478 The Physiology of Plants BOOK n 



curvature in half a minute. Perhaps a more extraordinary 

 observation was that the tendrils of Echinocystis lobata anc 

 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 ii 

 which a very fine powder was suspended. The repeatec 

 impact of these solid particles induced reaction, but no 

 the water alone. Pfeffer advanced as an explanation o 

 this behaviour the view that statical pressure alone doe 

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

 may be effective, definite points of limited extent in th< 

 sensitive region of the tendril must be affected by ; 

 push or a pull of sufficient intensity, simultaneously o: 

 in adequately rapid succession.' 



Darwin stated that the tendrils of Echinocystis refuse t( 

 climb round the stem of the plant, becoming straight am 

 non-sensitive when they approach it. Pfeffer combate.( 

 this, however, attributing the absence of coiling under sucl 

 circumstances to the smoothness and slenderness of th< 

 tendrils, together with their movements. 



Darwin showed in the course of his researches that th< 

 fixation of many tendrils is often followed by hypertro 

 phic growth. This was confirmed by many subsequen 

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

 species of Ampclopsis and Vitis, and the thickening of th 

 woody portion of the petioles of Solanum may be cited a 

 instances. More recently the hooks of Uncaria wer 

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

 example in the pulvinus of the leaflet of Dalbcrgia, th 

 woody cylinder of which becomes in a week double th 

 thickness of that of an unattached organ. 



