RELATIONS OF THE FUNGUS TO THE HOST. 41 



Asalroiidy iiidicatod, tlu> writer's work with spruyssooiiit'd toshow that 

 not more than a small porcciita.of of each year's intVctioiis ordinarily 

 arise from a perennial mycidiuni. In the Lovell orchai'd. wheie the 

 personally condiu-ted work was carried out, it would appear that not 

 to exceed '2 to 3 per cent of the infections could have arisen fi-om that 

 cause. On the other hand it would seem that at least 1)5 per cent of 

 the infections arose from spores, for, as already stated, i>5 to 98 per 

 cent of the spring infections could be prevented ])y a sinoflc spraying, 

 and this was actually accomi)lished where the spraying was done with 

 sufHcient thoroughness. It is believed by the writer, however, that 

 these percentages will vary within moderate limits in difl(>rent locali- 

 ties, with diti'erent varieties, and in ditferent seasons. The following 

 observations will explain these views. 



The mycelium of diseased leaves is found to be connected thi-ough 

 the leaf petiole with the mycelium of the infected limb. From the 

 writings of Sadebeck and many others it might be sui)posed that the 

 leaves were infected from the perennial mycelium in a majority of 

 cases, and that the mycelium met with in the petiole of the leaf origi- 

 nated from the perennial mycelium of the ])ranch. That such spring 

 infection really occurs from thc^ wintering mycelium of th(> branch 

 should perhaps ])e admitted, but that such is the connnon mode of 

 infection of the leaves is certainly dou))tful. The writer's studies have 

 shown that the mycelium in the ])ranch close to a cluster of infected 

 leaves diminishes in amount as it passes upward or downward in the 

 branch from such leaves. This fact is as obvious from microscopic 

 studies of the infested tissues as from the external hypertrophies 

 observable to the eye. A macroscopic examination of diseased and 

 swollen branches will show that the enlarged parts may extend upward 

 or downward along the branch from the base of the petioles of the 

 leaves, which seem to represent the center of infection. In a majority 

 of cases these swollen ridges terminate before reaching another leaf 

 bud, though in some instances they are seen to extend along the branch 

 throughout the entire length of one or more internodes, and in such 

 cases it is fair to suppose that the mycelium may have infected the 

 young leaves of a second or third bud in its course. It should be 

 remembered, however, that this mycelium, in a great majority of 

 instances, indicates no connection with a previous year's mycelial 

 growth, but has evidently just entered the branch from one or more 

 infected leaves. The microscopic evidence supports these conclusions, 

 which are, to some extent, in harmony with Bt^nton's observations, to 

 be hereinafter considered, but the writer is scarcely prepared to admit 

 the large percentage of spring infections arising from new mycelium 

 entering the branch which the observations of that writer seem to 

 imply. ^ The microscope shows that the hyphiv which pass away from 



' Pacific Rural Press, Aug. 2, 1890, p. 88. 



