MOVEMENTS OF PLANTS. 657 



free at the tip. If, during the course of its revolutions, it come in contact 

 with any object, such as a stem or a branch of another plant, it twines 

 around it. The cause of the movement is supposed to be due to rapid 

 growth in a longitudinal direction, the band of growth travelling round 

 the tendril and successively bowing each part to the opposite side. 



Leaf-climbers. These are plants whose leaf-stalks clasp round any 

 support that may come in their way, while, in other cases, the plants 

 climb by the tips' of their leaves, as in Gloriosa, p. 56 (fig. 67). The young 

 internodes of these plants revolve as in the former case, and with the 

 object of bringing the petioles into contact with surrounding objects. The 

 petioles in these cases are, when young, sensitive to a slight touch, bending 

 towards the touched side at different rates in different species. Such 

 petioles after a few days become much thickened, and assume a stem-like 

 character. 



Tendrils are thread-like modifications of leaves, peduncles, or branches, 

 possessing the power of spontaneous revolution, the whole length of the 

 organ exhibiting curvature except at the extreme tip, which is sensitive 

 to touch on one side or the other, or on both. When the end has caught 

 a support, the tendril winds round it in a spiral direction according to the 

 position of the support and the side first touched. Twining plants, on 

 the other hand, curl round their supports invariably in the direction of 

 their revolving movement. The tendrils of different species of the same 

 genus twine in different manners in some cases bending themselves 

 away from the light, so as to seek crevices in the bark, rocks, or walls 

 on which they are growing, and often developing adhesive disks at their 

 extremities, as in Ampelopsis tricuspidata ( Veitchii). The different move- 

 ments executed by tendrils may be referred to revolving nutation, bending 

 to or from the light and in opposition to gravity, in addition to those 

 caused by a touch, and spiral contraction ; and it seems probable that the 

 inducing causes may be different. Some are probably due, as stated by 

 Sachs and De Tries', to rapid growth along one side ; but Darwin doubts 

 whether the movements consequent on a touch can be so caused. The first 

 action of a tendril, says Darwin, is to place itself in a proper position so 

 as to be in the most favourable position for catching hold of any support. 

 Another wonderful property is the way in which they generally avoid 

 coiling themselves around the stem from which they themselves proceed. 

 When, says Asa Gray, " a tendril sweeping horizontally comes round so 

 that its base nears the parent stem rising above it, it stops short, rises 

 stiffly upright, moves on in this position until it passes by the stem, then 

 rapidly conies down again to the horizontal position, and moves on so 

 until it again approaches and again avoids the impending obstacle." 



Of the two kinds of motion, the spontaneous revolutions and that 

 induced by contact, the former may be compared with the spontaneous 

 movements of the leaflets of Desmodium, or the sleep-movements of leaves, 

 while the movements due to contact are analogous with the induced 

 movements of the sensitive plants. The tendrils of Passiflora gracilis and 

 of a gourd have been seen by Gray and others to coU up rapidly and 

 perceptibly when their extremities are gently rubbed, straightening again 

 after a short time, to be recoiled if again excited. Once a grasp has 

 been obtained the sensibility ceases. 



