8] MODIFICATION AND DISTRIBUTIONS OF CROPS 249 



the weeds which adulterate them, and which are apt all too easily 

 to be shipped and sown with them. It is apparently largely in this 

 manner — though further means were listed in Chapter IV — that, 

 among others, the following pernicious European weeds have become 

 dispersed to temperate America, South Africa, New Zealand, and 

 temperate Australia : Couch-grass (Agropyron repens), Crab-grasses 

 (species of Digitaria), Russian-thistle {Salsola kali var. tenuifolia), 

 Bindweeds (species of Convolvulus), Sheep Sorrel {Rumex acetosella 

 agg.). Dodders (species of Cuscuta), Plantains (species of Plantago), 

 Wild Carrot (Daucus carota), Prickly Lettuce {Lactuca scariola), and 

 Sow-thistles (species of Sonchus). Europe is not, however, by any 

 means the only source of weeds : although it has contributed some 

 500 to North America alone — including all present-day Americn 

 representatives of Lamium, Melilotus, Medicago, Malva, and some 

 other familiar genera — many weeds have been dispersed in the 

 opposite direction. Some have even gone much farther afield than 

 across the North Atlantic — e.g. Canadian Fleabane {Erigeron 

 canadensis) and Gallant Soldier (Galinsoga parviflora, a native of 

 South America). 



These and others among the most widespread of weeds are the 

 ' semi-cosmopolites ' mentioned in the last chapter, and it is notice- 

 able that some of them lack special means of dispersal — commerce 

 having gradually distributed them to all the major regions where 

 they can thrive. This is notably the case with such other types as 

 Shepherd's-purse, Common Chickweed, Annual Meadow-grass, and 

 Lamb's-quarters {Chenopodium album s.l.), which are among the 

 most widespread of all flowering plants because, primarily, of Man's 

 unwitting transport and, secondarily, of their variability which 

 enables them to grow in a wide range of diiferent habitats. How- 

 ever, as we have already seen, this is usually manifest only so long 

 as Man continues to interfere by clearing areas or at least keeping 

 the native vegetation at bay ; competition tends to be too much 

 for weeds which lack Man's help. This help often extends to 

 breaking up underground parts from which vigorous regeneration 

 can take place — for example in the case of the rhizomes of Couch- 

 grass and Bracken, and the roots of Dandelions. Such weeds are 

 particularly difficult to eradicate. 



Although the semi-cosmopolitan weeds tend to have such a wide 

 tolerance to environmental conditions that they are capable of 

 invading almost any agricultural or otherwise disturbed area, at least 

 within their normal climatic limits, other weeds are far more exacting 



