CHAP, xi Sensitivity and Stimulation 477 



Fitting observed that the changes in these distances set 

 in with very great rapidity, the elongation of the convex 

 side more than doubling its length in an hour, a rate of 

 increase which was more than twenty times that of an 

 unstimulated tendril. The markings on the concave side 

 showed a slight contraction of that side amounting to 

 about i per cent, per hour. Fitting concluded that these 

 results are attributable to growth changes only. 



The mechanism of the extension of the convex side of 

 a tendril which accompanies growth has been the subject 

 of a certain speculation. In 1887-9 Wortmann claimed to 

 have shown that the cell membranes of the concave side 

 of the organ are thickened and their rigidity increased by 

 the protoplasm, and that consequently the turgor of the 

 region stretches most strongly the thinner membranes of 

 the convex side. Kohl, in 1894, suggested different degrees 

 of elasticity of the cell walls in different directions, and 

 hence unequal extension under uniform turgor. Noll attri- 

 buted the change to a direct influence of the protoplasm 

 on the cell walls of the convex side, whereby they become 

 more plastic and extensible. These views, to which we 

 shall return later, differ rather strikingly from those held by 

 De Vries, according to which the action of the protoplasm 

 in all such cases causes local modifications of the turgor 

 of the cells and does not work directly upon the cell wall. 



On the general question of the behaviour of tendrils and 

 other climbing organs, the most notable investigations of 

 our whole period were those of Charles Darwin, the results of 

 which appeared in his Climbing Plants, in 1865. Among the 

 most striking discoveries he made may be noted the extreme 

 delicacy of perception which some tendrils display. A single 

 touch on the concave side of a tendril of Passi flora gracilis 

 caused a rapid curvature, which resulted in two minutes 

 in the formation of a helix. A loop of soft thread weighing 

 about half a gramme, placed three times on the tip, caused 



