Dec. 23, i9i8 Parasitism of Cronartium ribicola 649 



sori, typical column development is sometimes completely inhibited 

 by the pressure of the surrounding host tissue. All internal sori are to 

 be regarded as teratological phenomena of no special morphological 

 significance. Uredinia and telia on the petiole are common; occasion- 

 ally they develop on the upper surfaces of infected leaves. 



GENERAL DISCUSSION 



The writer's observations on the mycelium of Cronartium ribicola 

 in the bark of Pinus strohus confirm the conclusions of Klebahn (25) 

 in regard to the course of the hyphae of the parasite and add consider- 

 able new data on the inter relations of the host and parasite cells. 

 Hartig's paper {ig) on Peridermium pini and Wolff's article {64) on the 

 same fungus, which appeared shortly after Hartig's and in some places 

 is almost a direct copy of Hartig, are the only papers which have come 

 to the writer's attention which describe and illustrate the morphology 

 and parasitic relations of a bark-inhabiting rust and its pine host. 

 Hartig believes that the swelling of the bark of the host is due to the fact 

 that the cortex and phloem cells are forced apart by the abundant 

 intercellular mycelium of the parasite. This has been shown to be the 

 case with the hypertrophy produced by C. ribicola in the bark of P. 

 strobus. Observations on the bark of P. parviflora Sieb. and Zucc. 

 infected with the same fungus show that the swelling is produced in 

 the same way as it is in P. strobus. The same statement holds true for 

 the swelling caused by C. comptoniae Arth. in the bark of P. sylvestris 

 L., and P. ponderosa Laws. Neither C. ribicola nor C. comptoniae are 

 gall-forming rusts. Their mycelium is confined almost entirely to the 

 region of the cortex and phloem cells of their pine hosts, although the 

 hyphae do enter the wood along the rays, as Tubeuf (38) has reported 

 for Peridermium pmi, and occasionally work their way in between 

 tracheids. These hyphae have been traced in the case of C. ribicola 

 to a depth of three annual rings, counting in from the cambium. In 

 all probability it will be found that the myceUum may be found in the 

 ray cells of the annual ring laid down at about the time infection took 

 place, although it may not remain active after the ray cells die. While 

 the presence of the hyphae in a given annual ring may not mean that 

 the pine was infected during the year this annual ring was laid down, 

 because there must be some growth along the ray cells toward the center 

 of the tree, it will establish roughly the date of infection. The simi- 

 larity which evidently exists between the morphology and method of 

 parasitism of C. ribicola and C. comptoniae suggests that a close agree- 

 ment will probably be found among all non-gall-fonning caulicolous 

 Peridermia. 



The actual injury to the pine host cells from the irritation caused by 

 the invading mycelium is apparently very slight. The cells pierced by 



