144 THE VEGETATIVE ORGANS 



untwine. Observe the direction of twining in each case. The 

 stem of the Convolvulus, looked at from above, twines from 

 right to left (contra-clockwise). Most twining plants twine 

 in the same direction ; that of the Black Bryony, however, 

 twines from left to right (clockwise). By this means such 

 plants are able to climb many feet above ground, and to 

 reach the air and sunlight without the expenditure of 

 energy required in building up strong erect stems. 



Climbing organs sensitive to contact. Tendrils. In the 

 White Bryony (Bryonia dioic a) (Fig. 91), Clematis (Fig. 92), 

 Bush Vetch (Fig. 220), and Sweet-Pea (Fig. 131), slender 

 climbing organs called tendrils are developed, which differ 

 from climbing stems in being sensitive to contact. 



The Passion Flower has very sensitive branch-tendrils 

 whose movements are easily observed. Fig. 93 shows the 

 result of an experiment with one such tendril. At 3.10 p.m. 

 the concave side of the tendril was gently stroked with 

 a slender stick and records of its movements were taken, 

 with the result shown in the diagram. If, in describing 

 such a spiral, the tendril meets with a support, it twines 

 round this support and clings firmly. Spiral growth 

 continues, but being now fixed at both ends, the tendril 

 soon develops a reversed spiral. That this form of spiral 

 should be produced can be easily understood if you fix 

 a piece of string at both ends and turn the middle portion : 

 the part on the right turns in the opposite direction to 

 that on the left. Tendrils showing the reversed spiral are 

 also seen in the White Bryony (Fig. 91). The tendrils of 

 the Virginia Creeper (Fig. 94), which are also modified 

 branches, are peculiar in that they move away from the 

 light (negatively heliotropic). At the free ends disks 

 are formed, which, when stimulated by contact with 

 a rough surface, become coated with mucilage and are 

 thus cemented to the support. 



Tendrils are sensitive thread-like plant organs by which 



