28 MR. DARWrN OX CLTM15INO PLANTS. 



the petioles of the lateral and terminal leaflets are sensitive. A 

 loop of thread, weighing one-eighth of a grain, acted on the main 

 petiole ; but it took between two and three days to produce any 

 effect. The leaves have the remarkable habit and power of spon- 

 taneously revolving, generally in vertical ellipses, in the same 

 manner, but in a less degree, as will be described under O. micro- 

 phylla. 



Clematis calycina. — The young shoots are thin and flexible ; 

 one revolved, describing a broad oval, in 5 h. 30 m., and another in 

 6h. 12 m. : they followed the course of the sun ; but in all the species 

 of the genus the course followed, if observed long enough, would 

 no doubt be found to differ. This is a rather better twiner than the 

 two last species : the stem, when a thin upright stick free from 

 twigs was placed near, sometimes made two spiral turns round it ; 

 then, being arrested by the clasping of the petioles, it would run 

 up for a space straight and then generally reversed its course and 

 took one or two spiral turns in an opposite direction. This 

 reversal of the spire occurred in all the foregoing species. The 

 leaves are so small compared with those of most of the other species 

 that the petioles at first seem ill-fitted for clasping. Nevertheless 

 the main service of the revolving movement is to bring them into 

 contact with surrounding objects, which are slowly but securely 

 seized. The young petioles, which alone are sensitive, have 

 their ends bowed a little downwards, so as to be in a slight degree 

 hooked ; ultimately the whole leaf becomes flat. I gently rubbed 

 with a thin twig the lower surfaces of two young petioles ; and 

 in 2 h. 30m. they were slightly curved downwards; in 5h., after 

 being rubbed, the end of one was bent completely back parallel 

 to the basal portion ; and in 4 h. subsequently it became nearly 

 straight again. To show how sensitive the young petioles are, I 

 may mention that I put, in order to mark them, short streaks of 

 water-colour on their under sides ; an infinitely thin crust was 

 thus formed, but it sufficed in 21 h. to cause both to bend down- 

 wards. "Whilst the plant is young, each leaf consists of three 

 divided leaflets, which have barely distinct petioles, and these are 

 not then sensitive ; but when the plant is well grown, the two 

 lateral and terminal leaflets have long petioles, and these now- 

 become sensitive and are capable of clasping in any direction any 

 object. 



"When the petiole has clasped a twig, it undergoes some remark- 

 able changes, which occur with the several other species, but in a 

 less strongly marked manner, and will be here described once for 



