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Show 268 MODIFIED CIROUMNU'l'A.TION. CllAP. v. along the lower side of a part, w~ich is thus cn.nsed to bend down wards; and h yponusty 1s used for tho Iovorsed process, by which th.e r)art is mad.e . to bcnrl npwards. 1 h These actions come 1nto play _so 1 1 fr cquent ~ t at 'rth1e f the above two terms is h1gh y convcmcnt. e use o . l'l' 11' ts th,·1 movomen ~ s 1'nduccd result from . a Jnt)( . llOl 1orm of circumnutation; for, as we shall 1:nmcrlw.toly see, • ,·1 an organ ~ ncler the inf. luence. of epmasty docs not general l y move in a stra1ob ·ht lme d.o wn wa.rds, or1 u 1nd er that of hyponasty upwards, but _osmlhttcs up am ( ~wn 'th lateral movement: 1t moves, however, m a w1 some . . 'rl . h preponu_, erant manner in one chr . twn. ns s obw s that there is some gr wth on all sides of tl:o part, ut more on the upper side in th case of qnnasty, an(l more on the lower side in that of hypon<tsty, than ~n the other sides. At tho same time there may be m acl u_,.lt .1 0n, as De Vr·ies insists ' incr asecl grow. th on one side due to geotropism, and on another ~Hlc d.ue to heliotropism ; and thus tho off ct of cpmasty or of hyponasty may be either incr.utso~l or lc._s nrrl. . He who likes, mn.y speak of onlmary cucumnntatwn as beino- combined with epinasty, hyponasty, tho effects of gravitation, light, &c. ; but it seems to us, ftrotm reasons hereafter to be g1· von, t o b e m or · c corr·o c o say that circumnutation is modified by these sev~ra1 agencies. We will therefore speak of circnmllU~<\twn, which is always I· n progress, ash~oc1 · fied by epmasty, l , whether hyponasty, geotropism, or ot er agC'ncics, internal or extcrnal. f opinasty is that One of the commonest and simplest cases 0 · d' 1 ' together offereJ by lon.ves, whi. ch at an carl Y abn· o 'a r.c cldro w. o<S achs first round the buds, and diverge as they grow _0 1 erl. 0' thoupper remarked that thi.s was d ue t o m. ci.e ,a s· od "e"r r ow t l a onb 1 . 1own w ' d D V .. os has now s side of the petiole and blade; a_n ° n d aiJocl slightly by more detail that the movement 1s thus cause ' CHAP. v. EPINASTY AND HYPONAS'l'Y. 2G9 the weight of the leaf, and resisted as he believes by apogootropism, at least after the leaf has somewhat diverged. In our observations on the circumnutation of loaves, some were selected which were rather too young, so that they continued to diverge or sink downwards whilst their movements were being traced. This may be seen in the diagrams (Figs. 98 and 112, pp. 232 and 249) representing the circumnutation of the young leaves of Acantlws mollis and l'ela-l'gonium zonale. Similar cases were observed with Drosera. The movements of a young leaf, only J} inch in length, of Petunia violacea were traced during four days, and offers a better instance (Fig. 111, p. 248), as it diverged during the whole of this time in a curiously zigzag line with some of tho angles sharply acute, and during the latter days plainly circumnutated. Some young leaves of about the same age on a plant of this Petunia, which had been laid horizon tally, and on another plant which was loft upright, both being kept in complete darkness, diverged in the same manner for 48 h., and apparently were not affected by apogeotropism; though their stems were in a state of high tension, for when freed from the sticks to which they had been tied, they instantly curled upwards. The leaves, whilst very young, on tho leading shoots of the Carnation ( Dia1dlws ca1·yophyllus) are highly inclined or vertical ; and if the plant is growing vigorously they diverge so quickly that they become almost horizontal in a day. But they move downwards in a rather oblique line and continue fOl' some time afterwards to move in the same direction, in connection, we presume, with their spiral arrangement on tho stem. Tl10 course pursued by a young leaf whilst thus obliquely descending was traced, and the line was distinctly yet not strongly zigzag ; the larger angles formed by tbe successive lines amounting only to ~35o~ 154°, and 163°. The subsequent lateral movement (shown m Fig. 96, p. 231) was strongly zigzag with occasional circumnutations. Tho divergence and sinking of the young leaves of ~his P!ant seem to be very little affected by geotropism or heliotropism; for a plant, the leaves of which were growing rather slowly (as ascertained by measurement) was laid horizontally, and the opposite young leaves diverged from one anot~er ~Ymmetrically in the usual manner, without any upturnmg m the direction of gravitation or towards the light·. The needle-like leaves of Pinus p ·inaster form a bundle whilst Y?ung; afterwards they slowly diverge, so that those on tbe upright shoots become horizontal. The movements of one such • I / .. .. :·:J , .. |