CHAPTER IV 



TWINERS AND OTHER CLIMBING PLANTS 



§i. Twiners. 1 — The stems of many plants are so slender and so weak 

 mechanically that they cannot grow upright unless they climb upon supporting 

 objects. Without such mechanical support these plants always creep upon the 

 ground. Climbing plants grow up into the air by twining about, or attaching 

 themselves to, other plants or any available support, and they are thus able to 

 attain the best illumination. 



Twiners have long, slender stems, the growing tips of which twine about 

 suitable objects that happen to be near. Familiar examples of twiners are 

 the hop {Humulus lupulus), the scarlet-runner bean {Phaseolus multiflorus), 

 various species of Convolvulus (morning glory), and also some Polygonum 

 species, as P. dumetorum and P. convolvulus (bind- weed). The terminal por- 

 tion of a twining stem of Humulus lupulus is shown in Fig. 153. In all twin- 

 ing plants the growing tip moves about the axis of the older part, describing a 

 more or less circular path; the direction of this movement is clockwise in some 

 plants, and counter-clockwise in others (Fig. 154). In most plants the moving 

 portion consists of the last two or three internodes. The time required for a 

 complete revolution varies with the plant as well as with the environmental 

 conditions. In one experiment this time period was found to be one hour 

 and seventeen minutes for Scyphanthus elegans, one hour and forty-two min- 

 utes for Convolvulus septum, one hour and fifty-seven minutes for Phaseolus 

 vulgaris, and nine hours and forty-five minutes for Lonicera brachypoda. The 

 circular movement of the terminal region continues until some solid object, such 

 as the stem of another plant, is encountered and then the twiner begins to 

 wind itself about the support, providing this is of suitable shape and size. 



The turns of the resulting spiral are not closely applied to the support at 

 first, especially if the support is very slender; later, however, the spiral 

 elongates and becomes narrower, and the stem thus becomes firmly bound about 

 the supporting object. A firmer hold is effected by the stiff hairs that are 

 frequently present on the stems of twiners. 



Twining plants are able to wind about very slender objects, but the diameter 

 of the support must not be too great, or twining is prevented. The maximum 

 diameter of the support varies with different plants; Phaseolus multiflorus 

 twines about a support from 7 to 10 cm. in thickness, but twining fails to occur 

 if the diameter of the support is as great as 23 cm. Many tropical twiners 

 can twine about thick supports. 



1 Darwin, Charles R., Movements and habits of climbing plants. 2nd ed., revised. London, 1875. Bara- 

 netzki, J., Die kreisformige Nutation und das Winden der Stengel. Mem. Acad. Imp. Sci. St.-Peters- 

 bourg VII, 3i v/// : 1-73. 1883. Pfeffer, W., Zur Kenntnis der Kontaktreize. Untersuch. Bot. Inst. 

 Tubingen 1: 483-535. 1881-1885. Voss, Wilhelm, Xeue Versuche iiber das Winden des Pflanzensteng- 

 els. Bot. Zeitg. 6o 7 : 231-252. 1902. [MacDougal, D. T., Practical text-book of plant physiology, 

 XIV + 352 p. New York. 1901. Pringsheim, i<ji2. (See note 1, p. 253.)] 



311 



