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

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Abstract
Fronds of Asplenium bulbiferum in the "middle stages" of development, for which the summer responses to gravity stimuli had been worked out, were used to investigate the annual and diurnal rhythms of geotropism. The same stimulus, consisting in placing the frond horizontally for a "presentation time" sufficient to cause a curvature of about 5[degree] in 80% of the summer fronds, was used throughout, and the degree of curvature, % of fronds responding, and the "latent time," i.e., the time elapsing before 3[degree] curvature occurred, determined during different months. The lowest percentage responding, 20% below the summer value, associated with the greatest increases in latent time, 25% over the summer, occurred from Nov. to Jan. inclusive. The Feb. to Apr. period and Oct. fell about halfway between these and the summer values. Degree of curvature was not affected. In both summer and winter, the % responding fell by 33% at night, as compared with a 20% decrease in the day results from summer to winter. Latent time increased at night by 10%, much less than the increase of 25% found in the day winter values. Thus the mechanism of response is less affected by season than time of day, the mechanism of perception, measured by the number of fronds responding, more by time of day than season of year. Increasing the time duration of the stimulus increased the degree of curvature, decreased the latent time. These rhythms could not be correlated with growth rate, humidity nor temp. changes, nor does the curve of annual periodicity follow the annual light curve.