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Show 272 1\iODIFIED CIRCU:M:NUTATION. C HAP. Y. comes back to the point whence it started. This was ascertained by painting narrow lines with Indian ink along the convex surface of several hooks, and the line was found slowly to be· come at first lateral, then to appear along the concave surf:1ce and ultimately back again on the convex surface. In the ca ·e of Lonice1·a brud1yporla the hooked terminal part of the revolving shoot straightens itself periodically, but is 11ever revcrRed; that is, the periodically increased growth of the concave Ritle of t.ho hook is sufficient only to straighten it, and not to bend it over to i.he opposite side. The hooking of the tip is of service to twining plants by aiding them to catch bold of a support, and afterwards by cnauling this part to embrace the support much more closely than it could otherwise have done at first, thus preventing it, as we often observed, from being blown away by a strong wind. Whether the advantage thus gained by twining plants accounts for their summits being so frequently hooked, we do not lmow, as this structure is not very rare with plants which do not climb, and with some climbers (for instance, Vitis, Ampelopsis, Cissus, &c.) to whom it does not afford any assist-ance in climbing. With respect to those cases in which the tip remains always bent or hooked towards the same side, as in the genera just named, the mogt obvious explanation is that the bending is clue to continued growth in excess along the convex side. Wiesner, however, maintains"' that in all cases the hooking of the tip is the result of its plasticity and weight,- a conclusion which from what we have already seen with several climbing plants is certainly erroneous. Nevertheless, we fully admit that the weight of the part, as well as geotropism, &c., sometimes come into play. .Ampr:lopsis t?·icuspidata.-This plant climbs by tho aiel of adhel:'ive tendrils, and the hooked tips of tho shoots do not appear to be of any service to it. The hooking depends chiefly, as far as we could ascertain, on the tip being affected by epinasty and geotropism; the lower and older parts continually straightening themselves through hyponasty and apogeotropism. We believe that the weight of the apex is an unimportant element, because on horizontal or inclined sl1oots the hook is often extended horizontally or even faces upwn,rds. Moreover shoots frequently form loops instead of hooks; and in this case the * 'Sitzb. dcr k Alwd. dcr WisSE'nsch.,' Vienna, Jan. 1880, p. 16. CHAP. v. EPINASTY AND HYPONASTY. extreme part, instead of banging vertically down as would followifweightwas the efficient cause, extends horizontally or even points upwards. A shoot which terminated in a rathm: open hook, was fastened in a highly inclined downward ~sition, so that tho concave Fi~~:. 122. 1fr.50'a.mt4 Side faced upwards, and the result was that the apex at first curved upwards. This apparently was due to cpinasty and not to apogeotropism, for .~.~~·1/ the apex, soon after passing the. perpendicular, curved so mpidly downwards that wo . could not doubt that the move- ! ment was at least . d d : geotropism In the al e by i £ · course of I a ew hour.s the book was thus / converted mto a loop with the I apex of the shoot . . f : straight d pomtmg : i ownwards Th t )t : longer axis of th 1 • e y : first h . e oop was at trr ..top."ufi~'- omontal, but afte ·- ! tWhiasr ds became ver t.I cal. Durin1g :. same time th b i of the hook e asal part ! ofth I (and subsequently f Up , e doop) curved itself slowly ! .. ar s· andth' I bee n who' ll d 1s must h avo :: tropism in [ u~. to apogeo- : nast ppos1t10n to hypo- I y. The loop w fastened ups'd d as then IJ'p .m. its bao~l h lf1 e own, so that - ~o·.1.s• OIU a would b · • a.m. taneously acted e Simul-· nasty (if pres t)on by hypo- fil :JO'<t.mt.~rhj geotropism . end and by a po- A mpe zo ps1·s tr£cuspidata . h itself s ' an now it curved movement of hooked t·. yfpona~tfl· o greatly shoot t. 1the upwards i ' P o leaunw laced from 8 10 <" course of onl n 13th to 8 A.M. 15th . A.M: July there COuld h d Y 4 h. tha.t 5~ inches from th . A_P~x of shout that both £ ar ly be a doubt Plant illuminatetl ctlv~J tical g las . orces were acting light. Tcmp.1 ·7~o-19~~~g~.a skyreduced to one-third ofo 1.1:g m. aJ lasgeranJ Jcn. T 272 llfl''11 ,: .:.i.f.: : jP; :~J ,..1.,, :: ~:' |