HABITS OF CLIMBING PLANTS. 173 



natural families, beginning with Bignoniacece, which order 

 contains tendril-bearers, leaf-climbers, twiners, root-climbers, 

 and various combinations of these diverse modes. We, how- 

 ever, will first consider the tendrils of the Gourd, and Pas- 

 sion-flower families, regarding them as typical and simple 

 representatives of tendril-climbers. 



Passiflora gracilis., a delicate annual species, lately in- 

 troduced into the gardens, of the easiest cultivation, one 

 which differs from most of its relatives in the young inter- 

 nodes having the power of revolving , is said by Mr. Dar- 

 win to exceed all other climbing plants in the rapidity of 

 its movements, and all tendril-bearers in the sensitiveness of 

 its tendrils. In the latter respect it decidedly surpasses our 

 Echinocystis ; but it is nearly if not quite equalled by Sicyos, 

 in which the coiling upon contact was first noticed as a vis- 

 ible movement. The revolving internodes, when in the best 

 condition, make almost hourly revolutions, and the long, deli- 

 cate, straight tendrils revolve nearly in the same manner and 

 at the same rate. The sensitiveness of the tendril, when full- 

 grown, is correspondingly great, a single light touch on the 

 concave surface of the tip causing a considerable curvature. 

 " A loop of soft thread weighing J^^ o^ ^ grain, placed most 

 gently on the tip, thrice plainly caused it to curve, as twice 

 did a bent bit of thin platinum wire weighing -^^^th of a grain ; 

 but this latter weight, when left suspended, did not suffice to 

 cause permanent curvature." After touch with the twig, the 

 tip begins to bend in from 25 to 39 seconds. After coiling 

 into an open helix upon transient irritation, they soon 

 straighten again, recovering their sensibility ; but if left in 

 contact, the action continues. We found it a pretty experi- 

 ment, last summer, during the warmest days, to bring the upper 

 part of an outstretched tendril by its inner or concave side 

 against a twig or cord, and to see how promptly it would clasp 

 it, revolving its free apex round and round it. A curious dis- 

 crimination in the sensibility of such tendrils is mentioned 

 by Mr. Darwin, as follows : — 



" I repeated the experiment made on the Echinocystis, and 

 placed several plants of this Passiflora so close together that 



