MECHANICAL STRUCTURES IN THE VEGETATIVE ORGANS. 385 



carrj^ng heavy trains, &c. They thus resemble the alternate " cups " 

 of Bauhinia. 



A totally different principle is introduced in the cable-like lianes. 

 The stem develops a number of superficially projecting rod-like ridges. 

 If the whole stem be now twisted, it closely resembles in appearance 

 a cable of separate strands twisted together. If a piece of string be 

 held taut, but twisted by the hand at one end, and then slackened, the 

 force put into it must be met. To do this, the string instantly twists 

 upon itself until equilibrium is established. This twisting also takes 

 place in Nature, as in some Malpighiaceous lianes. There is much 

 superficial resemblance to wire cables ; but in the latter the wires are 

 all separate. 



Tendrils. — One of the most curious methods of strengthening a 

 tendril is seen in the Cucurbitaceae and Passion-flower. In the Bryony 

 and other members of the Cucumber family, the tendril is at first very 

 long, straight and stiff, with a hooked tip. As soon as the latter has 

 caught hold of, and coiled two or three times round a twig (for it is 

 highly sensitive to touch) , it begins to bend at some point and forms a 

 loop with somewhat straight sides. This loop then rotates, and the 

 result is that the tendril coils in opposite directions on either side of the 

 straight loop or " crank," as it might be called. As a rule, there will 

 be the same number of coils either way. Sometimes a second or third 

 crank is formed, presumably as strains are felt, which must be guarded 

 against, so as to avoid any rupture of the tendril. This crank-hke 

 motion does not appear to be known to modern botanists generally. 

 Neither Darwin nor Sachs alludes to it. Darwin's figure in his 

 " Climbing Plants " (p. 165, fig. 13) of the tendril of the Bryony is 

 not quite accurate, as will be seen by comparing it with " true " cranks 

 as taken from Nature. An interesting article on the action of the 

 tendril of a gourd, describing this process of coiling round a pencil 

 hanging freely, and then producing a " crank " to produce opposite 

 coils, will be found in " Harper's Magazine," July 1907, p. 296, with 

 illustrations. The author is Mr. H. T. Shannon. 



M. Leon, as far as I know, was the first to make observations on 

 the method of reversal of the coils of the tendrils of Cucurbitaceae, 

 Passi/iora, &c. (Recherches nouvelles sur la cause du mouvement spiral 

 des tiges volubiles, par M. T. Leon. Bull, de la Soc. Bot. de Fr. 1858, 

 p. 680.) He says : — 



" I have looked to see how the coils become reversed. Having 

 observed that they are formed only after the extremity of the tendril 

 was fixed to a support, I placed the end of a tendril of a melon-plant 

 in contact with an object fixed in the soil, and by an attentive examina- 

 tion I made certain that the reverse coilings w^ere simultaneous. The 

 tendril first curves itself in a certain part, and this curvature tends to 

 become pronounced, to form even a semicircle. This arc of a circle 

 then becomes the centre of a rotary but very slow movement, which 

 insensibly twists the tendril on the two sides of this arc, so that each 

 side describes a spiral, in an opposite way." 



