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TEXTBOOK OF BOTANY 



parts of the world. Since their roots and rhizomes are perennial, shoots 

 from buds on rhizomes begin to grow rapidly in spring and soon overtop 

 the annual plants, which start from seeds each year. Similarly cattails, 

 rushes, sedges, and water lilies frequently exclude many other plants from 

 certain habitats. Their rhizomes may grow several feet in length each 

 year, and the plants occupy new areas rapidly. 



Abandoned farans and denuded areas about cities are first occupied by 

 a mixed population of annual, biennial, and perennial weeds; but the 

 perennials increase their area year by year through vegetative multipli- 

 cation and finally exclude nearly all the annuals and biennials. 



For the prevention of soil erosion in gullies and on freshly made em- 



FiG. 181. A, uncontrolled wind erosion accompanied by crop failures and the 

 formation of deserts in Sherman County, Texas; B, same site after the soil had 

 been stabilized by grasses planted according to methods devised to control 

 this type of erosion. Photo from U. S. Conservation Service. 



