336 TEXTBOOK OF BOTANY 



importance in a country where trees are plentiful, because 

 there are many obstacles to their travels about the country. 

 Those perennial weeds which spread by means of underground 

 stems and roots offer a set of special problems. Usually such 

 weeds must be dealt with by thorough cultivation ; the exact 

 methods to be adopted will depend upon the nature of the 

 weeds that are making the trouble and upon the character 

 of the crop that is being raised on the same land. Even 

 though cultivation does not directly reach all the underground 

 parts of the weeds, it may nevertheless be successful in kill- 

 ing them by keeping down their top growth, because the 

 roots and underground stems are dependent upon the food 

 that is manufactured by the leaves, and will die in time if 

 this source of food is shut off. 



A proper rotation of crops, in addition to its other advan- 

 tages, offers means of dealing with weeds ; for instance, weeds 

 that become established in a field of wheat can be destroyed 

 the following year if cultivated crops be raised in the same 

 field ; or they may be largely overcome by a forage crop, 

 such as grass or alfalfa, which interferes with the growth of 

 weeds by shading them, and is cut for hay before most of 

 the weeds have ripened their seeds. It is found that some 

 weeds, especially those with rather broad leaves, such as the 

 wild mustard, horse nettle, dandelion, and Canada thistle, 

 growing in pastures or grain fields, can be killed by the use 

 of poisonous sprays. The grasses and grains have narrow 

 leaves which do not catch and hold much of the poisonous 

 solution, whereas the larger leaves of the weeds hold enough 

 of the poison to injure or kill them. The substances most 

 used in these sprays are iron sulphate, copper sulphate, and 

 common salt. Directions for weed spraying are given in the 

 bulletins of various state experiment stations. vSometimes 

 it is found advisable to smother weeds that occupy as yet but 

 a small patch by covering them ^^'ith building paper, or mth 

 straw or manure ; and many other special methods have been 



