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 ecidium phase 
of wheat rust in warm climates, 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 xcidia 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 bin 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.) 
