WHITE-PIiS^E BLISTER RUST. 51 



R. vulgare by Marshall. None were found by Unger (17G) on the upper surface of 

 leaves of E. alpinum nor by Taylor on R. americanum, R. aureum, R. carrierii, R. 

 culvenvellii, R. fasciculatum, R. nigrum, R. reclinatum, R. speciosicm, and R. (tenui- 

 fiorum) aureum. Stomata were found only on the lower surface of cotyledons of R. 

 fasciculatum and R. missouriense, the only ones examined by Taylor. Stomata were 

 found by Taylor to be not imcommon on petioles of R. americanum, R. aureum, R. 

 carrie^'ii, R. culverwellii, R. hirtellum, R. inerme, R. nigrum, R. odoratum, R. reclina- 

 tum, R. sanguineum, R. speciosum, R. succirubrum, R. (tenuiflorum) aureum, and R. 

 vulgare. None were found on petioles of R. curvatum and R. fasciculatum. Janczewski 

 (60) states that stomata are present on the young stems of R. petraeum. A few stomata 

 were foimd on young stems of R. aureum., R. hirtellum, R. nigrum, R. odoratum, R. 

 reclinatum, R. succirubrum., R. (tenuiflorum) aureum, and R. vulgare. None were found 

 on the stems of R. carrierii and R . fasciculatum. These findings compare well with 

 the inoculation results, if stomata are the avenue for infection. It is perhaps to be 

 expected that infection may be produced on the upper surface of the leaves, but only 

 very rarely. Colley (20) found young uredinia forming in the substomatal spaces, 

 which would indicate that infection took place in that ^dcinity and presumably 

 through the stomata. York ^^ found germ tubes of seciospores entering the stomata of 

 leaves of Ribes cynosbati, R. glandulosurfi, and R. nigrum. 



VARIATIONS IN APPEARANCE ON RIBES LEAVES. 



The study of great numbers of Ribes leaves infected in the green- 

 house and of very numerous specimens of diseased leaves collected 

 in the field from Maine to Minnesota, during the past ten years, has 

 revealed some distinct variations in the appearance of the fungus 

 and of the diseased leaves of different species and varieties of Ribes. 

 Previous study of such differences seem to have been made chiefly 

 by Hennings (52, 53). 



BLISTERY APPEARANCE OF THE UREDINIA. 



In August, 1916, there occurred a very hot, dry period in New 

 England. This was followed by the finding of a few very peculiar- 

 looking uredinia on Rihes nigrum and R. vulgare. Under a hand lens, 

 the uredinia were not of the usual mealy appearance, but looked more 

 like tiny blisters. Examination showed that the epidermis of the 

 leaves had become toughened, so that the uredinia did not burst 

 through it, as they usually do, but pulled it loose from the inner leaf 

 tissues and in this way actually formed small blisters. The uredinio- 

 spores never broke through it. From the weather conditions pre- 

 ceding and at the time this occurred it is believed that the dry, hot 

 weather rendered the epidermis tougher than usual. It is a rare 

 occurrence, as only a few cases have been noted. In 1916, one speci- 

 men came from each of the States of Massachusetts, Maine, and Ver- 

 mont. In 1917, a single case was found in New Hampshire. Hedg- 

 cock noted this in 1919 upon artificially inoculated Ribes bushes on 

 Block Island. Recently, blisters have been noted on R. aurucn) and 

 R. fasciculatum in the greenhouse. 



32 VTork, H. H. Op. cit. 



