Plants as Affected by Weeds. 179 



to soon overcome most cultivated plants, but for the aid 

 of the cultivator. As with harmful insects and fungi, 

 prompt and persistent efforts are essential to the control 

 of weeds in most cultivated grounds. 



337. Annual, Biennial and Perennial Weeds. With ref- 

 erence to their term of life, weeds and other plants are 

 divisible into three classes, viz., annual, those that liv^e 

 but one season; biennial, those that live only two seasons j 

 and perennial, those that live an indefinite number of sea- 

 sons. Weeds of the first class usually seed most abund- 

 antly, and hence they are most widely distributed and 

 appear in cultivated grounds in the greatest numbers. 

 Those of the third class are commonly most tenacious of 

 life and are therefore often most difficult to control. 



FIG. 80. Showing how plants of the sow thistle multiply from under- 

 ground stems. 



338. Annual and biennial weeds, since they have a defi- 

 nite life period and multiply almost exclusively by seed, 

 may be controlled by preventing seedage. To accom- 

 plish this with certainty, the plants should be destroyed 

 before bloom, as many species possess enough reserve 

 food to mature seeds sufficiently for germination, if cut 

 while in flower. 



339. Perennial weeds often multiply by suckers as well 

 as by seeds (Fig. 80). Since the roots or underground 



