OCCUPA riON OF LAND. 



377 



are constantly springing up, now flourish, bear seed, and take 

 more or less complete possession of the soil. Impoverished land, 

 abandoned by man, becomes nurtured by nature. Weeds, grass, 

 flowers, spring up in great variety often. Some can thrive but 

 little better than the abandoned crops, while others, peculiarly 

 fitted because of one or another adapted structure or habit, flour- 



Fig. 47 s. 



Abandoned field, in Alabama, growing up to broom-sedge and trees. (Photograph by Prof. 

 P. H. Mell.) 



ish. Crab-grass and other low-growing plants often cover and 

 protect the soil from the direct rays of the sun, and thus conserve 

 moisture. The clovers which spring up here and there, by the 

 aid of the minute organisms in their roots, gather nitrogen. 

 The melilotus, the passion flower, and other deep-rooted plants 

 reach down to virgin soil and lift up plant food. Each year 

 plant remains are added to, and enrich, the soil. In some places 

 grasses, like the broom-sedge (andropogon) succeed the weeds, 

 and a turf is formed. 



687. Seeds of trees in the mean time find lodgment. During 

 the first few years of their growth they are protected by the 



