APPENDIX. 407 



which were scattered irregularly over the diseased area. By the end of 

 .) une these elevations had increased considerably in size and in a few instances 

 the overlying epidermis had been ruptured so as to expose to view the cream- 

 colored mass of spores, which, however, soon became dark-colored. During 

 July, August, and September these acervuli became more and more abund- 

 ant and by the beginning- of October a very large proportion of them had 

 burst open for the purpose of discharging their spores. Spores which were 

 collected late in .Tune were immature and could not be induced to germinate. 

 Others which were gathered in July were also mostly immature, but in Octo- 

 ber I obtained an abundant supply of mature spores which germinated very 

 readily. The mass of spores in each acervulus can be easily seen with the 

 unaided eye, but the individual spores are so .small that they can only be 

 seen by the aid of a good microscope. They average about six by twenty-four 

 microns and are single celled, hyaline or with a greenish tinge, elliptical, 

 curved or geniculate and coarsely granular. Sections through a mature 

 acervulus show, under the microscope, a sub-epidermal stroma from which 

 arise comparatively long, closely compacted basidia, on the ends of which 

 the spores are born. It is the growth of this underlying mass that finally 

 ruptures the epidermis over it and thus sets free the spores. 



HOW THE FUNGUS VS^ORKS. 



As Stated above, the spores mature, and the acervuli burst open to set 

 them free in late summer and early fall. Thus exposed, the spores are 

 doubtless distributed by the rains and winds and possibly to some extent by 

 birds, insects and other agencies. A vast majority of the spores thus dis- 

 tributed undoubtedly fall in uncongenial places and fail to develop ; but 

 occasionally one lodges in a suitable place on the bark of some limb. We 

 found in our work that such spores germinated readily at a temperature of 

 22° c. ( 72° F. ), but that at a temperature of 29° c. i 84° F.) germination was 

 indefinitely delayed. It therefore seems certain that the spores do not 

 germinate during the summer, when the delicate germ tube would be killed 

 by the extreme heat and by lack of moisture ; but as we have seen that 

 mature spores are present in immense quantities early in October, and prob- 

 ably considerably earliei", it is fair to assume that they start to germinate 

 soon after the cool fall rains begin. Whether the mycelium of the germin- 

 ating spores penetrates the cuticle of the apple bark or whether it gains 

 access to the inner tissues through some slight crevice has not been detei^- 

 mined as yet. However, after gaining access to the living tissues the mycel- 

 ium ramifies through them, absorbing the nourishment upon which it grows, 

 and killing the surrounding cells. During the winter, as previously stated, 

 the growth of the fungus and the consequent spread of the disease is slow, 

 but in spring the mycelium takes on a renewed activity, which is shown by 

 the rapid spread of the disease. In May or early in June, the fungus reaches 

 the fruiting stage and from that time all its energies are devoted to the pro- 

 duction of spores and the diseased areas cease to spread. Whether the 

 mycelium, having accomplished the object of its existence, the accumula- 

 tion of nourishment for the production of spores, then dies, or whether it 



