78 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
last week in July, 1906, show a variation between 85° and 98° F., although 
it is to be said that this type of instrument responds somewhat slowly 
and it is quite possible that the maxima might have been as muchas 3 
or 4 degrees above that shown for a few minutes. When these instru- 
ments were first installed the bulb of one was placed with its center 3 
inches below the surface, with the result that the tracing pen rose above 
the slip and temperatures of 102° and 105° F. were indicated. Ata 
depth of a foot the temperature during the week indicated did not show 
a daily variation of over 2 degrees, but a continuous rise of 6 degrees 
occurred. 
It is not to be taken for granted that all plants native to the region 
are active under such conditions, however. The winter annuals and 
perennials come to bloom early in February in great number, and it is 
to be seen that the roots are embedded in a soil that ranges from 48° to 
55° F. at 6 inches from the surface, while at a foot the mean average is 
practically the same, with a narrower amplitude of diurnal variation. 
This temperature at the depth of a foot is practically the same as that 
recorded for midsummer in New York. 
A study of the mechanism of absorption would doubtless detect by 
comparison some important differences between the roots of winter 
annuals which take up soil solutions at 50° F. and those of the summer 
flora which habitually function at a temperature in the neighborhood of 
100° F., 50 degrees higher. Important correlations with the transpira- 
tory activities may also be expected. 
The temperatures of a number of plants were obtained by thrusting 
the bulbs of small mercurial thermometers into the fleshy stems and shad- 
ing the exposed portion of the instrument from the sun’s rays. The 
following data were recorded from tests of this character with an opuntia 
on July 17, 1898: 




Temperature | 7 20m 8h rom | gh ooM | oh 39m rr ooM | yyxh 30M | 2h 39m | 
of— A.M. A.M. A.M. A.M. A.M. A.M. P.M. 
Opuntia...... 79 .0° F.|93.5° F. | 93.8° F. | 9722° BF. i111. 2° F.|109 74" Bee 
Aign: doer 78.5 82.4 87.8 Q1.4 96.8 100.4 100.4 





The flattened fronds of the cactus were in an upright position, with 
the edges in the plane of the meridian, so that the angle of the incident 
rays of sunlight decreased with the altitude of the sun. Asa consequence 
of this insolation the resulting temperatures rise until about 10 a. m., and 
then decrease until the sun once more comes into a position where the 
rays might strike the surface at or near a right angle, reaching a second 
maximum at 2 p.m., though observations on this point were somewhat 
obscured by the daily clouding at the time of the experiments. 
