144 ENVIRONMENTAL CONDITIONS. 



which is proximately or immediately responsible for the peculiar 

 influence of gravitation, is pressure asymmetrically developed. That 

 this asymmetrical internal pressure that results when the position of 

 a plant is altered with references to the earth's center of inertia first 

 produces a molar movement of certain portions of the cell-contents, 

 and that this pressure and movement, with the new configuration of 

 protoplasmic particles when gravitational equilibrium is again estab- 

 hshed, is the cause of the altered cell activities known to be produced 

 by such change in the position of a plant, is the logical supposition 

 which has been developed into what of theory we as yet possess in this 

 general connection. Since the variations in the intensity of the gravi- 

 tational influence that occur over the surface of the earth are quite 

 negligible when considered with reference to the effect of this factor 

 on plant development, it is obvious that gravitation does not require a 

 thorough consideration at the hands of the ecologist or agriculturist. 

 Here is one external factor, at least, which is practically identical in all 

 habitats, as far as plant control is concerned. 



2. DESTRUCTIVE INFLUENCES OF MECHANICAL CONDITIONS. 



It seems probable that the pressures developed when roots grow 

 against or between rocks and other objects that they can not pene- 

 trate may sometimes be a considerable factor in determining the 

 success or failure of individual plants; but it is not at all likely that 

 this consideration is important in the distribution of plants among 

 different habitats. The only forms of mechanical influence from 

 without that appear to be generally important to plant distribution 

 and to agriculture are those due to (1) wind, (2) water, and (3) animals. 

 In relatively few cases soil-movements, such as landslides, the caving 

 of bluffs, etc., need to be brought directly into account in explaining 

 the vegetation of habitats of limited extent. Ice-movements, as at 

 the lower ends of some glaciers and at the margins of streams and 

 lakes, are often the source of plant destruction in such places. With 

 the mechanically destructive action of animals may be mentioned a 

 somewhat similar action of other plants or plant parts, but here rela- 

 tions other than mechanical are also frequently to be considered. 



The action of wind in differentiating the vegetation of different plant 

 habitats has often been dwelt upon in ecological literature. It must 

 be remembered, however, that this action is at least twofold; air-move- 

 ment not only exerts a deforming or breaking pressure upon the plant- 

 body, but it also profoundly affects the water-relation through increas- 

 ing the evaporating power of the air, as this is effective both upon 

 plant and soil. In connection with wind influence may be mentioned 

 the cutting action of blown sand, really a factor that should be con- 

 sidered with that of soil-movement (which includes the influence of 

 rolling stones), and with that of ice, as above mentioned. 



