28 THE PLANT LIFE OF MARYLAND 



or even mere traces of salts of calcium, magnesium, potassium, 

 sodium, sulphur and iron are also present in the above-named rocks. 

 These elements are all of vital importance in plant nutrition and 

 they commonly occur in the form of salts which are readily soluble. 

 The supplies of dissolved salts in the soil-water are being continu- 

 ously carried away through the washing, or leaching, of the soil by 

 rainfall, so that it is possible for a soil to become poor in a readily 

 soluble salt which is abundant in the rock from which the soil was 

 originally derived. 



The soils derived from the rocks mentioned in the last paragraph 

 are designated silicious, and they contain all the inorganic salts neces- 

 sary to plant growth, without containing any of these in such large 

 amounts as to be harmful or toxic, save in the case of soils derived 

 from serpentine rock, of which more will be said later. The soils 

 derived from limestone are so rich in calcium as to be distinctive in 

 their relation to plant nutrition, and are designated calcareous. The 

 influences of silicious, serpentine and calcareous soils on the metab- 

 olism as well as the distribution of plants will be noted at greater 

 length on a subsequent page. 



It must be evident that those relations which the vegetation bears 

 to local factors of soil, topography and the like, are most clear only 

 in areas which have not suffered modification by man. In virgin 

 forests and undisturbed marshes the vegetation is of such a character 

 that it will remain the same in its appearance and make-up from de- 

 cade to decade, — that is to say it has reached a condition of stability 

 which represents a nearly perfect balance between the life processes 

 and the physical conditions under which it exists. Where extensive 

 clearings exist and scarcely any areas of virgin forest remain, as is 

 the case in Maryland, it is extremely difficult to reconstruct a picture 

 of the virgin vegetation, and equally difficult to draw wholly satis- 

 factory conclusions as to the relation between natural vegetation 

 and the physical conditions. Particularly is the character of the 

 forests changed by clearing. This results in part from the selection 

 by the lumberman of certain tree species and the leaving of others, in 

 part from the altered conditions of soil due to clearing or to subse- 

 quent tires, and yet again to the chance conditions which may de- 

 termine the re-seeding of an area. 



