LEAF-CLIMBERS. 29 



all. The clasped petiole in the course of two or three days swells 

 greatly, and ultimately becomes nearly twice as thick as the 

 opposite leaf-stalk which has clasped nothing. When thin trans- 

 verse slices of the two are placed under the microscope their 

 difference is conspicuous : the side of the footstalk which has been 

 in contact with the support is formed of a layer of colourless cells 

 with their longer axes directed from the centre of the petiole, and 

 very much larger than any cells found in the opposite or unchanged 

 petiole ; the central cells, also, are in some degree enlarged, and 

 the whole is much indurated. The exterior surface generally 

 becomes bright red. But a far greater change takes place in 

 the nature of the tissues than that which is externally visible : 

 the petiole of the unclasped leaf is flexible, and can be easily 

 snapped, whereas the clasped footstalk acquires an extraordi- 

 nary toughness and rigidity, so that considerable force is re- 

 quired to pull it into pieces. "With this change, great durability 

 is probably acquired ; at least this is the case with the clasped 

 petioles of Clematis vitalha. The meaning of these changes is 

 plain, namely, that the petioles may firmly and durably support 

 the stem. 



lep tojpTiylla . — Tlie 



direc 



un 



irregular ellipses or large circles : four revolutions were com- 

 pleted within five minutes of the same average rate of 1 h. 51m. ; so 

 that this species moves more quickly than any other of the genus. 

 The shoots, when placed near a vertical stick, either twine round 

 it or clasp it with the basal portions of their petioles. The 

 leaves whilst yoxmg are nearly of the same general shape, and act 

 in the same manner like a hook, as will be described under C. 



and 



young 



much curved downwards and inwards ; so that the whole ] 

 readily catches and becomes entangled witli any neighbour 

 object. The petioles of the young terminal leaflets are acted 



^th of a gram 



^ „ ^ , will clasp a 



stick against which it presses. 



The whole leaf, wliilst yoimg, is in continual, spontaneous, slow 

 movement. The stem was secured close to the base of the leaves, 

 and, a bell-erlass beinar nlaced over the shoo 



days. A very irre- 



