THE SPECIAL IRRITABILITY OF TENDRIL-CLIMBERS 53 



whereas a piece of wood less than 2 mm. diameter and o-i gram weight 

 acts as a sub-minimal stimulus to Bauhinia tomentosa. The irritable hooks 

 of Uncaria^ Artabotrys, and Roucheria require the attachment of weights 

 of 100 to 1,000 milligrams according to whether a rough bamboo fibre 

 or hard twine, or a smooth copper wire or glass thread is used 1 . The 

 most sensitive tendrils may curve five to twenty seconds after stimulation, 

 whereas less sensitive ones may take one or more hours to respond 

 perceptibly 2 . The tendrils of Dalbergia linga begin to curve in five 

 minutes, those of Vitis discolor in one hour, those of Strychnos in twelve 

 hours, the root-tendrils of Vanilla planifolia in twenty-four hours, whereas 

 no increase of thickness resulting from stimulation can be detected until 

 after the lapse of one or more days 3 . 



The stimulus usually needs to act for some time to produce a response, 

 but in very sensitive tendrils a single strong contact is sufficient to produce 

 a slight curvature. This as well as more pronounced curvature is followed 

 by a straightening due to orthotropism if the contact stimulus no longer 

 acts 4 . Since the tendril remains irritable, Darwin was able to stimulate 

 the tendril of Passiflora gracilis twenty-one times in fifty-four hours, each 

 time the tendril being allowed to straighten after forming a hook-like 

 curvature. 



When a weak continuous stimulus is applied, the tendril first bends 

 beyond the ultimate curvature resulting from the antagonism between its 

 orthotropism and the applied stimulus 5 . Although we may say that the 

 tendril accommodates itself to the stimulus, it is not certain whether this 

 is due to the gradual awakening of opposing reactions, or to the decrease 

 of the excitability, or to a combination of factors. No decisive conclusion 

 can be made from the fact that the satisfaction of the contact irritability of 

 Cuscuta produces a periodic inhibition of this irritability. 



The minimal stimulus needs to be surpassed in order to cause the tendril 

 to coil completely around the support, and to maintain the coiling until 

 growth has ceased and the coils are permanent. The stimulus exercised 

 by a support is usually sufficient for this, and in fact slender sensitive 

 tendrils are able to form close coils around a thin thread. The less 

 sensitive and thicker tendrils of Vitis * are, however, only able to form 

 loose coils around supports less than 2 or 3 mm. diameter, while the hooks 

 of tropical climbers are usually unable to become firmly attached to 



1 Ewart, I.e., pp. 211, 223, 231. 



2 Darwin, 1. c., p. 172 ; Pfeffer, 1. c., p. 486 ; Miiller, 1. c., p. 109. 



3 Ewart, 1. c., pp. 209, 223, 229, 236. 



* Darwin, I.e.; de Vries, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 306; Fitting, I.e., 

 p. 6n. This straightening was first observed by Gray, Edinburgh New Phil. Journ., 1859, Vol. X, 

 P- 307. 



5 Pfeffer, I.e., p. 507 ; Darwin, I.e., p. 132. 



' Sachs, Lehrb. d. Bot., 4. Aufl., p. 872 ; de Vries, 1. c., p. 307. 



