2G 



MR. DARWIN ON CLIMBING PLANTS. 



and Mohl class these plants with those which bear tendrils ; but 

 as a leaf is generally a defined object, the present classification 

 has, at least, some plain advantages. There are other advantages, 

 as leaf-climbers are intermediate in many respects between twiners 

 and certain tendril-bearing plants. I have observed eight species 

 of Clematis and seven of TropcBolum in order to discover what 

 amount of difi'erence there may be within the same genus ; and 

 the differences, as we shall see, are considerable. 



Clematis. — (7. glandulosa. — The thin upper intemodes revolve, 

 moving against the course of the sun, precisely like those of a 

 true twiner, at an average rate, judging from three revolutions, 

 of 3 h, 48 m. The leading shoot immediately twined round a stick 

 placed near it ; but, after making an open spire of only one turn 

 and a half, it ascended for a short space straight, and then 

 reversed its spire and wound two turns in an opposite course. 

 This was rendered possible by the straight piece between the 

 opposed spires having become rigid. The simple, broad, ovate 

 leaves of this tropical species, so unlike those of most of the other 

 species of the genus, with their short thick petioles, seem but ill- 

 fitted for any movement. Whilst twining up a vertical stick, no 

 use is made of them. Nevertheless, if the footstalk of a young 

 leaf be rubbed with a thin twig a few times on any side, it will 

 in the course of a few hours bend to that side ; afterwards it 

 becomes straight again. The under side seemed to be the most 

 sensitive ; but the sensitiveness or irritability is but slight corn- 



Fig. 1. 



pared to that which we shall meet 

 with in some of the following spe- 

 cies ; for a loop of string, weighing 

 1*64 grain, hanging for some days 

 on a young footstalk, produced 

 a scarcely perceptible effect. A 

 sketch is here given of two young 

 leaves which had naturally caught 

 two twigs on each side of the stem. 

 A forked twig placed so as to 

 lightly press on the under side of a 

 young footstalk caused it, in 12 h., 

 to bend greatly, and ultimately to Clematis glanduhsa^ with two 



Qnf>h ATI PTtPTit that thplpifiia^^aPil J^^^g l^^es clasping twigs, with 



SUCH an extent tuat tne leai passed ^he clasping portions thickened. 

 to the opposite side of the stem ; 



the forked stick having been removed, the leaf slowly recovered 

 its proper position. 



