34 MR. DARWIN ON CLIMBING PLANTS. 



sixteenth of a grain is, these facts are remarkable. But I have 

 reason to believe that even a less weight causes a curvature when 

 acting over a broader surface than can be affected by thin thread. 

 Having noticed that the tail of a suspended string, which acci- 

 dentally touched a petiole, had caused it to bend, I took two 

 pieces of thin twine, 10 inches in length (weighing TG4 gr.), and, 

 tying them to a stick, let them hang as nearly perpendicularly 

 downwards as their thinness and flexuous nature, after being 

 stretched, would permit ; I then quietly placed their ends so as 

 just to rest on two petioles with their tips hanging about the 

 tenth of an inch beneath ; both these petioles certainly became 

 curved in 36 h. One of the ends of string, which just touched 

 the angle between a terminal and lateral sub-petiole, was in 48 h. 

 caught as by a forceps between them. In these cases the pressure, 

 though spread over a wider surface than that touched by the 

 cotton thread, must have been excessively slight. 



Clematis vitalba.—bly plants in pots were not healthy ; so that 

 I dare not trust my observations, which indicated much similarity 

 in habits with C.flammtda. I mention this species only because 

 I saw many proofs that the petioles of plants growing naturally 

 are excited to movement by very slight pressure. For instance, 

 I found petioles which had clasped thin withered blades of grass, 

 the soft young leaves of a maple, and the lateral flower-peduncles 

 of the quaking-grass or Briza: the latter are only about as thick 

 as a hair from a man's beard, but they were completely surrounded 

 and clasped. The petioles of a leaf, so young that none of the 

 leaflets had expanded, had partially seized on a twig. The petioles 

 of almost every old leaf, even when unattached to any object, are 

 much convoluted ; but this is owing to their having come, whilst 

 young, into contact during several hours with some object sub- 

 sequently removed. With the several above-described species, 

 cultivated in pots and thus carefully observed, there never was 

 any bending of the petioles without the stimulus of contact. When 

 winter comes on, the blades of the leaves of C. vitalba drop oft' ; 

 but the petioles (as was also observed by Mohl) remain, some- 

 times during two seasons, attached to the branches ; and, being 

 convoluted, they curiously resemble true tendrils, such as those 

 occurring in the allied genus Naravelia. The petioles which 

 have clasped an object become much more woody, stiff, hard, 

 and polished than those which have failed in this their proper 

 purpose. 



Trop^OLUM. — I observed T. tricolorum, T. azureum, T. penta- 



