100 NATURE STUDY AND AGRICULTURE 
ment. Annual weeds are more numerous than other classes. 
A list of the commonest may be found in the table that fol- 
lows. (See page 162.) 
Biennials and How to Kill Them.— Biennials do not 
thrive in cultivated fields as the annual visit of the plow 
destroys the plants before they can go to seed. In meadows, 
pastures, and waste places they may be exterminated by 
cutting them in season to prevent the formation of seeds 
for two years in succession. There are few biennials among 
our weeds. The commonest ones are the ordinary thistles, 
wild carrot, wild parsnip, and burdock. 
Perennials and How to Kill Them. — In the case of peren- 
nials, as with annuals and biennials, the first necessity is 
to prevent the production of seeds, but this alone is not 
sufficient. The entire plant must be killed, and the plow 
does that effectually except in case of those that have under- 
ground stems. Perennials of this type are the only ones 
that can flourish in cultivated fields. From these under- 
ground stems new plants persist in growing. As soon as the 
young plant appears above the surface of the soil it begins 
to make plant food and to send it down to the rootstock 
and thus replenish its strength. It is therefore necessary to 
prevent this growth by cultivation or otherwise until the 
-rootstock finally dies from starvation. It is so difficult 
to kill some of these plants that they are often considered 
the worst of all the weeds that we have. We refer especially 
to quack grass, Canada thistle, and perennial sow thistle. 
Other weeds of this type, though less harmful, are the wild 
morning glory, false sunflower, and common milkweed. 
In uncultivated ground we also find types of perennial 
weeds that are not quite so tenacious of life. The commonest 
of these is the dandelion. That has a fleshy root which gives 
