arrived in California via these ballast dumps. 



The first weeds known to arrive in California have been "fos- 

 silized" in adobe bricks. Filaree {Erodium cicutarium, Gerania- 

 ceae), Curly Dock (Rumex crispus, Polygonaceae), and Sow 

 Thistle (Sonchus asper, Compositae) are all Old World weeds 

 whose seeds have been found in adobe bricks of old Spanish 

 buildings constructed before the Mission Period (1769-1824). Ex- 

 amination of adobe bricks made in the Mission Period indicates 

 that an additional number of weedy species arrived and became 

 established during that time. These include Black Mustard (Bras- 

 sica nigra, Cruciferae), wild oats (Avena spp., Gramineae), and 

 Wild Carrot or Queen Anne's Lace (Daucus carota, Umbelliferae), 

 all from the Old World. Interestingly, however, the extensive and 

 rich botanical collections of such early plant explorers as David 

 Douglas, Thomas Nuttall, and John Fremont who visited the 

 state in the 1830's and 1840's contained only a single weed spe- 

 cies. In view of the fact that some of these men had to carry 

 their own collections and collecting gear on their backs, perhaps 

 they consciously avoided collecting plants that they knew to be 

 introduced, since they were more interested in new and unusual 

 native species that they were continually encountering. In the 

 early 1840's, the Russians made extensive collections in Sonoma 

 County, working out of their settlement at Fort Ross. These col- 

 lections reveal a varied assortment of introduced weeds, indicat- 

 ing that a number of these had been introduced long before ex- 

 tensive settlement of the area and suggesting that these intro- 

 duced plants were present but avoided by other collectors. 



By 1860 only about 100 species of weeds had been recorded 

 in California, but it is certain that many more had become well 

 entrenched by that time. After 1860, more attention was given 

 to recording the adventive flora of the state, with the result that 

 in the past century the arrival of new weeds and their spread 

 have been fairly well documented. This has been assisted by the 

 watchful eye of various county agents and by biologists in the 

 state Department of Food and Agriculture. 



It is difficult to predict whether or not an introduction will be 

 successful and, if so, whether the species will spread from its 

 point of introduction. For example, the yellow-flowered Uro- 



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