82 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
the previous discussion concerning the comparative transpiration and 
absorption during the day, it is to be seen that the heightened tempera- 
ture of the shoot must operate, in a simple physical way, to greatly aug- 
ment the amount of water thrown off, while the roots must take water at 
the same time to meet the loss, at a temperature as much as 40° F. lower. 
During the movement of the water from the roots to the leaves of 
grasses and other low-growing plants a total distance of no more than 50 
centimeters (20 inches) may be traversed, occupying a matter of a few 
minutes or an hour at most, during which time the temperature is raised 
the above amount. The warmingof the liquid as it passes upward through 
the living and non-living cells is attended by alterations in its solubility of 
mineral and organic substances and by a decreased capacity for holding 
gases in solution. The downward movement of solutions of sugars, 
acids, and nitrogenous substances from the leaves encounters the opposite 
set of conditions. This movement takes place almost entirely by osmose 
and diffusion, and is a much more complicated process, both chemically 
and physically, taking place in living cells only. The cooling of the liquid 
would entail alterations in its power of carrying substances in solution, and 
would also alter its physical relations to atmospheric gases. 
It may be said, in conclusion, that the facts disclosed as to the actual 
temperatures in the soil, the diurnal and seasonal change therein, lead to 
the belief that the differences in temperature of the aerial and under- 
ground portions of plants can not fail to be of very great importance in 
the physical and chemical processes upon which growth, cell-division, 
nutrition, and propagation depend. The determination of the effect of 
differences in temperature between the roots and aerial shoots has received 
but little consideration from the physiologist and the geographer. A 
careful analysis of the conditions and results of experimental observa- 
tions, carried on with plants under artificial conditions with the roots 
and shoots under abnormally similar temperatures, would no doubt 
result in the detection of many mistaken conclusions, especially in regard 
to absorption, translocation, and transpiration. 
That soil temperatures and their relation to those of the air must be of 
very great importance in the cultivation of economic plants is self-evident, 
especially in species in which the desired useful portion is formed under- 
ground and receives storage material formed by the activity of the aerial 
organs. Thus, in the case of such plants as the potato, certain mineral 
substances are absorbed from the soil at a comparatively low temperature, 
carried aloft into the heated leaves, where they participate in activities 
resulting in the formation of sugars, starches, and other carbohydrates, 
perhaps some nitrogenous substances as well, and then these complex 
bodies are slowly diffused downward, with many accompanying chemical 
and physical modifications, to underground cool-storage organs, where a 
condensation occurs and the products are stored in insoluble form in the 
tuber. 
