century ago, who would have predicted that there was danger to 

 the thousands of acres of Valley Grassland? Many species in this 

 area have been radically reduced in numbers, and although per- 

 haps few of them face immediate extinction, they certainly 

 should be placed on the list of endangered plant species. 



The eradication of many natural plant communities by man 

 has been accompanied by the replacement of these communities 

 with cities, freeways, suburbs, or other sorts of developed land 

 which are essentially devoid of vegetation. However, another 

 more subtle alteration of the CaUfornia flora has resulted from 

 the transformation of land occupied by native plants into grazing 

 land. Approximately thirty million acres of land in California are 

 cultivated or grazed. This land is largely occupied by agricultural 

 plants that require cultivation in order to survive. Grazing lands 

 and other lands which have been less severely disturbed often are 

 occupied by numerous species of introduced plants that have be- 

 come established as a part of the alien flora of the state. About 

 one-eighth of the present flora of the state consists of plant 

 species that have been introduced into the state from elsewhere 

 in the last 200 years. These are plants that mostly are classified 

 as "weeds". A gardener or agriculturalist might define a weed as 

 a plant that is growing where it is not wanted; a botanist might 

 add more specific biological qualifications to his definition. 

 Weeds possess several characteristics that enable them to survive 

 in disturbed habitats and, indeed, a number of weed species can 

 survive only in disturbed areas. Weeds, like cultivated crops, re- 

 quire man-made conditions in order to survive, although weeds 

 are not encouraged but survive as unwanted camp followers, 

 often thriving under the same conditions as cultivated plants. 



How did these weedy plants arrive? Some of them were intro- 

 duced into California consciously because they had ornamental 

 value. For example, the European Sweet Alyssum (Lobularia 

 maritima, Cruciferae) was introduced as a garden plant because 

 of its drought tolerance and attractive, scented white flowers. 

 It has become established locally as an inhabitant of poorly 

 kept gardens and empty lots. Likewise, Red Valerian {Centranthus 

 ruber ^ Valerianaceae) was introduced into the state as an orna- 

 mental, but has become locally established in waste places and 



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