LEAF-CLIMBERS. 



27 



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 

 1 line at right angles to the stem, and then become so much arched 

 downwards that the blade of the leaf points to the ground witli 

 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 origiual upturned position, which is 

 retained ever afterwards. The young leaves, being hooked, are 

 thus enabled to catch twngs 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 some- 

 times 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 mea- 

 sured 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 op- 

 posed 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 revol- 

 ving 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 twi- 

 ning, 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 pow r er of twining is like 

 that of the last species. Its leaves are nearly similar, except that 



* Our English grain equalg nearly, (55 milligrammes. 



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