870 MECHANICS OF GROWTH. 



the question arises whether the difference in growth of the two surfaces of the tendril 

 can reach to any given amount or not. The difference in length between the two 

 surfaces caused by unequal growth has, in fact, a limit, as is shown by experiment. 

 The slender tendrils of Passiflora gracilis twine firmly round threads of silk ; the 

 thick tendrils of the Vine on the other hand twine only round supports which are at 

 least from 2 to 3 mm. thick. The most strongly curved tendril of a Vine which 

 I could find had twined firmly round a support 3*5 mm. thick, and in a nearly circular 

 coil ; the mean thickness of the tendril at this spot was 3 mm. The concave surface 

 of a coil was nearly 11 mm., the convex outer surface nearly 29 mm. long, the 

 proportionate length of the two surfaces therefore nearly as i : 2*6. If this tendril 

 3 mm. thick were forced to twine round a support only 0*5 mm. in thickness, an 

 almost circular coil would have on the concave surface a length of i-6mm., on the 

 convex surface a length of 20*4 mm.; the relative length of the two surfaces would 

 therefore be as i : 13; and it does not seem possible for growth to cause so great 

 a difference in length between the two surfaces of a tendril. If, on the other hand, 

 the problem were to cause a tendril 0-5 mm. thick to twine firmly round a support 

 of the same thickness in nearly circular coils, it would only be necessary that 

 the inside of a coil should be i*6mm., the outside 47 mm, long, or that the pro- 

 portion between the two surfaces should be as i : 3. 



In order for a tendril to attach itself firmly to a support, it is not sufficient that 

 its coils should merely be in contact with it; they must be firmly appressed to it. 

 That this is actually the case is seen when a tendril is made to twine round a smooth 

 support, and the support is then withdrawn ; when, as de Vries has shown, the coils 

 become at once narrower and increase in number. This fact shows also that a 

 tendril which is irritated by contact with a support endeavours to form coils the 

 radius of whose curvature is less than that of the support, provided the support 

 is not too slender nor the tendril too thick. 



The cases are very instructive, in reference to the pressure which the coils of 

 tendrils exercise on their supports, where leaves are embraced by strong tendrils, and 

 are folded and compressed by them. 



What has now been said is merely intended to draw attention to the more important 

 mechanical principles which must be taken into account in the twining of tendrils. 

 The biology of climbing plants and of those furnished with tendrils, so fertile in extra- 

 ordinary adaptations, cannot be gone into in detail. On this subject the reader will 

 find in Darwin's treatise quoted above a mass of beautiful observations most admirably 

 described. 



Since the physiological function of tendrils is to take hold of supports (generally 

 other plants) in order to allow the slender-stemmed plant which is furnished with them 

 to climb up, the point of greatest importance is for the tendril to be brought into con- 

 tact with a support. This is usually effected with extraordinary perfection by the 

 revolving nutation not only of the tendril itself but also of the apex of the shoot that 

 bears it at the time when it is sensitive, thus causing every object anywhere within reach 

 of the tendril which could be used as a support to be brought almost inevitably into 

 contact with it. The apex of the shoot which bears the tendril usually describes an 

 ascending elliptic helix, the revolution being completed in from one to five hours. As in 

 the case of twining stems, a strong positive heliotropism would be injurious, as it would 

 often carry the tendril away from the supports. Some tendrils appear in fact to be 

 not heliotropic (those of Pimm according to Darwin), in others a weak positive helio- 



