776 



MECHANICAL LAWS OF GROWTH. 



scandens the anterior (upper) part of the pinnate leaf is transformed into slender 

 filiform tendrils inclined forwards, while the basal part of the leaf is rigid and divided 

 into leaflets ; sometimes, as in Lathyrus Aphaca, the whole of the leaf is replaced 

 by a filiform tendril. The morphological character of the tendrils of Cucurbi- 

 taceae is still doubtful, though they must probably be regarded as metamorphosed 

 branches. 



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 Cucurbitacese, 

 Ampelidea3, and Passifloreae. A typically developed tendril of this kind is repre- 

 sented in the mature state in Fig. 456, 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. 



Fig. 4SS.— Mode of climbing of Tropcrolum minits. The long petiole a of the leaf I is sensitive to 

 long-continued contact, and has clung round a support and round the stem of the plant itself j/ so as to 

 fix this stem firmly to the support ; z the shoot from the axil of the leaf. 



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 ; i. 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 was 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 



^ This and what follows is from Darwin, Movements and Habits of Climbing Plants, p. 100 



et seq. 



