A Technique for taking Successive Shadowgraphs of Oat Seedlings

Abstract

OWING to the great variability in growth-rate and tropic curvature of the coleoptile shown by oat (Avena) seedlings similarly treated, it is for many experimental purposes desirable to be able to take successive measurements of growth or curvature on the same individuals rather than on a fresh sample each time. Thus in experiments involving curvatures, in view of the difficulty in obtaining really straight seedlings to start with, doubtless partly due to nutation¹, it is most desirable to have initial records before treatment; it may also be useful to record the degree and kind of curvature at successive intervals after stimulation or application of growth substance. This is, however, unsafe with the usual technique, in which the shadowgraphs are taken with white light, since the exposure itself induces curvature. Schneider and Went¹ therefore constructed their ingenious “photokymograph” ; but this is only suitable for decapitated coleoptiles and for curvature but not growth measurements: it is, moreover, somewhat elaborate for routine or class use. The essence of the present technique is simply to use more-or-less panchromatic photographic paper for the successive shadowgraphs, and to make the exposures with light passed through an orange or red filter such that no phototropic or growth response can be detected even when the photographically necessary exposure is multiplied many times. These requirements have been met by the use of Kodak RP 15 “Reflex” paper and light from a tungsten filament lamp passed through a single layer of ‘Chromex 2202/C10’ orange cellulose acetate filter which has a cut-off at about 0.530 µ wave-length and a high transmission throughout the red. With a total intensity, at the position occupied by the seedlings, of 0.08 foot-candles, adequate shadowgraphs can be obtained with one minute's exposure. One hour's exposure of intact seedlings under these conditions has been found to cause no detectable growth response or subsequent curvature. It should therefore be safe to take a long series of successive records of the same seedlings.