S62 MECHANICS OF GROWTH. 



Sect. 24.— The Twining of Climbing Plants^ The stems of climbing plants, 

 composed of long internodes, have the power of twining spirally round upright 

 slender supports ; and the long petioles of the Fern Lygodium possess the same 

 property. This twining is a consequence of unequal growth, of a revolving nutation. 

 It is not caused, as Mohl held, by an irritation exercised by the support on the 

 growing internodes, and is therefore essentially distinct from the twining of tendrils 

 round supports, which depends on the irritation caused by constant and permanent 

 pressure'^. 



Only a few plants twine to the right {i. e. from right to left as one looks at the 

 support round which the plant twines), following the course of the sun or of the 

 hands of a watch ; among these are the Hop, Tamus elephantipes^ Polygonum scandens^ 

 and the Honeysuckle ; the greater number twine to the left, as Aristolochia Sipho^ 

 Thunbergi'a/ragrans, Jasminium gracile, Convolvulus Sepi'um, Ipomcea purpurea, Asck' 

 pias carnosa, Menispermum canadense, Phaseolus, &c. 



The first internodes of twining stems, whether they are primary stems as in 

 Phaseolus, lateral shoots from rhizomes as in Convolvulus, or from aerial organs as 

 in Aristolochia, do not twine but grow erect without any support. The succeeding 

 internodes of the same shoot twine ; they first of all elongate considerably, while 

 their leaves grow only slowly. The long young internodes incline to one side in 

 consequence of their weight, and in this position revolving nutation begins; the 

 overhanging part curves and executes a movement which causes the terminal bud to 

 describe a circle or ellipse. This circular motion is caused entirely by the curving 

 of nutation. If a black line is painted along the convex side of an internode of a 

 plant that twines to the right, like the Hop while the bud is pointing to the south, then, 

 when the bud points to the north it will be found on the concave side ; when to the 

 west or east, on the lateral surface between the convex and concave sides. Usually 

 two or three of the younger internodes are in a state of revolving nutation at the 

 same time ; and, since they are in different stages of growth, the curvature of the 

 older internode does not generally coincide with that of the younger one ; the whole 

 does not therefore form a simple arc, but often an elongated letter S, with the 

 different parts lying in different planes. As new internodes develope from the bud, 

 they begin to revolve, while the third or fourth internode ceases to do so, becomes 

 erect, and manifests another form of movement, becoming twisted, until its growth 

 ceases ^ 



^ [L. Palm, Ueber das Winden der Pflanzen : Preisschrift, Stuttgart 1827. — Mohl, Ueber den 

 Bau und das Winden der Ranken und Schlingpflanzen, Tubingen 1827. — Dutrochet, Comptes 

 rendus, 1844, vol. XIX, and Ann. des Sci. Nat. 3rd ser. vol. II. — Darwin, On the Movements and 

 Habits of Climbing Plants, London 1875). 



^ Darwin has already attempted to show that Mohl's view of the irritability of climbing inter- 

 nodes is untenable, without however bringing forward any convincing proof. But this proof has 

 been afforded by H. de Vries in a series of investigations carried on in the Wiirzburg laboratory, 

 published in the third part of vol. I of the Proceedings of the Wiirzburg Bot. Inst. (1873). The 

 description here given of the mechanical principles is based principally on his results. 



[See also Darwin, Movements and Habits of Climbing Plants, 1875, and Movements of 

 Plants, 1880.] 



3 Torsion is therefore not the cause of the revolution of the apex of the shoot, as is seen at once 

 from the fact that the number of revolutions of torsion in the same time is different from that of the 

 revolutions of nutation. 



