334 TEXTBOOK OF BOTANY 



undesirable grasses, are distributed with hay or straw. Weed 

 seeds are carried too by harrows and other implements used 

 in tillage ; by threshing machines ; in many stock feeds ; and 

 very extensively in fresh manures. Some plants, spoken of 

 as " tumble weeds," which have short roots, are easily torn 

 from the soil when they are dead, and are blown or rolled 

 by the wind from place to place, scattering seeds as they go. 

 This class'of plants includes the buffalo bur, the horse nettle, 

 the pigweed, the ragweed, ^and the Russian thistle. 



It has already been said that perennial weeds, while they 

 may depend largely upon seeds for their distribution (consider 

 the dandelion, for example) also have the possibility of mul- 

 tiplying their numbers by vegetative means. The common- 

 est of these means is that used by the Canada thistle, 

 namely, the branching of an underground" stem, the branches 

 becoming separate plants by the death of the older parts of the 

 stem. Some of the grasses, notably quack grass, spread in 

 the same way ; and many other weeds spread by means of a 

 rapid growth of underground stems and roots. Runners, like 

 those of the strawberry, are produced by some of the hawk- 

 weeds. Tubers are formed by a few weeds ; nut grass (a 

 member of the sedge family) is a tuber-bearing weed, and the 

 Jerusalem artichoke, though not usually growing so abun- 

 dantly as to be troublesome, may be placed in the same class. 

 The field garlic is an illustration of a weed whose perennial 

 underground part is a bulb instead of an elongated stem. 



353. The Control and Destruction of Weeds. What has 

 just been said regarding the ways in which weeds spread may 

 suggest methods by which they are to be controlled. So far 

 as concerns annual and biennial weeds, and indeed many per- 

 ennial weeds also, it is plain that they will be eliminated if 

 they can be prevented from forming seeds. This can be done 

 by cultivation, which kills the weeds before they have "gone 

 to seed," or even by cutting off their tops early and frequently 

 enough. This is why many of the weeds that appear in 



