoplas- 
mic activity is not passed. 
It is not to be taken for granted that the temperatures recorded above 
represent the maximum limit in the matter. Algez are found in certain 
springs in the Southwestern deserts which flourish at 128° F., and accred- 
ited records of air temperatures of 122° to 128° F. are available. The 
shoot of plants exposed to the sun under such conditions would doubtless 
be as warm as 135° or perhaps 140° F. It is evident therefore that the 
supra-maximum temperature of active protoplasm and of chlorophyl 
usually quoted in text-books are good for the laboratory only, and do 
not measure the range of endurance offered by desert forms. Capacities 
of this character would probably be found to depend upon the character 
and structure of the proteid constituents of living matter, as well as on 
the water content necessary for growth and for other functional and vege- 
tative activities. 
In comparison with the above, some data obtained by Messrs. Douglas 
and MacDougal on the summit of San Francisco Mountain, near Flagstaff, 
Arizona, are of interest. Observations were made at a camp at 11,500 
feet during the first week in August, 1898, and some recording instru- 
ments were kept in operation for a longer period. A consultation of these 
records shows that the temperatures did not range so widely during the 
course of a day as at lower altitudes. The difference between the soil 
and air seems very marked, however. Kerner estimates that at an ele- 
vation of 3,000 feet the mean temperature of the soil in humid localities 
