490 TRANSFORMATION OF ENERGY 



rolls itself up into a spiral, from the apex downwards. If a tendril which is 

 sensitive on all sides be stimulated first on one side and, at the same time or 

 shortly afterwards on the opposite side, no curvature results, provided the 

 stimulus on one side be equal to that on the other. If the same experiment be 

 tried with a tendril sensitive on one side only and if both upper and under sides 

 be stimulated, one would expect that the response would be the same as if the 

 under side only had been stimulated. That is, however, by no means the case, 

 for the tendril remains quite straight. If a small part of the upper side and 

 at the same time the whole of the under side be stimulated, curvature takes 

 place only at the places on the under side which lie opposite to the unstimu- 

 lated regions of the upper side. The sensitiveness to contact is thus as well 

 developed on the upper as on the under side, and the difference between the 

 two sides lies in the fact that while stimulation of the under side induces 

 curvature, stimulation of the upper side induces no visible result or simply inhibits 

 curvature on the under side, according to circumstances. Following Fitting, 

 we must therefore recognize tendrils which react eqiially on all sides and tendrils 

 which do not do so. As to the behaviour of the latter type we shall have a clearer 

 conception after we have analysed the stimulus movement into its different 

 phases. First of all let us examine the nature of the perception of the stimulus. 



On this subjfect Darwin has already made certain statements. He 

 assumed that the tendrils reacted to a definite pressure and therefore laid light 

 weights, such as wires, threads, &c., on the parts of the tendril capable of 

 movement, taking the utmost precautions against inflicting a shock. He found 

 that the tendril of Passiflora gave curvature-response to the pressure of 

 a piece of platinum wire 1-23 mg. in weight, and to a piece of cotton 

 thread 2-02 mg., while the tendrils of other plants required greater weights, 

 up to 4-9 mg., before any response could be recognized. According to 

 Pfeffer (1885), to whom we owe an elaborate investigation into the 

 phenomena of perception of contact stimulus by tendrils, these experiments of 

 Darwin's do not meet the case at all, since as a matter of fact much greater 

 pressures may be exerted on the plant without any visible response resulting. 

 A continuous or statical pressure generally never induces curvature, and even 

 in Darwin's experiments, in spite of every care being taken, vibrations could 

 not be avoided when the weights were placed on the tendrils or afterwards. 

 The shocks, slight as they were, operated as a stimulus. If shocks be not elimin- 

 ated, far lighter bodies than those employed in Darwin's experiments will induce 

 curvature, e.g. a particle of cotton thread 0-00025 mg., which was placed in posi- 

 tion simply by a draught of air. On the basis of Pfeffer's researches the 

 perceptive capacities of tendrils may be estimated in the following way. 



We may first of all inquire whether liquids as well as solids may act as 

 stimuli to the tendrils. This is obviously not the case, since if we inflict only 

 very slight blows on the tendrils with a solid body curvature appears at once, 

 while an equal shock from a liquid never induces any reaction. Pfeffer allowed 

 water, watery solutions of various substances, oil, and finally mercury to strike 

 the sensitive region of the tendril at greater or less velocities and obtained no 

 reaction, although the mercury had an injurious effect on the tendril. These 

 facts are of great importance, for they show that tendrils cannot be stimulated 

 by raindrops ; a capacity for reacting to such stimuli would be obviously quite 

 meaningless. If, however, there be any small solid particles in the liquid, such as 

 crystals in the oil, mud in the water or accidental impurities in the mercury, 

 stimulation is at once set up. It would appear therefore that tendrils are able 

 to discriminate between different conditions of aggregation of bodies and to 

 react to the solid but not to the liquid. Yet this is by no means the case, for 

 tendrils are unable to distinguish gelatine containing 10-14 per cent, of water 

 from a liquid, although that substance is solid at ordinary temperatures. This 

 fact suggested a whole series of interesting experiments, for Pfeffer was enabled. 



