Chap. II. TROP^OLUM. 63 



to move perceptibly in 5m, and another in 6 ra. 

 The former became bent at right angles in 15 min., and 

 became straight again in between 5 hrs. and 6 hrs. A 

 loop of thread weighing Jth of a grain caused another 

 petiole to curve. 



Tropaeolum pentaphyUum. This species has not the 

 power of spirally twining, which seems due, not so much 

 to a want of flexibility in the stem, as to continual 

 interference from the clasping petioles. An upper inter- 

 node made three revolutions, following the sun, at an 

 average rate of 1 hr. 46 m. The main purpose of 

 the revolving movement in all the species of Tro- 

 paeolum manifestly is to bring the petioles into contact 

 with some supporting object. The petiole of a young 

 leaf, after a slight rub, became curved in 6 m. ; another, 

 on a cold day, in 20 m., and others in from 8 m. 

 to 10 m. Their curvature usually increased greatly in 

 from 15 m. to 20 m., and they became straight again in 

 between 5 hrs. and 6 hrs., but on one occasion in 3 hrs. 

 When a petiole has fairly clasped a stick, it is not able, 

 on the removal of the stick, to straighten itself. The 

 free upper part of one, the base of which had already 

 clasped a stick, still retained the power of movement. A 

 loop of thread weighing ^th of a grain 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 produces no effect ; whereas 

 we have seen with Clematis montana that the anode 

 between the stem and petiole is sensitive. 



