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Show 500 MODIFIED CIRCUMNUTATION. CHAP. X. Fig. 186. P!talaris Canariensis: apogeotropic movement of cot yledvn, traced on a vertical and horizontal glass, from 9. 10 .A.,l\1. Sept. 19th t o 9 A .l\1. 20th. Figure here reduced to one-fifth of original scale. protrude whilst the fol· lowing observations were being made, was placed at 10° beneath the horizon and it rose only 59° i~ 24 h. It behaved rather differently from any other plant, observed by us, for during the first 4t h. it rose in a }jne not far from straight; during the next 61 h. it circumnutatod that is, it descended and again ascended in a strongly marked zigzag course; it then resumed its upward moYcment in a moderately straight line, and, with time allowed, no doubt would ha,·o hecome upright. In this case, after the first 4! h., ordinary circumnutation almost completely conq uercd for a time apogeetropism. Brassica oleracea.--The hypocotyls of several young seedlings placed horizontally, rose up ver· tically in the course of 6 or 7 h. in nearly straight lines. A seedling which bad grown in darkness to a height of 2! inches, and was therefore rather old and not highly sensitive, was placed so that the bypocotyl projected at be· tween 30° and 4-0° beneath the horizon. The upper part alone became curved CnAr. X. APOGEOTROPISM. 501 upwards, and rose during the first 3 h. 10 m. in a nearly straight line (Fig. 187); but it was not possible to trace the upward move- Fig. 187. ment on the vertical glass for the first 1 h.lO m., so that the nearly straight line in the diagram ought to have been much Jonger. During the next 11 h. the hypocoty I circumnutated, describing irregular figures, each of which rose a little above the one previously formed. During the night and following early morning it continued to rise in a zigzag course, so that apogeotropism was still acting. At the close of our observations, after 23 h. (represented by the highest dot in the diagram) the hypocotyl was still 32° from the perpendicular. There can be little doubt that it would ultimately have become upright by describing an additional number of irregular ellipses, one above the other. l.pogeotropism retarded by Heliotropism. -When the stem of any plant bends during the day towards a lateral light, the movement is opposed by a pogeotropism · but as the light gradually wane~ in the ev~ning the latter power slowly gams the upper hand, and draws Brassica olemcea: apogeotropic the stem back into a vertical movement of hypocoty I, traced position. Here then we hav on vertical glass, from 9.20 good . e a A .M . Sept. 12th to 8.30 A .M. oppor~umty for observing how 13th. The upper part of the apogeotrop1sm acts when very figure is more magnified t han nearly balanced by an opposing the lower part. If the whole force. For inst . th 1 cou~se had been traced, the of ~ ance, e P umule straight upright line would . ropreolum majus (see former have been much longer. Figlll'e Fig, .175) moved towards the dim here 1:e~u ced t o one-third of leiv emng light 1·n a sl1' g htly Zi.g zag the on gmal scale. ne until 6.45 P.M., it then returned on its course until |
