478 The Physiology of Plants BOOK in 



curvature in half a minute. Perhaps a more extraordinary 

 observation was that the tendrils of Echinocystis lobata and 

 Passiflora gracilis are able to discriminate between the con- 

 tacts made by falling raindrops and those of solid bodies, 

 refusing to bend to the former. This remarkable pheno- 

 menon, at first appearing to indicate a rudimentary con- 

 sciousness on the part of the plant, was subsequently investi- 

 gated by Pfeffer, who, in 1885, repeated the experiments, 

 and added to them by dropping on the tendrils water in 

 which a very fine powder was suspended. The repeated 

 impact of these solid particles induced reaction, but not 

 the water alone. Pfeffer advanced as an explanation of 

 this behaviour the view that statical pressure alone does 

 not act as a stimulus. He said, ' in order that a stimulus 

 may be effective, definite points of limited extent in the 

 sensitive region of the tendril must be affected by a 

 push or a pull of sufficient intensity, simultaneously or 

 in adequately rapid succession.' 



Darwin stated that the tendrils of Echinocystis refuse to 

 climb round the stem of the plant, becoming straight and 

 non-sensitive when they approach it. Pfeffer combated 

 this, however, attributing the absence of coiling under such 

 circumstances to the smoothness and slenderness of the 

 tendrils, together with their movements. 



Darwin showed in the course of his researches that the 

 fixation of many tendrils is often followed by hypertro- 

 phic growth. This was confirmed by many subsequent 

 writers. The discs developed on the tips of the tendrils of 

 species of Ampdopsis and Vitis, and the thickening of the 

 woody portion of the petioles of Solanum may be cited as 

 instances. More recently the hooks of Uncaria were 

 described by Treub, while Ewart added, in 1898, a further 

 example in the pulvinus of the leaflet of Dalbergia, the 

 woody cylinder of which becomes in a week double the 

 thickness of that of an unattached organ. 



