'/72 MODIFIED CIRCUMNUTATION. CHAP. V. 



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 surface, 

 and ultimately back again on the convex surface. In the case of 

 Lonictra brachypoda the hooked terminal part of the revolving 

 shoot straightens itself periodically, but is never reversed ; thftt 

 is, the periodically increased growth of the concave side of the 

 hook is sufficient only to straighten it, and not to bend it over 

 to the 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, &c.) to whom ib 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 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, &c., sometimes come 

 into play. 



Ampelopsis tricu?pidata. 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 straight- 

 ening 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 Wisseiiseh.,' Vienna, Jan. 1880, p. 1G. 



