yjH MECHANICAL LAWS OF GROWTH. 



round the support like the free apex; and therefore the irritation caused by the 

 contact extending to the portion that is not in contact produces a different form 

 of curvature consisting in a rolling up of this portion into the form of a corkscrew, 

 as shown in Fig. 456 u, zv, lu. This coiling is similar to that already mentioned as 

 taking place of its own accord in many tendrils which do not take hold of a support, 

 especially in the circumstance that the under or dorsal side of the tendril is always 

 the concave one ; but it differs from a spontaneous coiling in being always the 

 result of irritation, occurring invariably when tendrils take hold of a support, and 

 also in taking place some time (half a day to a day) after the attachment, at a time 

 when the tendril is still perfectly sensitive and growing rapidly in length ; while the 

 spontaneous coiling occurs only with the cessation of growth and of irritability. The 

 coiling which is the result of the irritation caused by contact also takes place much 

 more rapidly than that which is spontaneous ; both can be readily observed by 

 noticing older tendrils which are still straight and have not attached themselves, and 

 younger ones on the same shoot that are attached and already coiled up. The coil- 

 ing of tendrils attached to supports is therefore a result of irritability in the same 

 sense as the twining of the free portion round a support ; and it is only the physical 

 impossibility of also twining round the support that forces the portion of the tendril 

 between its base and the support to coil up like a corkscrew. The coiling of this 

 intermediate portion, like the curvature of a longer piece of a tendril in consequence 

 of the contact of a single point, is a proof that the local irritation is communicated 

 along the tendril. The whole consequence of irritation does not however end 

 with these phenomena ; for tendrils that are fixed to a support also increase sub- 

 sequently in thickness, sometimes very considerably, like the petioles of Solaniun 

 jasminoides ; they become woody, and have a longer term of life than those 

 which have coiled spontaneously, or generally than those that have not attached 

 themselves. 



There is still another point in which attached tendrils differ from those that 

 have coiled spontaneously. In the latter all the coils of the spiral run in one direc- 

 tion ; those of a tendril attached to a support have, on the contrary, points (Fig. 456, 

 w, w ) at which the direction changes ; between any two of these points is a number 

 of coils in the same direction, those beyond them being in the opposite direction ; in 

 long tendrils with close coils there are often as many as five or six of these points. 

 Darwin has already shown that this is no special property of tendrils, and still less 

 a specific result of irritation, but is rather a physical necessity; for if a body which 

 coils up is fixed at both ends so that one end is totally unable to twist, the coils 

 must necessarily be produced in opposite directions in order that the torsions 

 which are unavoidably produced may counterbalance one another. This behaviour 

 of fixed tendrils can be imitated by cementing a narrow stretched strip of india- 

 rubber firmly along another strip which is not stretched, and then releasing the 

 former ; it contracts and forms the inside of a spiral, the outer side of which is 

 formed by the strip that is not stretched. If the double strip is held at each end 

 and first stretched out straight and then relaxed, coils will be produced, some to the 

 right, others to the left, as in a tendril. If one end is now let go, the strip will twist 

 itself anew into a spiral. 



Since all the movements of tendrils that have been described are the result of 



