72 MR. DARWIN ON CLIMBING 1M.ANTS. 



reduced leaflets (a, b, c, d) are highly sensitive, for a loop of thread 

 weighing only the one-sixteenth of a grain caused thnn, in under 

 4h., to become greatly curved : when the loop was removed, the 

 petioles straightened themselves in about the same time. The 

 petiole («) was rather less sensitive ; and in another specimen, in 

 which the corresponding petiole bore rather larger leaflets, a loop 

 of thread weighing one-eighth of a grain did not cause curvature 

 until 18h. had elapsed. Loops of thread weighing one-fourth of a 

 grain, left suspended on all the lower petioles (/'to /) during several 

 days, produced no effect. Yet the three petioles/", g, and A are not 

 quite insensible, for when left in contact with a stick for a day or 

 two they slowly curled round it. So that the sensibility of the 

 petiole gradually diminishes from the tendril-like extremities to 

 the base. The internodes are not at all sensitive, which makes 

 Mold's statement that they are sometimes converted into tendrils 

 the more surprising, not to say improbable. 



The whole leaf, whilst young and sensitive, stands almost ver- 

 tically upwards, as we have seen is the case with many tendrils. 

 It is in continual movement, and one that I observed swept 

 large, though irregular, ellipses, sometimes narrow, sometimes 

 broad, with their longer axes directed to different points of the 

 compass, at an average rate of about 2h. for each revolution. The 

 young internodes also, which bear the revolving leaves, likewise 

 revolve irregularly in ellipses and spires ; so that by these com- 

 bined movements a considerable space is swept for a support. If 

 the terminal and attenuated portion of the petiole fails in seizing 

 any object, it ultimately bends downwards and inwards, and then 

 soon loses all its irritability and power of movement. This bend- 

 ing down is of a very different nature from that which occurs with 

 the extremities of the young leaves in many species of Clematis ; 

 for these, when thus bent or hooked, first acquire their full degree 

 of sensitiveness. 



Dicentra thalietrifolia. — In this allied plant the metamorphosis 

 of the terminal leaflets has been complete, and they are converted 

 into perfect tendrils. Whilst the plant was young, the ten- 

 drils appeared like modified branches, so that a distinguished 

 botanist thought this was their nature ; but in a full-grown plant, 

 there can be no doubt, as I am assured by Dr. Hooker, that the 

 tendrils are modified leaves. The tendrils, when of full size, are 

 above 5 inches in length ; they bifurcate twice, thrice, or even 

 four times ; their extremities are hooked, but blunt. All the 

 branches of the tendrils are sensitive on all sides, but the basal 



