{84} Never having had the opportunity of examining tendrils produced by the modification of branches, I spoke doubtfully about them in this essay when originally published. But since then Fritz Müller has described (Journal of Linn. Soc. vol. ix. p. 344) many striking cases in South Brazil. In speaking of plants which climb by the aid of their branches, more or less modified, he states that the following stages of development can be traced: (1.) Plants supporting themselves simply by their branches stretched out at right angles—for example, _Chiococca_. (2.) Plants clasping a support with their unmodified branches, as with _Securidaca_. (3.) Plants climbing by the extremities of their branches which appear like tendrils, as is the case according to Endlicher with _Helinus_. (4.) Plants with their branches much modified and temporarily converted into tendrils, but which may be again transformed into branches, as with certain Papilionaceous plants. (5.) Plants with their branches forming true tendrils, and used exclusively for climbing—as with _Strychnos_ and _Caulotretus_. Even the unmodified branches become much thickened when they wind round a support. I may add that Mr. Thwaites sent me from Ceylon a specimen of an Acacia which had climbed up the trunk of a rather large tree, by the aid of tendril-like, curved or convoluted branchlets, arrested in their growth and furnished with sharp recurved hooks. {85} As far as I can make out, the history of our knowledge of tendrils is as follows:—We have seen that Palm and von Mohl observed about the same time the singular phenomenon of the spontaneous revolving movement of twining-plants. Palm (p. 58), I presume, observed likewise the revolving movement of tendrils; but I do not feel sure of this, for he says very little on the subject. Dutrochet fully described this movement of the tendril in the common pea. Mohl first discovered that tendrils are sensitive to contact; but from some cause, probably from observing too old tendrils, he was not aware how sensitive they were, and thought that prolonged pressure was necessary to excite their movement. Professor Asa Gray, in a paper already quoted, first noticed the extreme sensitiveness and rapidity of the movements of the tendrils of certain Cucurbitaceous plants. {102} Fritz Müller states (ibid. p. 348) that in South Brazil the trifid tendrils of Haplolophium, (one of the Bignoniaceæ) without having come into contact with any object, terminate in smooth shining discs. These, however, after adhering to any object, sometimes become considerably enlarged. {111} Comptes Rendus, tom. xvii. 1843, p. 989. {113} Diagram showing the movement of the upper internode of the common Pea, traced on a hemispherical glass, and transferred to paper; reduced one-half in size (Aug. 1st)