NEGATIVE HELIOTROPISM OF UREDINIOSPORE GERM‐TUBES

Abstract

During the course of some germination tests with urediniospores of Puccinia Rhamni (Pers.) Wettst. in the spring of I9I3 an apparent negative heliotropic reaction by the germ-tubes was seen. The spores had been sown in a drop culture exposed to a unilateral illumination on a window sill and a high per cent. of the tubes had grown directly away from the light. Subsequent tests with controls in darkness substantiated the first observations. A search of the literature brought forth a single mention of a heliotropic reaction in germinating rust spores. Ward,2 referring to a series of germination studies with urediniospores of Puccinia dispetsa, has written: My reasons for varying the direction of incidence of the light in certain cases were based on some results (as yet inconclusive) that the germ-tubes exhibit heliotropic curvatures. That the sporidial germ-tubes of Puccinia malvacearum react negatively to daylight has been shown recently by Robinson,3 but aeciospore germ-tubes of Puccinia Poarum were found to be indifferent. Germ-tubes of conidia of Botrytis sp. also grew away from light but those of other non-rust fungi tested Alternatia sp., Penicillium glaucum and Peronospora parasitica, were indifferent according to Robinson. During the past fall, 1914, the study of the effect of light on germinating urediniospores of Puccinia Rhamni was again taken up. Urediniospores were obtained at first from the field and stored in gelatin capsules and later from cultures on oat plants grown in the greenhouse. The urediniospores of this species are especially suitable to daylight tests on account of their quick germination and the rapid growth of their germ-tubes. In the tests made the germ-tubes averaged in growth once to twice the spore length in as many hours and four to six times in three to four hours.