THE SOIL AND SOIL FORMATION 55 



obtaining some food from the dust and soil particles 

 continually being brought forward from adjacent soil 

 areas. Where the small plant comes in contact with 

 granite or other rock, it secures minute quantities of 

 mineral food. One lichen plant living and decaying 

 makes it possible for others to grow. These, decaying 

 on some rock surface and lodging in some crevice, result 

 in an accumulation of vegetable matter and make a soil 

 habitable for more highly organized bodies, as mosses. 

 The mosses, in their turn, gathering into their branches 

 more or less particles of crumbled rock and other ma- 

 terial driven by the wind or carried by the water, decay, 

 adding humus to the soil, and finally make a place 

 suitable for the germination of seeds and a feeding 

 ground for roots of some of the more highly organized 

 plants. Thus it is possible for the most inhospitable 

 surfaces to be made more or less productive. It is quite 

 possible that bacteria came before lichens and acted as 

 soil-forming agents before the latter plants were 

 evolved. 



Soil formation under difficulties. In gravel beds the 

 processes of disintegration and soil building have been 

 similar to those of solid rock surfaces. Small plants 

 that can exist in unfavorable conditions must grow first. 

 Their decaying bodies make it easier for the next genera- 

 tion, or for higher plants, and the soil is gradually en- 

 riched. If moisture is abundant in gravel beds, the sur- 

 face of the land is gradually filled with lichens, mosses, 

 and other simple plant organisms; and finally, flower- 

 ing plants find in the gravel hospitable nooks for a few 

 roots, where decaying plant materials hold moisture and 

 plant food for their use. Even where the gravel is so 

 large as to be called stones or bowlders, the mosses and 

 other plants have found an abode, and the result has 

 been that some of these soils have been so filled up with 

 decaying mosses and other plants that they are now 



