FACTORS OF ENVIRONMENT. 475 



tant, not only because of the effect they have upon the physical 

 condition of the soil, but also because of their activity in the 

 various metabolic processes in the plant. Some enter into the 

 production of protoplasm, some play a secondary part in stimu- 

 lating certain processes or activities of the plant. 



Within certain limits plants adapt themselves to changes in 

 the chemical condition of the soil, or to chemically different 

 soils, but there is great variation in this respect in different 

 species. To a nutriment salt like sodium nitrate in the soil, 

 plants will adapt themselves more readily to an increase in the 

 concentration, than to a non-nutrient salt like sodium chloride. 

 But when the concentration of the salt goes beyond a certain 

 grade, according to the species, it acts as a poison. 



921. Physical condition of soil. Rocky regions are least 

 suited for vegetation, except under certain conditions, and with 

 a limited number of plants. In mountain regions, large areas 

 of rock may be bare of vegetation, especially if the rock lacks 

 crevices. In the drier situations the vegetation may be limited 

 to rude lichens, or a few of the lower algae in moist situations. 

 Along the sea-coast, where the rocks are washed by wave 

 action, certain of the large marine algae are enabled to obtain 

 a foothold by means of holdfasts, even on the smooth surface 

 of rocks. 



Where the rocks are creviced, there is an opportunity for foot- 

 hold for a greater variety of plants, mosses, ferns, and the flow- 

 ering plants. The crevices also serve to catch and retain the 

 disintegrating remains of lichens and other vegetation which 

 mingle with the crumbling portions of rock, and form a different 

 soil which provides food for a greater variety of plants. Where 

 the rock is more broken in, the form of boulders or small stones, 

 the more exposed surfaces harbor lichens, while the interstices 

 give an opportunity for the accumulation of a greater mass of 

 finer soil. Where the rock is reduced to gravel or coarse sand, 

 the disintegrating vegetation eventually mixes with and covers 

 it, offering a still wider range for plant growth. With the 

 weathering and crumbling of the rocks, the admixture of decay- 



