322 PRACTICAL COURSE IN BOTANY 



is, of many forms. The habit is very common among the 

 lower forms of vegetation. The fact that one or more of 

 the phases are sometimes omitted, as the secidium phase 

 of wheat rust in warm cHmates, suggests the idea that it 

 may be of use in helping the plant to tide over difficult 

 conditions. Besides giving better chances of obtaining 

 nourishment, it probably has the same effect as cross fer- 

 tilization among flowering plants, in imparting increased 

 strength and vitality to the succeeding generation. Wheat 

 rust produced from barberry secidia is said to be much more 

 vigorous — and consequently more destructive — than when 

 derived from a uredo that has reproduced itself for several 

 generations. 



366. The damage done by rust to the host is through the 

 destruction of its tissues by the mycelium. The chlorophyll 

 is destroyed so that the plant can no longer manufacture 

 food, and is too starved to produce good grain. There are 

 many varieties of wheat rust, which have been found on 

 twenty-seven different kinds of grain. Most of them are 

 specialized to a particular host plant and will not, ordinarily 

 (364) , infest any other. Has this fact any bearing upon the 

 production of rustproof varieties ? 



Practical Questions 



1. Is a farmer wise to leave scabby and mildewed weeds and bushes 

 in the neighborhood of his grain fields? (364, 365.) 



2. Are there any objections to the presence of volunteer grain stalks 

 along roadsides and in fence corners during winter? (364.) 



3. Should cedar trees be allowed to grow near an apple orchard ? Give 

 a reason for your answer, (p. 317, Material.) 



4. Should diseased plants be plowed under ? (361.) 



5. What disposition should be made of them? 



6. Ought diseased fruits to be left hanging on the tree? 



7. Why is it necessary to pick over and discard from a crate or bm all 

 decaying fruits and vegetables? 



8. Does a rotation of crops tend to prevent the spread of disease in 

 plants ? Give reasons for your answer. 



9. Are rustproof varieties to be relied on indefinitely ? (364.) 



