Plants as Affected by Weeds. 173 
class are commonly most tenacious of life and are therefore 
often most difficult to control. 
338. Annual and biennial weeds, since they have a defi- 
nite life period, and multiply almost exclusively by seed, 
may be effectually controlled by preventing seedage. To 
accomplish this with certainty, the plant 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. Since the underground stems or roots, whence 
the suckers grow, are hidden beneath the soil and are often 
extremely tenacious of life, weeds of this class are fre- 
quently very hard to eradicate. 
Persistent prevention of leafage, by starving the proto- 
plasm of the roots, is always a sovereign remedy, though it 
Fie. 79. Somiaie how plants of the sow thistle multiply from underground stems. 
is often extremely difficult to apply in practice, since the 
suckers of some species grow with great rapidity. Yet, asa 
whole, no better remedy is known. Frequent plowing and 
cultivation of the infested ground is usually the most effect- 
ual means of preventing leafage. 
Certain very tenacious perennial weeds, as the Canada 
thistle,* and the sow thistle,t when growing on deep, rich 
* Cnicus arvensis. + Sonchus arvensis. 
