96 ME. DA.EWIX ON CLIMBING PLANTS. 



separated by sliort straight portions. This curious and symmetrical 

 structure has been noticed by several botanists, but has not been 



Fig. 13. 



i 



V 



^ 



t 



A caught tendril of Bryonia dioica^ spirally contracted in reversed directions. 



explaiiied*. It occurs without exception witli all tendrils which 

 after catching any object contract spirally, but is of course most 

 conspicuous in the longer tendrils ; it never occurs vnth uncauglit 

 tendrils ; and when this appears to have happened, it will be found 

 that the tendril had originally seized some object and had after- 

 wards been torn free. Commonly all the spires at one end of a 

 caught tendril run in one direction, and all those at the other end 

 in the opposite direction, with a single short straight portion m 

 the middle.; but I have seen a tendril with the spires alternately 

 turning j&ve times in opposite directions, with straight portions 

 between them ; and M. Leon has seen seven or eight such alter- 

 nations. Whether the spires turn several times in opposite di- 

 rections, or only once, there are as many turns in the one direction 

 as in the other. Tor instance, I gathered ten long and short 

 caught tendrils of the Bryony, the longest w^ith 33, and the 

 shortest with only 8 spiral turns ; and the number of turns in one 

 direction was in every case the same (within one) as in the oppo- 

 site direction. 



The explanation of this curious little fact is not difficult ; I vrul , 

 not attempt any geometrical reasoning, but will give only prac- 

 tical illustrations. In doing this, I shall first have to allude to a 

 point which was almost passed over when treating of Twinmg- 

 plants. If we hold in our left hand a bundle of parallel strings, 

 we can with our right hand turn these round and round, and 

 imitate the revolving movement of a twining plant, and the strings 

 do not become twisted. But if we now at the same time hold a 

 stick in our left hand, in such a position that the strings become 



* See M. Isid. Leon in Bull. Soc. Bot. de France, toiu. v. 1858, p. 680. 



