242 PASTORAL AND AGRICULTURAL BOTANY 



beneficial, because it destroys the weeds, as water-robbers, and secondly, 

 it forms a dust mulch. 



3. Weeds absorb the mineral and other food materials upon which the 

 crop brought, into competition with weeds, depends. If the weeds are 

 plentiful, very considerable amounts of food substance are removed 

 from the soil and are thus not available to the planted crop. 



4. Certain weeds like the morning-glory and bindweed prostrate the 

 cultivated plants by climbing up them and by their weight causing the 

 growing crop plants to fall over. 



5. The weedy plants form fruits and seeds, which become mixed with 

 those of the growing economic plants, and are difficult to remove in the 

 cleaning of such seeds for market. Corn-cockle becomes mixed with 

 wheat, chickweed with alfalfa seeds and the like. Such weed seeds are 

 impurities and lower the agricultural and^commercial value of such 

 economic seeds. 



6. Weeds, as the host plants of injurious crop insects, harbor the 

 insects from planting time to planting time of the crop plants on which 

 they prey. The injurious Colorado beetle, which destroys the potato, 

 lives on- the hedge mustard, thistles, goose-foot and other weedy herbs. 



7. Weeds are injurious because they harbor parasitic fungi. The 

 finger-and-toe organism of the turnip lives on the charlock as a weed. The 

 white rust of cabbages (Cystopus candidus) is found on the shepherd's 

 purse. 



8. Weeds interfere with the proper cultivation of the soil, as their 

 presence mechanically obstructs the use of plow and harrow. 



9. It has been suggested without proof that weeds may poison the 

 soil, so as to inhibit the growth of other plants. This relation has not 

 been properly investigated. 



10. The roots of weeds penetrate the interior of the drains causing a 

 stoppage, which can be removed only with difficulty. 



IT. Weeds are unsightly and objectionable because of their smell, 

 rankriess and prickly fruits, etc. 



12. Weeds are injurious to man and the domestic animals because 

 they are poisonous. This topic has been dilated upon in the earlier chap- 

 ters of this book and need not be discussed here. 



13. Weeds sometimes render hay and other harvested crops of less 

 financial value, and frequently reduce the yield, so that the financial 

 returns may be such that the crop is grown at a serious monetary loss. 



