LEAF-CLIMBERS. 31 



whole petiole ultimately grows straight. Both the medial and 

 lateral petioles are sensitive ; and the three branches, into which 

 the basi-lateral petioles are generally subdivided, likewise are sen- 

 sitive. The basal portion of the main petiole between the stem 

 and the first pair of leaflets is less sensitive than the remainder, 

 but it will clasp a stick when in contact. On the other hand, the 



Fig. 2. 



A young leaf of Clematis vitieella. 

 inferior surface of the rectangularly bent terminal portion (carry- 

 ing the terminal leaflet), which forms the inner side of the end of 

 the hook, is the most sensitive part ; and this portion is manifestly 

 best adapted to catch distant supports. To show the difference 

 in sensibility, I gently placed loops of string of the same weight 

 (in one instance weighing - S2 of a grain) on the several lateral and 

 on the terminal sub-petioles; in a few hours the latter were bent, 

 but after 21 h. no effect was produced on any of the lateral petioles. 

 Again, a terminal sub-petiole placed in contact witli a thin stick 

 became sensibly curved in 45 m., and in 1 h. 10 m. had moved 

 through ninety degrees, whereas a lateral petiole did not become 

 sensibly curved until 3 h. 30 m. had elapsed. In this latter case, 

 and in all other such cases, if the sticks be taken away, the petioles 

 continue to move during many hours afterwards ; so they do after 

 a slight rubbing ; but ultimately, if the flexure has not been very 

 great or long-continued, they become, after about a day's in- 

 terval, straight again. 



The gradation in the extension of the sensitiveness in the 

 petioles of the several above-described species deserves notice. In 

 C. montana it is confined to the main petiole, and has not spread 

 to the sub-petioles of the three leaflets ; so it is with young plants 



