HOW WEEDS INCREASE 529 



neglected to the injury of crops. This cultivation serves 

 the double purpose of keeping down weeds and preventing 

 the loss of soil moisture. Another benefit from weeds is 

 that they often force the use of a system of crop rotation 

 which might not be adopted if it were not made necessary 

 by their prevalence. Some weeds which are practically 

 impossible to control in grain fields soon disappear when a 

 cultivated crop is grown or the land is seeded to grass. When 

 mustard, wild oats, or other weeds become very plentiful 

 in fields where small grain is grown continuously, they can 

 best be checked by growing a crop of corn or potatoes and 

 cultivating it thoroughly. This is good practice, even when 

 no weeds are present, but it might not be adopted if the weeds 

 had not compelled its use. Many weeds of meadows and 

 pastures are easily killed by cultivation. Thus the rotation 

 of crops is an efficient means of subduing weeds. 



HOW WEEDS SPREAD 



698. Agencies. Weeds spread in many ways. Some 

 weeds have few or limited means of distribution, while others 

 are provided with many agencies of dissemination. Natural 

 agencies, such as the movement of wind and water, play a 

 large part in the spread of weeds.. Animals, both wild and 

 domestic, carry the seeds from place to place. The activities 

 of man, however, are perhaps more efficient in spreading 

 weeds than any other factor. 



699. Natural Agencies. One of the most important of 

 the natural agencies by which weeds spread is the movement 

 of air currents. The seeds of many plants are so light or 

 they are provided with appendages of such a nature that they 

 are easily carried long distances by the wind. The seeds of 

 the milkweed, thistle, and dandelion spread more widely by 

 the agency of winds than by any other means. Some plants, 

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