WEEDS AND WEED CONTROL 243 



14. The presence of weeds in such abundance as to attract attention 

 reduces the selling value of the land on which they are found. 



INTRODUCTION AND DISTRIBUTION 



If the list of American weeds is scanned carefully and analytically, 

 it will be found that most of the injurious and troublesome weeds have been 

 introduced from Europe. A few have come from elsewhere. The ques- 

 tion may be asked, why this fact is so? Before the natural conditions 

 were much disturbed by white men from Europe, eastern America was a 

 densely forested country in which most of the herbaceous plants grew 

 on the forest floor in the shade of the dominant forest trees. When the 

 forests were removed, these ground plants of the woods were subjected 

 to the action of the full sunlight, to the drying effects of the wind, and to 

 a soil deprived of its superficial layers of water-retentive leaf mold. They, 

 therefore, were destroyed in large numbers of species, except the more 

 hardy forms which adjusted their growth to the new conditions. The 

 introduced plants, removed from the inhibition of their European 

 competitors, insect and fungous foes and accustomed for at least a thousand 

 years to open field cultivation and growth along roadsides and ether 

 open places, found the new environment favorable to their rapid spread 

 and occupancy of the soil vacated by the native species of plants. It has 

 been suggested also that the European species were more plastic than the 

 native American plants and better able to adjust their growth to their 

 new surroundings. Some of the weeds, however, came from the west, 

 but were introduced later than the advent and spread of the overseas army 

 of weeds. These western weeds came in when the cultivated areas were 

 extended westward beyond the forested areas, so as to occupy the open 

 prairie and steppe country to the westward. Opportunity was thus 

 presented for the native plants of the prairies and steppes bordering on the 

 cultivated districts to contribute somewhat to the weed flora of the 

 east, because with the plowing of the land these western weedy plants 

 found the conditions very favorable for their eastern spread, such as 

 the carpet weed (Mollugo verticillata) , daisy fleabane (Erigeron cana- 

 densis), cocklebur (Xanthium), rag weed (Ambrosia artemisiafolia) , 

 vervain (Verbena hastata, V. urticifolia) , horse nettle (Solanum carolinense) 

 and others. Of late and in consequence of increased communication 

 with the prairies and the country beyond the Mississippi River, the west- 

 ern plants are moving eastward by rapid strides. Such are fetid mari- 



