TWINING OF TENDRILS. 



Ill 



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 in contact with the support increases for some time, 

 then remains stadonary, 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 

 curves easily is sensitive only on the concave 

 under surface ; others, as those of Cobsea 

 and Cissiis discolor, are sensitive on all sides ; 

 in Muiisia Clematis the under and lateral sur- 

 faces are sensitive, but not the upper surface. 



While the revolving nutation and sensi- 

 tiveness 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. in Bignonia, 

 Vitis, 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 Cardio- 

 spermum and Mutisia) or more often a helix 

 narrowing conically upwards (as in Cucur- 

 bitacese, Passifloreaa, &c.) in which state they 

 then dry up and become woody. 



These processes must however be con- 

 sidered as abnormal, the tendrils having 

 failed of performing their purpose of coming 

 into contact, by means of their revolution, 

 with a support during the period that they 

 are sensitive and still in a growing state. 

 If this contact takes place on the sensitive 

 side, a curvature arises at the spot, and the 



tendril clings to the support ; fresh sensitive spots are thus constantly brought 

 into contact with it, and the free apex twines firmly round the support in a larger 

 or smaller number of coils (Fig. 456), The nearer the spot where contact first 

 takes place to the base of the tendril the larger are the number of revolutions 

 round the support, and the stronger the attachment ; though even a small number of 

 revolutions is sufficient to attach it with considerable force. The portion of the 

 tendril between its base and the point of attachment is obviously unable to twine 



Fig. 456.— Coiling of a tendril oi Bryonia dioica. B a 

 portion of the branch from which the tendril springs by 

 the side of the petiole h and the axillary bud k; the 

 lower part of the tendril u is straight; the upper part x 

 has coiled round a twig 4; the long intermediate 

 part between the rigid basal portion ii and the point of 

 attachment x has coiled spirally, and thus raised the 

 branch B ; -w lu' the two spots where the direction of the 

 coil is reversed. 



