Studies in the geotropism of pteridophyta VI—On rhythm in graviperception and reaction to gravity

Abstract

Recurrent phenomena in living things such as the yearly fall of the leaf, or the daily opening and closing of certain flowers, is a matter of common observation. Many other less obvious cases have been disclosed in biological research; such as the annual opening and closing of lenticels, and the diurnal periodicity of secondary thickening (Kaiser, 1879) and of osmotic pressure (Ursprung and Blum, 1916); but so far as I am aware, annual rhythm has not been demonstrated for any form of irritability, and neither annual nor diurnal rhythm for any tropism. The present work attempts to show that the perception of, and response to, gravity are not of the same intensity at all periods, but vary both with the time of day and season of the year. During the course of a series of researches on geotropism in the Pteridophyta the first of which was communicated to the Royal Society in 1921 (Prankerd, 1922) the impression was gained that the plants were less responsive during the winter, and it was decided to investigate the matter quantitatively for the fern whose behaviour to gravity was best known, namely Asplenium bulbiferum . The work was performed in daylight, partly because a dark room at constant temperature for long periods of time was impracticable, but more especially because it was desired to keep the plants under perfectly natural conditions, and thoroughly healthy. That they were so was shown by their general appearance, growth, and reproduction. Precautions against heliotropic curvature were taken as fully explained previously (Prankerd, 1922). The temperature of the research greenhouse in which the plants were grown was usually near 20° C, and all experiments recorded in this paper were performed at 20° ± 1°, and at a relative humidity of about 80%. Hence, with the exception of a some­what lower average temperature at night, and of course less daylight in the winter, every external condition to which the plants were subjected would seem to have been the same throughout the year, and their behaviour as far as could be seen, for example, in growth and reproduction, was fairly uniform. Previous work has shown that the irritability of the frond to gravity varies with its development (Waight, 1923). It is very low in the early stages, requiring long periods of stimulation, and as response is also slow, experiments are very lengthy. In its later stages the frond is somewhat uncertain in its behaviour, and more sensitive to light stimulus. The middle stages, i.e ., “late infant” and early “adolescent” ( see below), have therefore always been employed. They have been more fully studied, are the most reactive, and are more constant, at least than the later stages.