36 ME. DARWIN ON CLIMBING PLANTS. 



clasp either their own stem or the supporting stick. The petioles 

 and filaments often become, after a time, in some degree spirally 

 contracted. In these spontaneous movements, and in the abortion 

 of their laminae, the sensitive filaments present a much nearer 

 approach to the condition of tendrils than do the petioles of any 

 other leaf-climber observed by me. 



Tropceolum azureum. — ^An upper internode made four revolu- 

 tions, following the sun, at an average rate of Ih. 47 m. The stem 

 twined spirally in the same irregular manner as in the last species ; 

 it produced no filaments or rudimentary leaves. The petioles of 

 the young leaves are very sensitive : a single very light rub with a 

 twig caused one to move perceptibly in 5 m., and another in 6 m. ; 

 the former petiole became bent at right angles in 15 m., and 

 became straight again in between 5 h. and 6 h. A loop of thread 

 weighing -^th of a grain caused a petiole to curve. 



Tropcdolv/m pentajphyllum. — The plant observed by me had not 

 the power of spirally twining, which seemed due, not to the want 

 of flexibility in the stem, but rather to continual interference from 

 the clasping petioles. An upper internode made three revolu- 

 tions, following the sun, at an average rate of Ih. 46 m. The main 

 purpose of the revolving movement in all the species is mani- 

 festly to bring the petioles into contact with some supporting ob- 

 ject. The petiole of a young leaf, after a slight rub, became curved 

 in 6 m, ; another, on a cold day, in 20 m. ; but others generally in 

 from 8m. to 10m.: the curvature usually increased greatly in from 

 15m. to 20m. The petioles became straight again in between 5b. 

 and 6 h., and on one occasion in 3 h. When a petiole had fairly 

 clasped a stick, it could not on the removal of the stick recover 

 itself; but the free upper part of a petiole, which had already 

 clasped a stick by its basal part, still had the power of movemenr. 

 A loop of thread weighing ^th of a grain certainly caused a petiole 

 to curve ; but the stimulus was not sufficient, the loop remaining 

 suspended, to cause a permanent flexure. If a much heavier loop 

 be placed in the angle between the petiole and the stem, it pro- 

 duces no eff'ect ; whereas we have seen that the angle between the 

 stem and petiole of Clematis monfana is sensitive. 



Tropceolum peregrinum. — In a very young plant the inter- 

 nodes did not revolve, resembling in this respect a young twin- 

 ing plant. The four upper internodes in an older plant made 

 three irregular revolutions, in a course opposed to the sun, 

 at an average rate of 1 h. 48 m. It is remarkable how nearly the 

 same the average rate of revolution (taken, however, but from fe^ 



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