RUSTS 285 



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 aecidium 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. Blackberry, anemone, groundsel, buckthorn, 

 and many other common plants are known to harbor 

 secidia, but what particular species of uredo and puccinia 

 and aecidium belong together in any one case, it is impos- 

 sible to determine without continued observation and 

 experiment. 



408. Difference between Polymorphism and Alternation 

 of Generations. — These two processes must not be con- 

 founded. A polymorphic plant, so far as we know, may 

 reproduce itself indefinitely by means of simple spores 

 without the intervention of gametes and oospores, but to 

 constitute an alternation of generations there must inter- 

 vene somewhere in the life history the union of two unlike 

 spores (gametes) to form an oospore, with the alternate 

 appearance of sporophyte and gametophyte. 



PRACTICAL QUESTIONS 



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

 in the neighborhood of his grain fields? (407.) 



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

 along roadsides and in fence corners during winter ? (405.) 



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

 Give a reason for your answer. (406.) 



4. Should diseased plants be plowed under? (402.) 



5 . What disposition should be made of them ? 



6. Ought diseased fruits 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 ? 



FIELD WORK 



The study of fungi can be carried on only to a very limited extent 

 without the use of a compound microscope, and all serious work of the 

 kind must be conducted in the laboratory. The general observer, how- 

 ever, may do some practical work by learning to recognize the various 



