EN V IRONIC FACTORS 425
led, as might be expected;, to the result that one plant which would live parasitically on another must have a more highly concentrated sap. Not all plants with a high concentration of sap may become parasitic on all those of low concentration, however, for other reasons, some of them seasonal, morphological, etc. (see Fig. 5).
���Fig. 5. A Joint of Opuntia Blakeana Rooted Parasitically in a Cavity in the Body of Carnegiea gigantea, under which condition it has existed for three years
The difficulties in dealing with the mechanical features presented by the soil are such that it has not yet been possible to construct an instru- ment which would give data analogous to absorption by roots as does the evaporimeter for transpiration by leaves. Developments in this matter are to be hoped for, however. Meanwhile the studies of Dr. Cannon on root-systems and the distribution of water in the soil have yielded some generalizations of no little value in the consideration of the aspects of the vegetation of a region. Among these it is to be men- tioned that the treelessness of the immense stretches of western prairie and probably of steppes everywhere is a matter dependent upon the dis- tance below the surface at which the so-called "ground water" or "water table" lies. Trees and forests may be established in such re- gions when the supply of moisture in the upper layers of the soil are increased by irrigation, conservation of rainfall or whatever artificial means may be employed.
Living matter is a thermal engine in which the energy of various substances is released very slowly by oxidation processes. It is also self- organizing and substances of various kinds entering into its solutions may be reduced and their components rearranged in the form of char- acteristic constituents and products and in turn become fuel for the engine. Many of these reducing processes are carried on in the presence