182 TENDRIL-BEARERS. Cuap. IV. 
a tendril when touched is due to rapid growth, seems to 
be that tendrils lose their sensitiveness and power of 
movement after they have grown to their full length; 
but this fact is intelligible, if we bear in mind that all 
the functions of a tendril are adapted to drag up the 
terminal growing shoot towards the light. Of what 
use would it be, if an old and full-grown tendril, 
arising from the lower part of a shoot, were to retain 
its power of clasping a support? This would be of 
no use; and we have seen with tendrils so many in- 
stances of close adaptation and of the economy of 
means, that we may feel assured that they would 
acquire irritability and the power of clasping a support 
at the proper age—namely, youth—and would not 
uselessly retain such power beyond the proper age. 
