10 MASS. EXPERIMENT STATION BULLETIN 368 



discolored areas. The vessels were clogged with gummy, brown tyloses through- 

 out the infected areas. The result of infection of the host by the fungus is, there- 

 fore, a typical vascular mycosis (Fig. 6). 



With the possibility in mind that the fungus might be spread by spores within 

 the vessels, measurements of the width of vessels and tracheids in the elm wood 

 were made, in the early summer of 1935. It was found that in the spring wood the 

 average width of the vessels, between the two inner walls, was 32.5 microns, 

 while the corresponding measurement of the tracheids was 20.6 microns. These 

 figures were computed from measurements made of 35 tracheids and 35 vessels. 

 The widths of these elements were found to be relatively constant, so that it 

 was not considered necessary to make a greater number of measurements. Since 

 the average size of the spores was found to be 4.6 by 11.4 microns, the figures 

 arrived at, together with observations of the fungus spread in experimental trees 

 also reported in this paper, seemed to justify the supposition that the fungus 

 might easily be spread inside the tree by means of spores. 



In a paper entitled "The Distribution of Spores of Wilt-Inducing Fungi 

 Throughout the Vascular System of the Elm by the Sap Stream," which he 

 presented at the meetings of the American Association for the Advancement of 

 Science in December 1936, Banfield (3) arrived at a similar conclusion after ex- 

 periments regarding the spreading of spores within elms. 



INFECTION EXPERIMENTS 



Cultures Used 



Spore suspensions were made from fungus cultures by pouring sterile water 

 into the petri dish containing a culture and transferring the spore suspension 

 thus formed to sterile test tubes by means of sterile pipettes. From this suspen- 

 sion hanging drops were prepared in sterile water and in dextrose solution; the 

 suspension was also smeared on thin discs of potato dextrose agar in Van Tieghem 

 cells under sterile conditions. All of these preparations were kept in sterile 

 petri dishes with distilled water in the bottom to prevent the drying out of the 

 media. The spores in the dextrose solution and those in the sterile water germi- 

 nated in 72 hours (Fig. 7). The growth was very slow and ceased after a few days. 

 There was no evidence of branching hyphae from these spores. On potato dex- 

 trose agar the spores produced hyphae in 24 hours, and normal branching growth 

 followed (Fig. 7). 



Spores which were incubated on agar in darkness germinated readily. Spores 

 which were incubated on agar where it was light during the daytime, but where 

 exposure to direct sunlight was limited to a short time each day, showed no 

 hyphal growth during a period of two weeks. Spores which had already germinat- 

 ed were exposed to the above conditions at the same time, and at the end of two 

 weeks the mycelium had grown profusely and extended to the edge of the Van 

 Tieghem ring. The spores which had been incubated in the light for two weeks 

 without germinating were then placed in the dark and incubated there for ten 

 days, but no hyphal growth resulted. 



In an effort to determine the thermal death-point of the spores, 5 cubic centi- 

 meter portions of the spore suspension previously described were placed in sterile 

 test tubes and subjected to heat in water baths at various temperatures ranging 

 from 50° to 80° C. for ten minutes each. The tubes were then cooled rapidly and 

 the contents poured into petri dishes containing potato dextrose agar, and in- 

 cubated for five days. There was a marked decrease in the percentage of spores 



