TEMPERATURES OF PLANTS IN THE DESERT. SI 
It may be seen from these that the maximum daily temperatures occurred 
between 8 and 11 p.m., and the minima 12 hours later, or between 8 and roa. m. 
The optimum temperature for absorption by roots lies well above that of the soil, 
at the depth at which the observations were made. It follows, therefore, that the 
temperature of the soil approaches this optimum most nearly, and offers most 
favorable conditions for the taking up of watery solutions at a time of the day 
when the amount of water thrown off by the shoot and of mineral matter used in 
metabolism are nearing the minimum by reason of the absence of light, lowered air 
temperature, and consequent increased humidity of the air. These inharmonious 
conditions account almost wholly for the profusion of guttation excretions, or ‘dew - 
drops,’’ formed on the tips and margins of grass blades and leaves of low-growing 
plants early in the evening. 
Absorption by the root continues quite vigorously after sunset by reason of the 
favorable temperatures; and the augmented amount of fluid in the cortex of the 
roots sets up a pressure which ultimately forces the water into the central cylinder 
and up through the woody cells faster than it may be used and transpired by the 
thin-walled cells of the leaves. The vessels become filled with water which is 
forced out in form of drops through the excretory openings. In some species the 
amount of water coming away from the plant in this manner may reach quite an 
appreciable quantity. 
On the other hand, the forenoon witnesses the rapid acceleration of transpira- 
tion by all parts of the shoot at a time when the soil temperature is decreasing to a 
minimum. The increase of transpiration continues until mid-afternoon, while the 
temperature of the soil reaches a minimum two or three hours earlier, and then 
begins to rise, but does not do so sufficiently to favor absorption to any great extent. 
It is true, of course, that the needs of the leaves may be partially met by the activity 
of the rootlets which lie nearer the surface. 
When plants are cultivated in pots in greenhouses, the small volume of soil 
around the roots responds much more rapidly to changes in the temperature of the 
surrounding air and to the influence of streams and sprays of water than does the 
upper layer of soil in the open. In general, the soil in the greenhouse will show 
a much higher average temperature, which, with the other conditions mentioned, 
makes necessary special treatment on the part of the gardener. If the natural 
conditions of water-supply by precipitation were complied with, the needs of the 
plant would by no means be met under the altered conditions of temperature. 
Another point of interest in the present connection is the fact that 
such notable differences are found between the temperatures of the sub- 
terranean and aerial portions of the bodies of plants at almost all seasons. 
During June, 1902, the shoots of herbaceous plants were in an atmosphere 
that varied between 8° C. (46.5° F.) and 34° C. (92.5° F.), while the roots 
were between 8°C. (46.4° F.) and 13°C. (55.4° F). As the maxima and 
minima were not synchronous the actual difference between the tempera- 
ture of twigs and leaves on the upper part of the plant and roots on the 
lower amounted to as much as 22°C. (nearly 40° F.) at certain times of the 
day. Such conditions occur, though slightly less accentuated, during the 
entire summer in this locality. It is evident without further discussion 
that such differences in the temperature conditions of the two poles of the 
plant must exert a more or less important influence on the transport of 
fluids and solutions from one part of the plant to another. Referring to 
