48 LEAF-CUMBERS. Chap. II. 



position, which is preserved ever afterwards. The 

 petioles which have clasped any object soon become 

 much thickened and strengthened, as may be seen 

 in the drawing. 



Clematis Montana. The long, thin petioles of the 

 leaves, whilst young, are sensitive, and when lightly 

 rubbed bend to the rubbed side, subsequently becom- 

 ing straight. They are far more sensitive than the 

 petioles of C. glandulosa ; for a loop of thread weighing 

 a quarter of a grain (16*2 mg.) caused them to bend ; 

 a loop weighing only one-eighth of a grain (8*1 mg.) 

 sometimes acted and sometimes did not act. The 

 sensitiveness extends from the blade of the leaf to 

 the stem. I may here state that I ascertained in 

 all cases the weights of the string and thread used 

 by carefully weighing 50 inches in a chemical balance, 

 and then cutting off measured lengths. The main 

 petiole carries three leaflets ; but their short, sub- 

 petioles are not sensitive. A young, inclined shoot 

 (the plant being in the greenhouse) made a large 

 circle opposed to the course of the sun in 4 hrs. 20 m., 

 but the next day, being very cold, the time was 

 5 hrs. 10 m. A stick placed near a revolving stem was 

 soon struck by the petioles which stand out at right 

 angles, and the revolving movement was thus arrested. 

 The petioles then began, being excited by the contact, 

 to slowly wind round the stick. When the stick was 

 thin, a petiole sometimes wound twice round it. 

 The opposite leaf was in no way affected. The atti- 

 tude assumed by the stem after the petiole had 



