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 



