^66 MECHANICS OF GROWTH. 



The distinguishing properties of tendrils are more perfectly developed the more 

 exclusively they serve as organs of attachment for the sole purpose of climbing, the 

 less therefore they partake of the normal character of leaves or parts of the stem ; in 

 other words, the more perfectly the metamorphosis is carried out. To this category 

 belong especially the simple or branched filiform tendrils of the Cucurbitaceae, 

 Ampelidege, and Passifloreae. A typically developed tendril of this kind is repre- 

 sented in the mature state in Fig. 487, after it has seized hold of a support by its 

 apex and then coiled up. What is said here refers especially to true tendrils of 

 this description. 



The characteristic properties of tendrils are developed when they have com- 

 pletely emerged from the bud-condition, and have attained about three-fourths of 

 their ultimate size. In this state they are stretched straight; the apex of the shoot 

 which bears them usually revolves, the tendril itself exhibiting the same phenomenon, 

 curving along its whole length (with the exception usually of the oblique basal 

 portion and the hooked apex) in such a manner that the upper side, the right side, 

 the under -side, and the left side become in turn convex. No torsion takes place. 

 During this revolution the tendril is rapidly growing in length and is sensitive to 

 contact; z.e. any contact of greater or less intensity on the sensitive side causes 

 a concave curvature first of all at the point of contact, from which the curvature 

 extends upwards and downwards. ' If the contact is only temporary, the tendril 

 again straightens itself. The degree of sensitiveness ^ is very different in different 

 species ; in Passiflora gracilis a pressure of i milligram is sufficient to cause curva- 

 ture in a very short time (25 sec.) ; in other species a pressure of 3 or 4 milligrams 

 is required and the curvature does not take place so soon (30 sec. in Sicyos)\ the 

 tendrils of other species curve, when slightly rubbed, in a few minutes ; in the case 

 of Dicenira thalictrifolia in half an hour ; in Smilax only after more than an hour ; 

 in Ampelopsis still more slowly. 



The curvature on the side which has been touched increases for some time, 

 then remains stationary, and finally (often after some hours) the tendril again 

 straightens itself, in which state it is once more sensitive. A tendril the apex of 

 which is slightly curved is sensitive only on the concave under surface ; others, as 

 those of Cobcea and Ctssus discolor, are sensitive on all sides; in Mutisia and 

 Clematis the under and lateral surfaces are sensitive, but not the upper surface. 



While the revolving nutation and sensitiveness last the tendril attains its full size 

 in a few days; the revolving motion then ceases, and with it the sensitiveness; and 

 further changes then follow, differing in different species. In some the tendrils 

 remain straight after they have completely developed and become motionless ; in 

 others they become abortive and fall off, as e.g. Bignonia, Vilis, and Ampelopsis. It is 

 more common for the tendrils to roll up from the apex slowly to the base, when 

 growth has ceased with the concave side undermost, so that they at length form 

 a spiral (as in Cardiospermum and Mutisia) or more often a helix narrowing conically 

 upwards (as in Cucurbitaceae, Passifloreae, &c.), in which state they then dry up and 

 become woody. 



^ This and what follows is from Darwin, Movements and Habits of Climbing Plants (1875), 

 p. 171 et seq. 



