Thigmotropic Responses Research Articles

Behaviour of uredospore germ-tubes of Puccinia graminis tritici in relation to the fine structure of wheat leaf surfaces

Studies of wheat leaf surfaces by transmission and scanning electron microscopy have revealed a regular lattice of wax crystals which overlies and entirely covers the cuticle. When uredospore germ-tubes of Puccinia graminis Pers. f.sp. trictici Erikss. & Henn. contact this lattice, they become orientated parallel to the short axis of the leaf. It is suggested that thigmotropic responses of the germ-tube to the lattice is the orientation mechanism which maximizes the probability of the hypha contacting a stoma. A technique for preparing fragile biological specimens for the scanning electron microscope is described.

Dew and the growth of the uredospore germ tube of Puccinia graminis on the wheat leaf

SUMMARYWater droplets simulating dew, formed on wheat leaves held in a condensation chamber, appeared first on the leaf hairs and coalesced into lines along the ridges of the leaf surface in close proximity to the stomata. On leaves thus wetted, germ tubes from uredospores of Puccinia graminis showed a thigmotropic response, growing at right angles to the ridges: when they reached the stomatal region the penetration of the stoma may be further assisted by chemotropic or hydrotropic attraction.Studies on plastic surfaces showed that germ‐tube growth took place only between and not within water droplets.

DORMANCY AND GERMINATION IN SEEDS OF ECHINOCYSTIS LOBATA

IN THE extensive literature on dormancy in seeds which has appeared during the last twenty-five years, no exact study of this condition in Echinocystis lobata seems to have been reported. Rose (1915) included cucumber in a list of seeds which need after-ripening but gave no suggestion as to how this process may best be brought about. Sempers (1918) reported the inducement of germination by clipping the seed coats or by freezing. This latter method was suggested to him by finding seeds germinating in December in a sand pile of the previous year with which the seeds had been thrown out. Full growth of these plants followed in the spring. Charles (1919) reported successful germination in the spring following fall planting, a practice now recommended by certain seed dealers in their catalogues. She also reported an instance where seeds which had lain on a cement floor under a shed for a year germinated without further planting. This lack of attention to seed germination of Echinocystis has been due probably to the fact that the plant has no economic use and is of little horticultural value. It is, however, a useful plant for teaching purposes. Its rapid growth makes it excellent material for studies in growth rates, while its tendrils provide exceptionally good opportunities for studies in thigmotropic responses. Its use has been greatly limited, however, by the inability to grow plants from seeds stored under ordinary laboratory conditions when needed. It has, therefore, seemed worth while to make a somewhat careful study of the factors involved in the dormancy and after-ripening of these seeds, both from the point of view of securing good material for teaching purposes and for the sake of extending our knowledge of dormant seeds in general. MATERIAL AND METHODS.-Echinocystis lobata (Michaux.) T. and G., Wild Balsam Apple, Wild Cucumber, is native in the northeastern and central United States and has been introduced into the far West. It is sometimes grown as a cultivated plant. The seeds are produced in the fall and may be shed at the time of the bursting of the turgid fruit or mav remain within the capsule until both are fully dry. Material for this study was obtained from several sources, primarily from wild material collected during successive years at Essex, Massachusetts, and from plants grown in the Botanical Garden, Smith College, Northampton, Massachusetts. No significant differences were observed in the results obtained from these two sets of material. During the latter part of the study, material obtained from Bodger Seeds, Ltd., El Monte, California, and from H. A. Dreer, Philadelphia, Pennsylvania, was used. Interesting, though possibly not important, differ-