772 MECHANICAL LAWS OF GROWTH, 



a reverse position. The effect of geotropism alone or combined with heliotropism 

 would be simply to cause the petiole to curl upwards in a vertical plane ; but the 

 weight of the lamina is scarcely ever equally distributed on the two sides of the plane 

 of curvature ; one side is more heavily weighted, and causes the plane of curvature 

 of the petiole to bend obliquely to that side, and other parts of the petiole to be 

 thus exposed to the influence of gravitation and heliotropism. Complicated curva- 

 tures and torsions of the petiole and of the lamina itself are caused in this way, the 

 final result being again to reverse the lamina, so as to bring its proper upper side 

 uppermost and expose it to the light as much as possible. 



It will be seen therefore that a distinction must be drawn between two kinds of 

 torsion ; firstly, that of erect organs ; and secondly, that of organs which grow in a hori- 

 zontal or oblique position. In the former case the torsion results from internal con- 

 ditions of growth, and especially from the outer layers growing more rapidly than the 

 inner ones ; the arrangement of the internal parts — in the internodes of higher plants 

 . probably the course of the fibro-vascular bundles — determines the direction of the 

 torsion. 



Torsions of the second kind are caused in quite a different way. The outer layers 

 of the growing organ are in a state of passive tension, and there is no internal tendency 

 to torsion ; but the weight of the parts attached to it causes a torsion of the growing 

 organ, which is rendered permanent by growth and by the very imperfect elasticity 

 of the organ. 



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 grow- 

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

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

 pressured 



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 elephaiitipes, Polygonum scandens, 

 and the honeysuckle ; the greater number twine to the left, as A7'isiolochia Sipho, 

 Thunbergia fragrans, Jasumiium gracile, Convolvolus sepium^Ipomcea ptwpurea, Ascle- 

 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 



* 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.11. — Darwin, On the Movements and 

 Habits of Climbing Plants, Journal of the Linnean Soc, (Bot. vol. IX, London 1865). 



2 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, 

 which will be published in the third part of the Proceedings of the Wiirzburg Bot. Inst. The 

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



