SOIL-FORMERS 



209 



which to grow. Whenever ground is cleared of vegetation, 

 plants again spring up. The farmer plows the meadow 

 or pasture, and immediately a horde of weeds appears. 



375. The same landscape in winter and in s 



Any breach or break in the earth's surface makes room 

 for a new group of plants. Note how the railway embank- 

 ments and the newly graded roadsides take on a covering 

 of vegetation. Observe the ragweed. Whenever soil is 

 formed at the base of a cliff, plants at once secure a foothold. 

 (Fig. 372.) 



363. Plants Aid in the Formation of Soil.— This they 

 do in two ways: by breaking down the rock; by passing into 

 earth when they decay. Even on the hardest rocks, lichens 

 and mosses may grow. (Fig. 373.) The rhizoids eat away 

 the rock. A little soil is formed. Ferns and other plants gain 

 a foothold. The crevices are entered and widened. Slowly 

 the root acids corrode the stone. Leaves and stems collect 

 on the rock and decay. Water and frost lend their aid. As 

 the centuries pass, the rock is eaten away and pulverized. 

 Note the soil that collects on level rocks in woods where 

 wind and rain do not remove the accumulations. 



364. In bogs and marshes and on prairies, the remains 

 of plants form a deep black soil, In bogs the vegetable 

 matter is partially preserved by the water, and it slowly 

 becomes solidified into a partially decayed mass known as 



