Cuar. 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 Hchinocystis, 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. Passtflora 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 synd of a grain (2:02 mg.) With the 
tendrils of several other plants, loops weighing 1th of 
