CLIMBING PLANTS 



117 



In the shade of hedges the stems become etiolated and 

 weak, and the plant climbs. Polygonum dumetorum is 

 another imperfect twiner. 



3. Tendril- Climbers. Tendrils are organs specialized 

 for climbing. While growing, they are sensitive to con- 

 tact, and their free ends circumnutate. On touching a 

 support, the tendril twines round it. The free portion 

 between the support and the plant then takes on a spiral 

 twist, a reversal- point naturally occurring in the middle, 

 since both ends are fixed and incapable of movement 

 (Fig. 36). The object of this is to bring the stem closer to 

 its support, and at the same time to increase the strength 



FIG. 34. Convolvulus (BINDWEED), 

 TWINING COUNTER - CLOCKWISE. 

 (REDUCED.) 



FIG. 



35. HOP, TWINING CLOCK- 

 WISE. (REDUCED.) 



of the anchoring cable. The closer the plant is to its 

 support and the stronger the cable, the less liable is it 

 to be wrenched away from it by the wind and damaged. 

 When twining has ceased, the tendril thickens and 

 becomes woody, thus making the connection permanent 

 and durable. 



The hooks and bristles of scrambling plants are oppor- 

 tune outgrowths from the surface of the plant. They are 

 neither leaves, roots, nor shoots. Tendrils, on the other 

 hand, represent definite plant- organs leaves or portions 

 of leaves, stem- structures, or even roots. We mean by 

 this that the rudiment of the tendril started as one of 



