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Show 264 1\!0DIFIED CIRCUMNUTATION. CIIAI'. v. vVe shall now consider the many cases in which c·u.c umn utation has been modified fodr v.a rious spocit~.l pm.p o ses ,· that is ' a movement alrca. y m. :progress 1s temporarily increased in s~mc one chrc~twn, and ~em-porarily diminished or quite_ ~IT st?d m other .chrcc~ t . These cases may be chvH.l dm two suL-clttsses, lOllS. . in one of which the modification dcpcn<ls on.mnate or constitutional causes, and is im.lepcndcnt of external conditions, excepting in so far that the proper ones for growth must be present. In the sccon<l sub-class the modification depends to a large e~tcnt on. external <tgencies, such as the daily alt rnatwns of hght ~nd darkness, or light alone, temp raturc, or tho attr~ct10n of gravity. The first small sub-class will bo cons~derecl in the present chapter, and the second sub-clttss m the remainder of this volume. THE Cmcul\INUTA'riON OF CLil\IBING PLANTS. The simplest case of modified circumnutation ~s that offere l by climbing plants, with t~c xceptwn °~ those which climb by the <1i<l of motwnlos~ ho~ks 01 of rootlets: for the moc.lific<ttion consists cluofly 11~r~e greatly increased amplitude of the movement. 1~ would follow either from grct~tly incr asod growth o:ei a small lcncrth, or more probt~bly from moderately m· creased growo th spread over a cons1e. ,l erc.. t ble lono'th of the b . movincr orcran preceded by tnrg sconce, and actt~g sue· cessivebl y o0 n a'l l si• des. rr ho c•u cumnut ·a .t " n of climbers 10 . is more regular than t h at of or dm. ar.y plants.·' b.ut ill 1 't almost every other respect there is a clos~ simi an y between their movements, namely, in thou te1ndc~cty to describe ellipses directed successi.v e1 Y to .a l p.o mt .s of the compass-in their courses being often m erli ruptecl by zi. gzag lm. e , tn.a ng· 1 es, loor)s ' or sma CRAP. v. CLIMBING PLANTS. 2G5 ellipses-in the rate of movement, and in different species revolving once or several times within the same length of time. In the same internorlc, the movements cease first in the lower part and then slowly upw~rds .. In both sets of cases the movement may be modified m_ a clo~cly analogous manner by geotropism an~ by h~hotro1nsm; t~ough few climbing plants are heliotropic. Other pomts of similarity mio·ht be pointed out. b ~hat th_e movements of climbing plants consist of ?rdmar~ c1rcu~nutation, modified Ly being increased m amphtude, Is well exhibited whilst the plants are very !oung ; for at this early age they move like other seedlmgs, Lut as they grow older their movements gradually increa~c without undergoing any other change. That tlus power is innate, and is not excited by any extern_al agc~10ies, beyond those necessary for gr?wth and VIgour, IS obvious. No one doubts that t~Is P?wer has been gained for the sake of enabling chmbmg plants to ascend to a hei<rht, and thus to reach the light. ~rhis is effected by t~vo very different methods; first, by twining spirally round a support but to do so their stems mu t be lono· and flexible ~ and, secondly, in the case of leaf-climbc~·s and tendril~ bearers ' bY .b n : ngm· g t h esc organs I·n to contact with a support, wluch is then seized by the aid of their sensitiveness. It may be here remarked that these !atter movements have no relation, as far as we can Judge, ~ith circumnutation. In other cases the tips oftendnls, after having been brought into contact with adhsupport, become developed into little discs which a ere firmly to it. 1 We h~ve said that the circumnutation of climbino· ~~ants drffers from that of ordinary plants chiefly b; 1 8 greater amplitude. But most leaves circumnutate |