Chap. II. CLEMATIS. 51 



thick as the opposite one which has clasped nothing. 

 When thin transverse slices of the two are placed 

 under the microscope their difference is conspicuous : 

 the side of the petiole which has been in contact with 

 the support, is formed of a layer of colourless cells with 

 their longer axes directed from the centre, and these 

 are very much larger than the corresponding cells 

 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 

 visible : the petiole of the unclasped leaf is flexible 

 and can be snapped easily, whereas the clasped one 

 acquires an extraordinary degree of toughness and 

 rigidity, so that considerable force is required 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 vitalba. The meaning of 

 these changes is obvious, namely, that the petioles may 

 firmly and durably support the stem. 



Clematis mierophylla, var. leptophyUa. The long 

 and thin internodes of this Australian species revolve 

 sometimes in one direction and sometimes in an op- 

 posite one, describing long, narrow, irregular ellipses 

 or large circles. Four revolutions were completed 

 within five minutes of the same average rate of 

 1 hr. 51m.; so that this species moves more quickly 

 than the others of the genus. The shoots, when placed 

 near a vertical stick, either twine round it, or clasp it 



