MARYLAND W E A 111 ER SERVICE _i 



the atmospheric conditions and the wide differences in soil-water 

 supply from habitat to habitat, goes to show that it is in the specific 

 peculiarities of leaf size and leaf structure that we must look for the 

 chief means for the maintenance of the balance. An abundant sup- 

 ply of soil-water renders easy the maintenance of the balance and 

 favors the occurrence of plants with broad thin leaves (hygrophytrs), 

 the scarcity of soil-water renders difficult the maintenance of the 

 balance and conditions the occurrence of plants with small or leathery 

 leaves and various peculiarities of anatomical structure (xerophytes). 

 We thus see the fundamental importance of the amount of soil-water 

 in determining the character of vegetation, and it is in turn depend- 

 ent upon the physical texture of the soil and upon the topography. 



Not only is the amount of the soil-water of importance but also 

 the character of its dissolved contents. The existence of a consider- 

 able percentage of common salt in the water of salt marshes renders 

 the existence there of ordinary plants an impossibility. It serves 

 also to render absorption slight even in salt plants, because the salt 

 is useless to the plant and cannot be got rid of, therefore accumula- 

 ting in the tissues to an amount that is ultimately sufficient to be 

 toxic. Although water is so abundant in salt marshes it is unavail- 

 able to plants and they are thus subjected to difficulty in maintaining 

 the balance between absorption and transpiration in much the same 

 manner as are plants of dry habitats. In the upland too, there are 

 lesser differences in the chemical character of the soil-water due to 

 the nature of the rock from which the soil particles have been de- 

 rived, and these differences are of some importance in determining 

 the local distribution of plants. The great generality of rocks con- 

 tain a variety of chemical elements, such that the soils derived from 

 their disintegration yield all the inorganic substances necessary to 

 the growth of plants. The soil-water, which is in siich intimate con- 

 tact with the mineral particles of the soil, exerts a continuous solvent 

 action upon these particles and thus brings into solution a variety of 

 inorganic salts in amounts varying with their solubility. The com- 

 pounds of silicon and aluminum, which make up such a large per- 

 centage of the composition of granites, shales, schists, gneiss and other 

 rocks, are highly insoluble and accordingly remain as the chief solid 

 constituents of the soils derived from these rocks. Small percentages 



