| OCR Text |
Show 432 MODIFIED OIRCUMNU'I'ATION. CUAP. VIII. ellipse was completed botwocn 3 P.M. and about 5.30 P.M., the hypocotyl still bending towards the light. The hypocotyl was straight and upright in the morning, but by 6 P.M. its upper half was bowed towards the light, so that the chord of the arc thus formed stood at an anglo of 20° with the perpendicular. After 6 P.M. its course was reversed through the action of apogeotropism, and it continued to bend from the window during the night, as shown by tho broken line. On tho IJoxt day it was kept in the dark (excepting when each observation was made by the aid of a taper), and the course followed from 7 A.M. on the 8th to 7.45 A.M. on the 9th is here likewise shown. The diiTcrencc between tho two parts of the figure (177), namely, that described during the daytime on the 7th, when exposed to a rather dim lateralligh~, and that on the 8th in darkness, JS striking. The difference consists in the lines during the first day having been drawn out in the direction of the light. The movements of tho other seedling, traced under the same circumstances, Fig. 178. were closely similar. . Apheliotropism.-We succeeded .m observing only two cases of apheliotropism, for these are somewhat rare; and the movements are generally 80 slow that they would have been very Bi{Jnonia capreolata: aphc- troublesome to trace. f liotropic movement of a N gan o tendril, traced on a hori- JJignonia capreolata.- 0 or ds zontal glass from 6.45 any plant, as far as wo have s?en, ben do A.M. July 19th to 10 A.M. away so quickly from the ll?ht as 20th. Movernent11 as the tendrils of this Bignoma .. They originrtlly traced, little aro also remarkable from circum· magnified, here reduced 1 than to two-thirds of the nutating much less regular Y .. original scale. most other tendrils, often r~mm~ng stationary; they dopend on apheli·o tropi·s m for commg mto CHAP. VIII. APHELIO'I'lWPISl\f. 433 contact with the trunks of trees.* The stem of a young plant was tied to a stick at tho base of a pair of fino tendrils, which projected almost vertically upwards; aucl it was placed in front of a north-east window, being p1·otected on all other sides from the light. The first dot was made at 6.45 A.M., and by 7.35 A.l'rr. both tendrils felt tho full influence of the light, for they moved straight away from it until 9.20 A.M., when they cir~umnutated for a time, still moving, but only a little, from the light (see Fig. 178 of tho left-hand tendril). After 3 P.M. they again moved rapidly away from tho light in zigzag lines. By a late hour in the evening both had moved so fu1·, that they pointed in a direct line from the light. During tho night they returned a little in a nearly opposite direction. On the following morning they again moved from the light and converged, so that by the evening they had become interlocked, still pointing from the light. Tho right-hand tendril, whilst converging, zigzagged much more than the one figured. Both tracings showed that the apheliotropic movement was a modified form of circumnutation. OyclamenPersicum..-Whilst this plant is in flower thepeJuncles stand upright, but their uppermost part is hooked so that the flower itself hangs downwards. As soon as the pods begin to swell, the peduncles increase much in length and slowly curve downwards, but the short, upper, hooked part straightens itself. Ultimately the pods reach the ground, and if this is covered with moss or dead leaves, they bury themselves. We have often seen saucer-like depressions formed by the pods in damp sand or sawdust; and one pod (' 3 of inch in diameter) buried itself in sawdust for threo-qua.rters of its length. t We shall have occasion hereafter to consider the object gained by this burying process. The peduncles can change tho direction of their curvature, for if a pot, with plants having their peduncles already bow~d downwards, be placed horizontally, they slowly bend at nght angles to their former direction towards the centre of the earth. We therefore at first attributed the movement to geotropism; but a pot which had lain horjzontally with the pods /0lim'.Th~ Movements and Habits 0 bmg Plants,' 1875, p. 97. t The peduncles of several f~~er speoie~ of Cyclamen twist ~lvcs ltlto a spire and accordtng to Era&mus Dar~in (' Bo-tanic Garden,' Canto., iii. p. 12G), the pods forcibly penetrate the em·tlt. See abo Grenier and Oodron, • Flore do France,' tom. ii. p. 4fi9. 2 F |
