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 mullein, 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 hav- 

 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. 



