RUST OF ANTIRRHINUM. 43 



Occurrence of Spore Stages. 



Urediniospores occur at all times on the diseased snaiidragons. In the 

 greenhouse these are normally the only tj^pe of spores produced.' Telio- 

 spores occur only rarely in New England. Many infected plants bear only 

 urediniospores, even on the advent of killing frosts. Occasionally telio- 

 spores may be found outdoors in November, occurring more often on the 

 stems than on the leaves. In November the writer placed several snap- 

 dragons bearing uredinia in wire baskets and allowed them to winter over 

 out of doors in this way. Examination the following March showed only 

 one telium on all the material. 



In the greenhouse there is no lowering of temperature to stimulate the 

 formation of teliospores, but their formation is stimulated if the host plant 

 dries out very slowly. When plants were suddenly dried out, no teliospores 

 were formed ; but when plants were gradually deprived of water, teliospores 

 were formed in five weeks. Under normal conditions of culture the 

 teUospore may be eliminated as a factor in the greenhouse. No greenhouse 

 snapdragons seen by the writer showed teliospores except those plants 

 gradually deprived of water, as above mentioned. 



Sj)ore Germination and Infection Experiments. 



The first attempts made to germinate urediniospores were not uniformly 

 successful. As it proved later, this was because the room temperature 

 at which the tests were made was above the maximum temperature for 

 spore germination. The method by which the minimum, optimum and 

 maximum temperatures for the germination of these spores were deter- 

 mined is here described. This method has been previously described by 

 the writer (Doran, 1919). 



The spores to be used were removed from infected leaves by a stream of 

 water from a pipette. In this way only mature spores were obtained, 

 while scraping with a brush or wooden instrument would also detach 

 j^oung and immature spores. The spores were shaken in distilled water 

 until they were uniformly distributed through it. Drops of this water 

 containing the spores were then placed on clean slides, and the latter 

 were placed on culture plate benches in moist chambers. These were 

 placed in biological incubators at constant temperatures. About twelve 

 hours later the germinated and ungerminated spores were counted. Most 

 of the spores which germinated did so, however, in five to eight hours. 

 Throughout these tests it was noticeable that no spores in the interior 

 of the drop ever germinated. Only those spores in contact with the air 

 as well as the water germinated, so spores in the interior of the drop were 

 not counted as being present. The distilled water used was not aerated. 

 This aerotropism was not further investigated. Throughout the tests one 

 lot of spores was always run at 10° C. The percentage of germination at 

 10° C. was taken as a standard, raised to 100, and the other percentages 

 at the different temperatures raised proportionately. This was done in 



