863 
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 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^ 
Thunbergia fragrans, Jasminiiim gracile, Convolvulus Sepium, IpomcBa purpurea, 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 
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, lieber den 
Bau und das Winden der Ranken und Schlingpflanzen, Tübingen 1827. — Dutrochet, Comptes 
rendus, 1844, vol. XIX, and Ann. des Sei. 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 Würzburg laboratory, 
published in the third part of vol. I of the Proceedings of the Würzburg 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.] 
^ 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. 
