470 PKACTICAL BOTANY 



434. How weeds injure the farm and garden.^ Althougli 

 some weeds are of use as food for man or the lower animals 

 and a few have medicinal properties, their presence in the 

 farm or garden is on the whole most harmful in the follow- 

 ing ways : 



(1) Weeds take soil moisture needed by useful plants. 



(2) Weeds rob the soil of valuable salts, such as nitrates 

 and potash compounds, and it is probable that they may add 

 secretions that are injurious. 



(3) Weeds weaken other plants by shading them, thus hin- 

 dering photosynthesis. 



(4) Parasitic weeds, like the flax dodder and the clover 

 dodder (Fig. 351), rob their hosts of plant food. 



(5) Some weeds harbor parasitic fungi or insects injurious 

 to useful plants. 



(6) Poisonous or intoxicating plants injure horses, cattle, 

 and sheep. 



(7) Some spiny plants, such as the smaller cacti, and burs 

 like the sand bur, may lame the feet of domestic animals. 

 Thorny shrubs are very troublesome to woolgrowers, pulling 

 out much wool, and burs greatly injure the quality of the fleece. 



(8) Certain weeds, when eaten by cows, render milk un- 

 palatable or ill-scented. 



(9) Weed seeds injure the quality and affect the price 

 of clover and other seeds that are raised for sale, and thus 

 diminish the value of the grain with which they are mixed. 



The harm done by weeds in shading crops is most notice- 

 able in the case of rapidly growing species which spring up 

 among delicate seedlings such as flax and onions. In extreme 

 cases the \\^eeds may almost ■ entirely prevent the growth of 

 the crop. 



The most important example of fungi harbored by weeds 

 is that of wheat rust on barberry bushes (Sect. 233). The 

 potato beetle feeds on many plants of the Nightshade family, 



1 See Bulletin 175, "A Suaond Weuil Manual," Ohio Agr. Exp. Sta., for a 

 fuller discussion. 



