182 TENDRIL-BEAEBBS. Chap. 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. 



