Cuap, IL. CLEMATIS. 59 
slowly increased until the petiole moved through nearly 
ninety degrees: beyond this it did not move; nor did 
the petiole, the loop remaining suspended, ever become 
perfectly straight again. 
When we consider, on the one hand, the thickness 
and stiffness of the petioles, and, on the other hand, 
the thinness and softness of fine cotton thread, and 
what an extremely small weight one-sixteenth of a 
grain (405 mg.) is, these facts are remarkable. But 
I have reason to believe that even a less weight excites 
curvature when pressing over a broader surface than 
that acted on by a thread. Having noticed that 
the end of a suspended string which accidentally 
touched a petiole, caused it to bend, I took two 
pieces of thin twine, 10 inches in length (weighing 
1-64 gr.), and, tying them to a stick, let them hang as 
nearly perpendicularly downwards as their thinness 
and flexuous form, after being stretched, would per- 
mit; I then quietly placed their ends so as just 
to rest on two petioles, and these certainly became 
curved in 36 hrs. One of the ends touched the angle 
between a terminal and lateral sub-petiole, and it was 
in 48 hours caught between them as by a forceps. In 
these cases the pressure, though spread over a wider 
surface than that touched by the cotton thread, must 
have been excessively slight. 
Clematis vitalba.—The plants were in pots and not 
healthy, so that I dare not trust my observations, which . 
indicate much similarity in habits with C.flammula. I 
mention this species only because I have seen many 
