650 



NATURE 



rjULY 22, 1920 



side of the organ induces at first an increase of 

 turgor on the distal side and that this first effect 

 may be neutralised and reversed by transverse 

 conduction of excitation are seen strikingly ex- 

 hibited in the accompanying record (Fig. 6), 

 where a narrow beam of light was applied at a 

 point of the stem diametrically opposite to the 

 motile leaf which was to serve as the indicator of 

 the induced variation of turgor under the unilateral 

 action of light. That this indirect stimulation 

 caused an enhancement of turgor of the opposite 

 side was soon demonstrated by the erectile move- 

 ment of the leaf. When the stimulus is moderate 

 and of short duration, the response is only erectile 

 or positive. But when the stimulation is con- 

 tinued the excitatory impulse is conducted to the 

 distal side, giving rise to diminution of turgor, 

 contraction, and the fall of the leaf. 



Fig. 6.— Increased turgor due to indirect stimulation inducing erectic 

 a, diagram of experiment ; h, erectile response (shown by down-curve 

 fall (up-curve) due to iransveise conduction of excitation. 



Table II. — Showing Responsive Effects Common 

 to Pulvini and Growing Organs under Uni- 

 lateral Stimulation. 



Effect of direct stimulation on 

 proximal side • 



Diminution of turgor. 



Contraction and con- 

 cavity. 



Galvanometric n e g a- 

 tivitv. 



Effect of indirect stimulation on 

 distal side 



Increase of turgor. 

 Expansion and convexity. 



Galvanometric 

 tivity. 



p o s 1- 



When stimulus is strong or long-continued, the 

 excitatory effect is conducted to the distal side, 

 neutralising or reversing the first response. 



Space does not allow my entering into the ques- 

 tion of Nyctitropism, which will be found fully 

 explained in the "Life Movements in Plants," 

 vol. ii. 



Geotropism. — No phenomenon of tropic re- 

 sponse appears to be so inexplicable as the oppo- 

 site effects of stimulus of gravity on the root and 

 the shoot. As regards the mechanism of the up- 

 curving of a horizontally laid shoot, it may be due 



NO. 26.'i.7^ VOL. 105] 



either to the expansion of the lower side or to an 

 active contraction of the other. In order to decide 

 the question I devised the method of geo-electric 

 response whereby the state of excitation (which is 

 attended by contraction) is independently detected 

 by the induced electromotive change of galvano- 

 metric negativity. Displacement of the shoot from 

 the vertical to the horizontal position is found to 

 be immediately followed by the clearest electric 

 indication that the upper is the excited side. The 

 electrical response is found to increase as the 

 sine of the angle of inclination. This excitation 

 of the upper side involves its contraction and the 

 resulting geotropic curvature upwards. 



Localisation of Geo-perceptive Layer by Means 

 of the Electric Probe. — The new investigation was 

 carried out by means of my electric probe, which 

 consists of an exceedingly fine platinum wire en- 

 closed in a capillary glass tube, the 

 probe being thus electrically insulated 

 except at the extreme tip. When the 

 probe, suitably connected with a gal- 

 vanometer, is slowly thrust into the 

 stem, so that it enters one side and 

 comes out at the other, the galvano- 

 meter deflection shows by its indica- 

 tion the state of irritation of every 

 layer of cells throughout the organ. 

 When the stem is held in a vertical 

 position the probe during its passage 

 shows little or no electric sign of irrita- 

 tion. But when the stem is displaced 

 from the vertical to the horizontal 

 position, the geotropically sensitive 

 layer now perceives the stimulus and 

 becomes the focus of irritation, and 

 the probe on reaching this point gives 

 the maximum deflection of galvano- 

 metric negativity. This electric indication of irrita- 

 tion disappears as soon as the geotropic stimulus 

 is removed by restoration of the stem to a vertical 

 position. I was thus able to map out the contour 

 lines of physiological excitation inside a living 

 organ. The geo-perceptive layer was thus local- 

 ised at the endodermis. 



In geotropic response the only anomaly that 

 remained was in regard to the response of the 

 root being opposite to that of the shoot. Every 

 cut portion of the growing region of the shoot 

 responds to the stimulus of gravity by bending 

 upwards. The growing region of the shoot is 

 therefore both sensitive to stimulus and responsive 

 to it. Hence geotropic stimulation of the shoot 

 is direct. But this is not the case with the root ; 

 here it is the tip of the root which perceives the 

 stimulus, the geotropic bending taking place at 

 some distance from the tip. From the results of 

 electric investigation I find that the root tip be- 

 comes directly stimulated, while the responding 

 growing region some distance from it becomes 

 indirectly stimulated. Hence geotropic stimu- 

 lus acts indirectly in the responding region 

 of the root. I have shown that the effects of direct 

 and indirect stimulus on growth are antithetic ; it 



1 of Mimosa leaf: 

 I followed by rapid 



