340 INTRODUCTION TO BOTANY 
very different one the next year) often rids the ground of the 
most troublesome weeds. Such weeds as wild oats (fig. 246), 
foxtail grass, and wild mustard are very injurious in fields 
of the small grains; they do little damage in cornfields, and 
changing the crop from wheat to corn for a year or two helps 
to reduce these weeds. Many of the worst weeds in grasslands 
and pastures, such as the common sorrel (fig. 241), wild car- 
rot, wild parsnip, buttercup, moth mulein, common mullein, 
orange hawkweed, oxeye daisy, and yellow daisy, do little 
harm in cornfields. At times these weeds and many others 
become very harmful in grasslands. If such grasslands are 
plowed, and a cultivated crop, as corn or potatoes, is grown 
for one or two years, these weeds may be effectively removed. 
PROBLEMS 
1. Name five of the worst weeds of cornfields in your region; five 
of fields of small grains; five of grasslands. Which, if any, of these 
weeds are not natives of this country? 
2. What is the most troublesome weed in the gardens that you 
know? Why? What is the best method of destroying it? 
3. Give an example of a weed that thrives best in wet soil; of one 
that will grow in very dry soil; of one that is little injured by trampling ; 
of one that is so offensive to grazing animals that it is never eaten by 
them; of one that is not killed by being uprooted and left exposed. 
4. Describe the way in which the seeds or fruits of ten common 
weeds are dispersed. 
5. Which of these kinds of seeds are very likely to be bought mixed 
with many weed seeds: corn, ordinary grasses, wheat, clover, beets? 
Explain. How is the difficulty to be avoided? remedied? 
6. Give an example of a weed that is troublesome in spite of hay- 
ing no very efficient means of seed dispersal. 
7. Try to give some reasons for the fact that a majority of our 
worst weeds are of foreign origin — largely European. 
8. Explain why rotation of crops, such as plowing a mowing field 
and seeding it to corn, tends to destroy weeds. 
9. Give instances of useful plants of the farm or garden that you 
_have found growing like weeds among other crops. 
