WEEDS 



473 



species. If we look through a list of a hundred of the worst 

 weeds over the continental area of the United States (exclud- 

 ing Alaska), 1 it appears that almost exactly half of the num- 

 ber are from Europe. Nine others are from tropical America 

 or from India, so that a clear majority of these hundred nota- 

 ble weeds are foreigners. It is rather difficult to give all the 

 reasons why so many of our common weeds come from Europe, 



but it is certain that of the 

 prevalent weeds on that con- 

 tinent many represent the 

 result of a gradual sifting- 

 out process 

 which has 

 lasted for 

 tens of cen- 

 turies. Dur- 

 ing all that 

 long period 



FIG. 355. Pokeweed, a common weed of waste ground the tilled 

 A, a flowering branch; B, flower; C, fruit lands of Eu- 



rope have 



gradually become populated by such European plants as proved 

 able to live in cultivated ground in a temperate climate against 

 human opposition. Together with these are found such other 

 persistent species as may have found their way in from Asia 

 and Africa. When the soil of temperate North America first 

 began to be cultivated by the whites, it was inevitable that 

 great numbers of European weeds should be brought in along 

 with farm and garden seeds, in the ballast of vessels, and in 

 other ways, and rapidly gain a foothold on the new continent. 

 The history of the spread of many weeds has been preserved, 

 and it forms a most interesting chapter of economic botany. 2 



1 Farmers' Bulletin 28, U. S. Dept. Agr. 



2 See the essay of Dr. Gray already cited ; also Farmers' Bulletin 28 and 

 Bulletin 15, Division of Botany, U. S. Dept. Agr. Consult also all the attain- 

 able weed reports of the state agricultural experiment stations. 



