CLIMBING PLANTS. 



689 



in the following autumn. Only the Bitter-sweet {Solanum dulcamara) and several 

 species of honeysuckle (e.g. Lonicera caprifolium. and Periclym^emiTn), which exist 

 in comparatively inclement regions, possess twining stems which increase in thick- 

 ness from year to year. But in many of these species the twining is not very 

 conspicuous, and the Bitter-sweet forms, so to speak, a link between plants with 

 twining and those with interweaving stems. In tropical regions, on the other 

 hand, long-lived twining stems are by no means rare. Obviously the coils of a stem, 

 firmly attached round a thin support and increasing in thickness, must approach one 

 another very closely; thus arise those strange lianes which excite the astonishment 

 of all visitors to tropical forests. Stems are quite common of a diameter of 4 cm., 



Fig. 161.— Portion of a Liaue stem, twisted like a corkscrew, from a tropical forest; natur:il size. 



wound like a corkscrew round the thin stems of other lianes, and sometimes such 

 structures — of which a small portion is represented in natural size in fig. 161 — are 

 seen stretching right up to the summits of the trees in hundreds of uniform twists, 

 like a thick ship's cable many metres long. 



The tendril-bearing stem, {stirps cirrhosa) climbs up into the sunlight by the 

 help of special organs known as tendrils. The tendrils are filamentous structures 

 when young; sometimes of exceeding delicacy, sometimes thick and stifi". In some 

 cases they are simple, in others forked, but always sensitive, and so constructed 

 that they can grasp any body with which they come in contact, hold it fast, and 

 use it as a support. Before the tendril adheres to a support it is straight, and 

 extends in the direction in which there is the greatest probability of reaching a 

 .support. It also performs movements the aim of which is to strike against some 

 firm object. If this end is attained, the support which it has encountered is 

 firmly gripped by the tip of the tendril, whilst the part lying immediately behind 

 the point of attachment contracts together spirally. By this spii-al contraction the 



VOL. I. 44 



