{9} The view that the revolving movement or nutation of the stems of twining plants is due to growth is that advanced by Sachs and H. de Vries; and the truth of this view is proved by their excellent observations. {10} The mechanism by which the end of the shoot remains hooked appears to be a difficult and complex problem, discussed by Dr. H. de Vries (ibid. p. 337): he concludes that "it depends on the relation between the rapidity of torsion and the rapidity of nutation." {11} Dr. H. de Vries also has shown (ibid. p. 321 and 325) by a better method than that employed by me, that the stems of twining plants are not irritable, and that the cause of their winding up a support is exactly what I have described. {12} Dr. H. de Vries states (ibid. p. 322) that the stem of Cuscuta is irritable like a tendril. {13} See Dr. H. de Vries (ibid. p. 324) on this subject. {14} Comptes Rendus, 1844, tom. xix. p. 295, and Annales des Sc. Nat 3rd series, Bot., tom. ii. p. 163. {15} I am much indebted to Dr. Hooker for having sent me many plants from Kew; and to Mr. Veitch, of the Royal Exotic Nursery, for having generously given me a collection of fine specimens of climbing plants.