384 NINETEENTH REPORT. 



Whether these differences in spore size and parasitic habit are sufficient 

 to differentiate species or sub-species^ must be left for further work. 

 With abundant material from Europe and America, and by cultural and 

 infection studies this problem could readily be solved. For the present, 

 having recorded the observed differences, the writer retains the old name, 

 Macrosporium sarcinae forme Cav., for the etiological factor in this dis- 

 ease. 



INFECTION PHENOMENA. 



How the fungus enters the host: 



The following method was used in determining how the fungus enters 

 the host: A leaf attached to the living plant was inserted through the 

 opening in the stage of the microscope from which the sub-stage had 

 been removed. One of the leaflets was clamped in place by means of 

 clips and a small drop of spore suspension placed upon its surface. The 

 germination of the spores could thus be observed under the low power 

 of the microscope. A fresh drop of water was added from time to time to 

 prevent drying. 



Usually germination begins within two to three hours after inoculation. 

 Witliin ten to fourteen hours the germ tubes have attained a length of 

 four to six times that of the spore, after which, penetration begins. This 

 is best observed by focusing the high power directly into the drop, using 

 for illumination light from a small arc directed at the sub-stage mirror. 



The germ tube appears to enter between the epidermal cells. A few 

 cases of stomatal entrance have been observed, but this is not character- 

 istic. 



Where it was desirable to examine more closely the means of penetra- 

 tion, the leaf was treated in the following manner: The spores were 

 permitted to dry down on the leaf and a piece of the tissue in which 

 penetration had occurred was fixed in 95% alcohol for one hour. This 

 treatment removed the chlorophyll and made the tissue almost trans- 

 parent. The material was tlien stained in eosin, 5-10 minutes, cleared 

 in plienol-turpentine, and mounted in balsam. The germ tubes were 

 stained a deep pink and the leaf tissue a lighter shade of the same color. 

 Tissues stained in this manner showed plainly the mode of entrance of 

 the germ tube, and the mycelium which has already begun to grow 

 through the leaf tissue could be easily observed. 



Infection Experiments: 



In performing these experiments three general methods of inoculation 

 were used. These will be referred to by number in the subsequent dis- 

 cussions, where convenient. 



