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Show 270 MODIFIED CIUCUMNU'l'A 'l'IO~. CnAr. v_, young leaf was traced dUl'ing 4~ (layA, a1:u th? tracing hero gi_ven (Fin-. 121) shows that it desccndo<l at firf.lt m a nearly strmght b line, hut aftcrwar<ls zigzagged, Fig. 1'3 t. r mn.k:i11p; one or two little loops. t The diverging and descending moYcmenh; of a rather old 'r leaf were alHo tmcetl (Reo former Fig. ll0, p. 251): ' it descended during the first day an<l night in a some- ! what zigzag line; it then cir- \ ~~:::; 11 ~~~cl a~;~i~~l(~lc~:e;~~~:i~ .. <J ~;u~~i:. s~=~d ~~: fi!1°~;r Jl~~~ / .// tion, and now pbinly circum-nutatcd. AH in the ca .. eof the C[trnation, the len.ves, whi!Rt very yom1g, do not Rcem to be much affected by geotropism or hel:iotropiRm, for thoRc on a young plant bid horir.ontally, ,..__/ al1<l thoHe on another plant G·\, 1 ft upright, bo 1 th kc1~1t in tl.te dark, continuet to ( 1vergc m i the usual manner without fi bending to either side. / With ColHI'a scnndens, the i young leaves, as they succes· ~, ~ivcly <livergc from the lead-ing shoot which is bent to one side, rise up so as to pr~ jcct vertically, and they rct.am this position for ome t~me < whilst the tendril is rovolvlil~g. Tho divercring and ascenc ng movement~ of tho petiole of .?:_ one such o.leaf, were tlaceJ on Pitw$ pinaster : epinastic U.ownwar!l d sky· 1 f a Ver·tica.l glass un er a movement of a young :a ' pro- ed duce<.l by a young plant m a pot, light; and the course pursu traced on a vertical gla~s untlcr a was in most parts nearlY skyliO'ht from ti.45 A.M. June 2nd straight, but there were two to 10~401 P.M. 6th. CHAP. v. EPINAS'l'Y AND IIYPONAS'l'Y. 271 well-marked zigzags (ono of them forming an angle of 112°), and this indicates circumnutn.tion. The still closed lobes of a young leaf of Dionroa projected at right angles to the petiole, and were in tho act of slowly rising. .A glass filament was attached to tho under side of tho midrib, and its movements were traced on a vertical glass. It circumnutatcd once in tho evening, and on the next day rose, as already described (sec Fig. lOG, p. 240), by a number of acutely zigzag lines, closely approaching in character to ellipse. . 'l'his movement no doubt was <lue to epinaRty, aided by apogeotropism, for the closed lobes of a very young loaf on a plant which had been placed horizontally, moved into nearly tho same line with tho petiole, as if tho plant had stood upright; but at tho same time the lobes curved laterally upwards, and thus occupied. an unnatural position, obliquely to tho plano of tho foliaceous petiole. As the hypocotyls and epicotyls of somo plants protrude from the seed-coats in an arched form, it is doubtful whether tho arching of these parts, which is invariably present when they break through the ground, ought always to bo attributed. to epinasty; but when they are at first straight and afterwards become arched., as often happens, the arching is certainly due to epinasty. As long as the arch ifl surrounded by compact earth it must retain its form; but as soon as it rises above the surface, or even before this period if artificially freed from tho sunounding pressure, it begins to straighten itself, and this no doubt is mainly due to hyponasty. 'rho movement of tho upper and lower half of the arch, au<l of the crown, was occn,sionally traced ; and the course was more or loss zigzag, showing modified circumnutation. With not a few plants, especially climbers, the summit of the shoot is hooked, so that the apex points vertically downwards. In seven genera of twining plants* the hooking, or as it has been called by Sachs, tho nutation of the tip, is mainly due to an exaggerated form of circumnutation. 'l'hat is, tbe growth is so great along one side that it bends tho shoot completely over to the opposite side, thus forming a hook; the longitudinal line or zone of growth then travels a little laterally round tho shoot, a.nd the hook points in a slightly different direction, and so onwards until the hook is completely rcvcrscc1. Ultimately it * 'The Movements and I;Ia.bits of Climbing Plants,' 2nd edit. p. 13. |