200 WHERE PLANTS GROW 



themselves to widely different environments. Weeds are ex- 

 amples. Many plants have become so specialized in habitat 

 as to be parasitic, saprophytic, or epiphytic. Chap. XIII. 



333. Common plants often grow in most unusual 

 and difficult places. Note that some weeds grow not only 

 in fields, but often gain a foothold in chinks in logs, on 

 rotting posts, in crotches of trees, on old straw stacks, in 

 clefts and crannies of rocks. In moist climates, as Eng- 

 land, plants often grow on thatched roofs. 



334. Plants may be said to be seeking new places in 

 which to grow. Whenever ground is cleared of vegeta- 

 tion, plants again spring up. The farmer plows the 

 meadow or pasture, and immediately a horde of weeds 

 appears. Any breach or break in the earth's surface 

 makes room for a new group of plants. Note how the 

 railway embankments and the newly graded roadsides take 

 on a covering of vegetation. Observe the ragweed. When- 

 ever soil is formed at the base of a cliff, plants at once 

 secure a foothold. Fig. 345. 



335. 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 will grow. Fig. 34G. 

 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 which 

 collects on level rocks in woods where wind and rain do 

 not remove the accumulations. 



336. 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 



