THE MOVEMENTS AND SENSITIVITY OF PLANTS 333 



97. Movements of Twining and Climbing Plants. — The twin- 

 ing and climbing plants form a peculiar ecological group. In spite 

 of their great length, they are characterized by very thin stems in 

 which mechanical tissue is almost completely absent. Hence they 

 are unable to sustain themselves in a vertical position and are 

 in need of an external support. This is supplied by other plants, 

 upon which they climb. According to the exact meaning of the 

 word, the twining and climbing plants are not really parasites, since 

 they develop no haustoria and feed quite independently. Never- 

 theless, they may do a great amount of harm to plants which they 

 use as supports, by depriving them of light. This occurs the more 

 easily, since plants of this group need but little substance for the 

 building of their slender stems, and therefore develop very rapidly, 

 easily outgrowing their supports. 



The necessity to find a support upon which to lean has led the 

 twining and climbing plants to develop a high degree of movement. 

 They are, indeed, the most mobile plants among the representa- 

 tives of our flora. According to the nature of the movements 

 and the method of attachment to their support they may be divided 

 into two different groups: the climbing plants, which attach 

 themselves by means of special organs, the tendrils; and the twin- 

 ing plants, which coil around the support by means of their stems. 



The most typical representatives of the climbing plants are 

 the various cucurbits and legumes. Morphologically, their ten- 

 drils are either metamorphosed lateral shoots, or leaves or parts of 

 leaves. These morphological distinctions are, however, of small 

 physiological importance. In all instances the tendril consists of a 

 thin lash, which is originally coiled as a spiral, but then extends and 

 describes peculiar circular movements in the air (Fig. 137). If in 

 the process of these movements the tendril meets with a firm sup- 

 port, it will clasp firmly around it and then, coiling up like a spring, 

 will draw the shoot to the support. If they meet with no support, 

 they wither. 



Two kinds of movements are thus found in tendrils: the circular 

 movement, while seeking a support, and the coiling movement 

 around it. The first is autonomic. It is dependent on internal 

 causes and is produced by an accelerated growth of the tendril on 

 its various sides. Similar differences in the rate of development 

 may be observed in all growing organs, producing a swaying of the 

 growing tip, which is not possessed of any peculiar orientation. 



