168 REVIEWS. 



6 inches in diameter, but they could not, though aided by me 

 in many ways, wind round it. This apparently is owing to 

 the flexure of the shoot, when winding round an object so 

 gently curved as this post, not being sufficient to hold the 

 shoot to its place when the contracting force creeps round to 

 the opposite surface of the shoot ; so that it is at each revolu- 

 tion withdrawn from its support." — (pp. d-1^, passim.^ 



The successive shifting of the contracting side of the shoot, 

 which explains the revolution or bowing in turn in every direc- 

 tion, no less explains the twining round a proper support, 

 leaving however some idiosyncrasies unexplained. Some ten- 

 drils and some petioles of leaf-climbing plants equally possess 

 this revolving power ; but their usefulness depends mainly 

 upon additional and more special endowments, — mainly upon 

 the power of directly responding by curvature to the contact, 

 more or less prolonged, of an extraneous body. 



Of Leaf-climbers, no instance is more familiar than that of 

 Clematis or Virgin's Bower. Little more was known of them 

 than that they climbed by curling their petioles (common or 

 partial) around neighboring objects. Mr. Darwin made obser- 

 vations upon eight species of Clematis, seven of Tropieolum, 

 the common species of Maurandia, Lophospermum, Fumaria, 

 etc., as also upon Gloriosa and Flagellaria, which climb by a 

 tendril-like production of the tip of the leaf. From the sum- 

 mary it appears that plants which belong to eight families are 

 known to have clasping petioles, and those of four families 

 climb by the tips of their leaves. In almost all of them the 

 young internodes revolve, in some of them as extensively as 

 in twining plants, — the movement being plainly serviceable 

 in bringing the petioles or the tips of the leaves into contact 

 with surrounding objects. Those whose shoots revolve most 

 freely are also capable of twining spirally around a support ; 

 but when the stem twines (as in Clematis Sieholdii and C. 

 calycina, but not in C. Viticella')., it has the peculiarity of wind- 

 ing first in one direction for two or three turns, and then in the 

 opposite direction. The petioles are principally efficient in these 

 ])lants, and that by means of an endowment which is not shown 

 to belong to twining stems, with one or two exceptions. That 



