Page:Darwin - On the movements and habits of climbing plants.djvu/28

 The young leaves change their position in a rather odd manner: when first developed the petioles are upturned, parallel to the stem; they then slowly bend downwards, remaining for a short time at right angles to the stem, and then become so much arched downwards that the blade of the leaf points to the ground with its tip curled inwards, so that the whole petiole and leaf together form a hook. If they come into contact with no object, they retain this position for a considerable time, and then bending upwards they reassume their original upturned position, which is retained ever afterwards. The young leaves, being hooked, are thus enabled to catch twigs when brought into contact with them by the revolving movement of the internodes. The petioles which have clasped any object soon become much thickened and strengthened, as may be seen in the diagram.

Clematis montana.—The long and thin petioles of the leaves, whilst young, are sensitive, and when lightly rubbed bend to the rubbed side, subsequently becoming straight. They are far more sensitive than the petioles of C. glandulosa; for a loop of thread weighing a quarter of a grain caused them to bend; a loop weighing only one-eighth of a grain sometimes acted and sometimes did not act. The sensitiveness extends to the angle between the stem and leaf-stalk. I may here state that I ascertained the weights of the string and thread used in all cases by carefully weighing 50 inches in a chemical balance, and then cutting off measured lengths. The main petiole carries three leaflets; but the short petioles of these leaflets 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 h. 20 m., but the next day, being very cold, the time was 5 h. 10 m. A stick placed near the revolving stem was soon struck by the petioles which stand out at right angles, and the revolving movement was arrested. The petiole then began, being excited by the contact, to slowly wind round the stick. When the stick was thin, the petiole sometimes wound twice round it. The opposite leaf was in no way affected. The attitude assumed by the stem after the petiole has clasped a stick, is that of a man standing by a column, who throws his whole arm horizontally round it. With respect to the stem's power of twining, some remarks will be made under C. calycina.

Clematis Sieboldi.—A shoot made three revolutions against the sun at an average rate of 3 h. 11 m. The power of twining is like that of the last species. Its leaves are nearly similar, except that