io8 



General Botany 



in diameter. These long, slender stems grow rapidly and enable 

 the plants to spread their leaves quickly over the tops of large 

 trees. With a proper support these stems have all the advantages 

 of upright stems, without having to use so mucn material in 

 building woody supporting tissue. 



Some climbers — for example, 

 the morning-glory — gain sup- 

 port by twining. Others, like the 

 grape, have tendrils, specialized 

 organs developed in place of 

 branches or leaves. A tendril 

 responds to contact with a sup- 

 port by coiHng tightly about it 

 (Fig. 57). When attached, me- 

 chanical tissues develop within, 

 greatly increasing their strength. 

 Then, due to unequal growth on 

 the two sides, the tendril twists 

 spirally and draws the plant 

 close to the support. 



In some vines, hke the Boston 

 ivy (Fig. 59) the tendrils have at 

 their tips sensitive disks which, when rubbed against a support, 

 secrete a sticky substance and become cemented to it. This 



Fig. 57. Tendrils of wild cucumber. Note 

 the coiling of the tendril by which the plant 

 is drawn nearer the support and the rever- 

 sal of the spiral in different parts of the 

 tendril. Is there always such a reversal? 



