WEEDS AND PLANT DISEASES 201 



In this way plants may be obtained that are immune 

 from certain diseases. 



QUESTIONS 



1 . What is a weed ? 



2. What are the most common weeds in your locality? 

 Which of these are regarded as the worst pests ? 



3. In what ways are weed seeds carried from place to place? 



4. What are some of the most common plant diseases? 



5. What methods are used to destroy or prevent plant dis- 

 eases ? 



PRACTICAL EXERCISES 



1. Dig up a burdock in order to study the whole plant. 

 Examine the leaves and the roots. Do grazing animals eat 

 the leaves ? Does cutting off the stem at the top of the ground 

 kill the plant? How are the seeds distributed? 



2. Make a study of the dandelion. Count the number of 

 seeds in a ripe dandelion head. Through how many months 

 of a year do dandelions bloom ? How are the seeds distributed ? 

 Where are the leaves placed? Dig up several plants and 

 examine the roots. What advantages have dandelions that 

 make them persistent weeds ? 



3. Collect the seeds of a large number of weeds and keep them 

 in small bottles. 



4. Bring to school specimens of plants with such diseases as 

 potato scab, smuts of grain crops, pear blight, downy mildew 

 of the grape, peach leaf curl, apple scab, apple rust, brown rot 

 of the peach, black knot of the cherry and plum, " pod spots 

 of beans, etc. 



5. Kill the smut spores that may be present in seed oats 

 by the use of formaldehyde. In a barrel place 40 gallons of 

 water and add one pint of formaldehyde of standard strength 



