206 THE STUDY OF PLANT COMMUNITIES • Chapter VIII 



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FlG. 99. An introduced weed, tumble mustard (Sisymbrium altissmtum), 

 dominant over the entire extent of a sagebrush burn, one year after the fire. 

 Washoe County, Nev.— Photo by W. D. Billings. 



more drastic nature. It is hard to believe that 60 percent or more 

 of our weeds are not native but introduced species that have come 

 from all parts of the world. 176 Some were brought in as orna- 

 mentals and almost immediately escaped and spread from gardens. 

 Others came in accidentally with seeds of desirable plants. Many 

 introductions have been useful and extremely valuable. Most of 

 our cultivated plants have been improved by crossing with strains 

 of foreign varieties at some time, or they were themselves original- 

 ly introduced. In recent years, such introductions are not made 

 haphazardly. 



Unfortunate experiences with unconsidered or accidental intro- 

 ductions can be listed for all parts of the world. The water hy- 

 acinth, introduced from South America, has spread throughout 

 the lowland waterways of our southern states where it chokes 

 canals, impedes drainage and navigation, and destroys wildlife. A 

 similar problem has resulted with the introduction of Elodea in 

 the low countries of Europe. Animals may cause similar difficul- 

 ties, as the spread of the introduced English sparrow and the star- 

 ling in the United States. The muskrat has become a pest in central 



