opposite side.  The hooking of the tip is of service to twining plants by aiding them to catch hold of a support, and afterwards by enabling 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 know, as this structure is not very rare with plants which do not climb, and with some climbers (for instance, Vitis, Ampelopsis, Cissus, etc.) to whom it does not afford any assistance 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 most obvious explanation is that the bending is due 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, etc., sometimes come into play.

Ampelopsis tricuspidata.—­This plant climbs by the aid of adhesive tendrils, and the hooked tips of the 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 shoots the hook is often extended horizontally or even faces upwards.  Moreover shoots frequently form loops instead of hooks; and in this case the

* ‘Sitzb. der k.  Akad. der Wissensch.,’ Vienna, Jan. 1880, p. 16. [page 273]

Fig. 122.  Ampelopsis tricuspidata:  hyponastic movement of hooked tip of leading shoot, traced from 8.10 A.M.  July 13th to 8 A.M. 15th.  Apex of shoot 5 ½ inches from the vertical glass.  Plant illuminated through a skylight.  Temp. 17 1/2o — 19o C. Diagram reduced to one-third of original scale.

extreme part, instead of hanging vertically down as would follow if weight was the efficient cause, extends horizontally or even points upwards.  A shoot, which terminated in a rather open hook, was fastened in a highly inclined downward position, so that the concave side faced upwards, and the result was that the apex at first curved upwards.  This apparently was due to epinasty and not to apogeotropism, for the apex, soon after passing the perpendicular, curved so rapidly downwards that we could not doubt that the movement was at least aided by geotropism.  In the course of a few hours the hook was thus converted into a loop with the apex of the shoot pointing straight downwards.  The longer axis of the loop was at first horizontal, but afterwards became vertical.