Chap. IV. SUMMARY. 171 



tremities of twining shoots, which not only reverse 

 their curvature, or at least become periodically straight, 

 but curve themselves in a greater degree than the 

 lower part. In most other respects a tendril acts as if 

 it were one of several revolving internodes, which all 

 move together by successively bending to each point 

 of the compass. There is, however, in many cases this 

 unimportant difference, that the curving tendril is 

 separated from the curving internode by a rigid 

 petiole. With most tendril-bearers the summit of the 

 stem or shoot projects above the point from which 

 the tendril arises ; and it is generally bent to one side, 

 so as to be out of the way of the revolutions swept by 

 the tendril. In those plants in which the terminal 

 shoot is not sufficiently out of the way, as we have 

 seen with the Echinocystis, as soon as the tendril 

 comes in its revolving course to this point, it stiffens 

 and straightens itself, and thus rising vertically up 

 passes over the obstacle in an admirable manner. 



All tendrils are sensitive, but in various degrees, to 

 contact with an object, and curve towards the touched 

 side. With several plants a single touch, so slight as 

 only just to move the highly flexible tendril, is enough 

 to induce curvature. Passiflora gracilis possesses the 

 most sensitive tendrils which I have observed : a bit 

 of platina wire ^-th of a grain (1*23 mg.) in weight, 

 gently placed on the concave point, caused a tendril 

 to become hooked, as did a loop of soft, thin cotton 

 thread weighing ^nd of a grain (2*02 mg.) With the 

 tendrils of several other plants, loops weighing ^th of 



