334 THE FUNGI WHICH CAUSE PLANT DISEASE 



rounding tissues of the seedling as these develop on germination 

 of the seed. 



Infection Experiments. Since the method of studying the 

 rusts by observing their Ufe histories in the laboratory where 

 they are imder complete control of the observer has assumed 

 such prominence of late years the technique deserves notice. 

 The first step is to find associated in the field the aecia and 

 other stages of a rust in such way as to suggest relation- 

 ship between two forms hitherto unknown to be connected. 



Material of the rust is then collected and healthy host plants 

 are also removed to the laboratory. If the teliospores are col- 

 lected in the fall they are kept out of doors in cheese cloth bags 

 till germination time in the spring. Whether collected in spring 

 or fall the viability of the spores must be tested by sowing in a 

 hanging drop of water. If germination is plentiful then the infec- 

 tion experiment is made. First the suspected alternate host is 

 sprayed with water to give the spores proper conditions for ger- 

 mination, then masses of spores are placed directly on the plant 

 by a scalpel and a bell jar is placed over the plant to assure a hiunid 

 atmosphere. In from five to eight days yellow spots should indi- 

 cate where the infection has taken place and in a short time 

 pycnia and aecia or other sori follow. In all infection work it is 

 imperative to know that the plants used be not already infected 

 in the field from another source. 



The secium is by some regarded as a structure whose function 

 is to restore vigor to the rust fungus.'"* On the other hand, 

 Freeman and Johnson '"* found that in fifty-two generations 

 of the fungus, without the intervention of aecia or telia there 

 was no apparent diminution in the vitality of the uredinial 

 generation. 



Form Genera. The telial stage is regarded as the highest 

 stage of the rust fungus and is the one on which classification is 

 often based. Thus an aecium, uredinium, caeoma, etc., that is 

 known to possess a telial form is regarded as part of the species 

 indicated by its teUospore, e. g., iEcidium berberidis being part 

 of Puccinia graminis has no specific identity but is regarded as 

 a stage of P. graminis. 



There are numerous uredima, aecia and other non-telial forms 



