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Show 49-! MODIFIED CIRCUMNU'f AT ION. CIIAP. X. tation and heliotropism, wo had the groat ad vantage of Loing able to lesson tho light ; but with geotropism analogous experiments wore of cour. e impossible. vVe could, however, oLs rvo tho movements of stems placed. at first only a little from the pcr1Jen<licular, in which case geotropism did not act with nearly so much power, as when the stems wore 1orizont;1] an<l at right angles to the force. Plants, also, were se]cetcd which were but feebly geotropic or apogootropi<~, or Lad become so from having grown rather ohl. Another plan was to place the stems at first so that they pointed 30 or 40 degrees beneath the horizon, and then apogeotropism had a great amount of work to <lo before the stem was rendered. upright ; awl in this cu. e ordinary circumnutation was often not wholly obliterated. Another plan was to observe in the evening plants which during tho day bad become greatly curved heliotropically; for their stems under t]w gradually waning light very slowly became n prigh t t hrongh the action of apogeotropism; and in this case modified circumnutation was sometimes well displayc<l. Apoyeotropism.-Plants wore seleeted for observation almost by chance, excepting that they were htkon from widely different families. If the stem of a plant which is even moderately sensitive to apogeotropism be placed horizontally, the upper growing part bends quickly upwards, so as to become perpendicular; and the line traced by joining tho <lots suecessiv~ly made on a glass-plate, is generally almost straight. li'or Jll· stance, a young Gytisusfmgr-ans, 12 inches in hejght, was pln.c?d so that the stem projected 10° beneath tho horizon, n.nc~ Jts course was traced during 72 h. At first it bent a very httle downwards (Fig. 182), owing no doubt to tho weight of the stem, as this occurred with most of tho other pbnts observetl, thouo·h as they were of course circumnutating, the short dowu· 0 ' • h ward lines were often oblique. After throe-quarters of an our the stem began to curve upwards, quickly d.uring the first. two hours, but much more slowly during the afternoon and mght, CHAP. X. APOGEO'l'ROPISM. 495 and on the following day. During the second night it fell a little, and circumnutatod during tho following clay; but it also moved a short distance to the right, which was caused by a little light having boon accidentally admitted on this side. The stem was now inclined GQ0 above the horizon, and had therefore risen 70°. With time allowed it would probably have become upright, and no doubt would have continued circumnutating. The sole 1·emarkable feature in the figure here given is the straightness of the course pursued. The stem, however, did not move upwards at an equable rate, and it sometimes stood almost or quito still. Such periods probably represent attempts to circumnutate in a direction opposite to apogeetropism. The herbaceous stem of a l'erbena melindres (?) laid horizontally, roso in 7 h. so much that it could no longer be observed on the vertical glass which stood in front of the plant. The long line which was traced was almost absolutely straight. After the 7 h. it still continued I I ! Fig. 182. I to rise, but now circumnutated Cytisusj1·ag1·ans: apogeotroJ•ic mow-slightly On th f ll · d ment of stem from 10° beneath to . · e 0 owmg ay 60° above horizon traced on Yer- It stood upright, and circum- tical glass, from S.30 A.l\1. l\1arch n~tated regularly, as shown in 12th to 10.30 1'.111. 13th. The suiJFig. 82, given in the fourth sequent circumnutating movement ch te is likewise shown up to 6.45 A.l\l. ap r. The stems of several on the 15th. Nuctumal cour~e othe~ .Plants which were highly represented, as u~ual, by a Lroken sen~1tlve to apogeotropism rose line. .Thlonment not greatly magup m almost straig·ht lines, and nifie<.l, and tracing re<.luceu to two-thirds of original scale. |