WEEDS 



335 



312. How weeds injure the farm and garden. 1 Although 

 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 following ways : 



1. Weeds take 

 moisture needed 

 by useful plants. 



2. Weeds rob 

 the soil of val- 

 uable salts, such 

 as nitrates and 

 potash compounds, 

 and it is probable 

 that they may add 

 secretions that are 

 injurious. 



3. Weeds shade 

 other plants, thus 

 weakening them 

 by hindering pho- 

 tosynthesis. 



4. A few para- 

 sitic weeds, like 

 the clover dodder 

 (fig. 243), rob 

 their hosts of 

 plant food. 



5. Some weeds 

 harbor parasitic 

 fungi or insects 

 (such as the potato 

 beetle) which are 

 injurious to use- 

 ful plants. 



FIG. 242. The common ragweed (Ambrosia 

 artemisiaefolia) 



The group of flower clusters at the left and the leaf at 

 the right are considerably reduced, and the central plant 

 is much reduced. The slender stem is characteristic of 

 individuals grown in the shade; plants grown in the 

 sunlight are much shorter and very robust 



1 See also Bergen and Caldwell, Practical Botany. Ginn and Company, 



Boston. 



