OCR Text |
Show 424 MODIFIED CIRCUMNU'rATION. CHAP. VIII. similar to the previous one. In these two cases there could be no doubt that the ordinary circumnutating movement of the hypocotyl was modified and rendered heliotropic. Fig. 171. D'pun. B. 1fT :MJ'a.m. A. Ap!O. s gra1·eolens: heli. otropi. c movement an d c·u cumnu ta' tion of hth e. . hyptoal-cotyls of two seedlings towards a dim lateral light, t1:aced on a ori:O~lnal glass during the day The broken lines show the1r return noc ~~ ced courses. Height of h;pocotyl of A ·5, and of B ·55 inch. Figure re u to one-half of original scale. Brassica oleracea.-The hypocotyl of tl1 e ca bba g e' whe.n nteodt disturbed by a lateral light, circumnutates in a compllca CHAP. VIII. HELIOTROPISM. 425 manner over nearly tho same space, and a figure formerly given is here reproduced (Fig. 172). If the hypocotyl is exposed to a moderately strong lateral Hght it moves quickly towards this side, travelling in a straight, or nearly straight, line. But when the lateral light is very dim its course is extremely tortuous, and evidently consists of modified circumnutation. Seedlings were placed before a north-east window, protected by a linen and muslin blind and by a towel. Tho sky was cloudy, and whenever the clouds grew a little lighter an additional muslin blind was temporarily suspended. The light from the window was Braasica oleracea: ordinary circumnutating movement of the hypocotyl of a seedling plant. thus .so much obscured that, judging by the unassisted eye, the seedlings appeared to receive more light from the interior othf e the room than from the window·' but this was not really case, as was shown by a very faint shadow cast by a pencil on 8 card. Nevertheless, this extremely smaJI excess of light sotno oodn e sid.e caused the hypocotyls ' which in tho morning bad upr1ght, to bend at right angles towards the window so that in th · . ' t d e evenmg (after 4.23 P.M.) thmr course had to be ~~~ on a vertical glass parallel to the window. It should bo d k that at 3.30 P.M., by which time the sky had become m~~r, ~~e towel was removed and replaced by an additional n hnd, which itself was removed at 4 P.M., the other two |