TWINERS AND OTHER CLIMBING PLANTS 315 



growing organs, he failed to find any circumnutation. Where it occurs it is 

 caused merely by inequalities in the rate of elongation on different sides of the 

 axis. 



Summary 



1. Twiners. — Climbing plants gain support by twining about objects or by forming 

 mechanical attachments in other ways. Twiners have very slender stems, with long 

 internodes. After the seedling stage of development is past, each new intcrnode 

 elongates very rapidly at first and the leaves of the youngest two or three nodes are 

 retarded in their enlargement. Also, the youngest two or three internodes and the 

 retarded leaf buds are pale in color. Consequently, this youngest region of the twining 

 plant appears much as if it were etiolated, as has been said, although grown under 

 natural day-night conditions of light. The rapidly elongating, younger portion 

 enlarges in such a way that the tip is made to swing in a more or less circular path about 

 the vertical axis, clockwise in some species and counter-clockwise in others. The time 

 required for a revolution is, in general, between one and ten hours. If a suitable 

 support is encountered (this must be nearly vertical and must not be of too great 

 diameter), the elongating stem twines about it, each turn of the stem being higher than 

 the preceding one. As elongation is about to cease, each internode straightens as 

 much as possible, thus tightening the spiral on the support. At about this time the 

 leaves expand, and the stem becomes green. Twining results from more rapid elonga- 

 tion on one side of the stem than on the opposite side, the lateral region of most rapid 

 elongation migrating regularly around the stem. The regularity of this change is in- 

 ternally controlled, but twining ceases when the pull of gravitation is equalized about 

 the plant axis by clinostat rotation. 



2. Non-twining Climbers. — The stems of non-twining climbers behave much like 

 those of twiners, except that they do not twine. They bear tendrils, aerial 

 roots, etc., by which they become attached to any suitable support with which they 

 happen to come into contact. In some forms tendrils correspond to leaves (pea), in 

 other forms they correspond to branches (grape). Tendrils are sensitive to contact. 

 They may grow around a suitable support by thigmotropic response, or they form 

 adhering disks at their tips when stimulated by contact. After the tip is attached 

 the main part of the tendril elongates very much throughout a narrow lateral region, 

 thus producing a double spiral, which acts like a spiral wire spring. Many tendrils 

 have contact papilke, thin spots in the epidermal walls of the contact-sensitive region. 



3. Circumnutation. — Charles Darwin showed that the tips of all vertically elonga- 

 ting stems swing slightly away from the vertical and around it, as elongation proceeds. 

 He called this swinging circumnutation. To a very slight and frequently almost 

 imperceptible degree, and in a very irregular manner, ordinary plant tips therefore act 

 somewhat like the tips of twiners. Circumnutation is caused by an irregular and 

 spasmodic migration, about the elongating portion of the shoot, of a lateral region of 

 more rapid enlargement. 



Circumnutation is often very difficult to demonstrate, but it is doubtful if any shoot 

 ever elongates with its own axis exactly vertical for more than momentary periods of time. 

 It seems safe to regard circumnutation as universal in elongating stems. It is very irregular in 

 many cases, and frequently very slight indeed. — Ed. 



