TWINING OF CLIMBING PLANTS. 773 



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 the revolving movement of nuta- 

 tion 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 con- 

 cave sides. Usually two or three of the younger internodes are in a state of revolv- 

 ing 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 w'hole 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 \ 



The direction of revolving nutation and of torsion is, in all climbing plants, the 

 same as that in which they twine round their support^. If a point in the terminal 

 region exhibiting nutation is prevented from moving by some external cause, as by 

 being fixed, the revolving movement of the free part will continue for some time, 

 but the free part will then grow in a spiral ascending in the direction of nutation. 

 The revolving movement of nutation then combines with a new torsion of the lower 

 parts which are already coiled spirally ; but this torsion is opposed in its direction to 

 the revolving nutation, and therefore also to the torsion previously mentioned. It is 

 probably occasioned by the weight of the free overhanging apex of the shoot ; at 

 all events this causes the concave side of the part in a state of revolving nutation 

 to face from that time the axis of the spiral which has been formed. 



The most common case in which a revolving movement of nutation is averted 

 in this way is when the apex of a shoot comes, in consequence of this motion, into 

 contact with an erect support. If the support is not too thick, it forms the axis 

 of the spiral curvatures which the climbing stem makes round it ; when the support 

 is very slender, the stem winds in such large coils that they do not touch the support 

 at all, or only accidentally at a few places. 



But the revolving nutation can also be artificially interfered with in various 

 other ways ; as, for example, by placing a support on the posterior side of the shoot 

 as respects its revolution, and fastening it by means of gum to the apex, which would 

 otherwise become detached from it. The first spiral coil is in this case formed in 

 precisely the same manner as if the support were in its normal position, but the 

 support stands outside the coil which does not embrace any support. Spiral coils 



^ 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 lime is different from that of the 

 revolutions of nutation. 



^ What follows is from de Vries. 



