ULY 22, 1920] 



NATURE 



649 



tional, I find that they check growth or bring 

 about an " incipient " contraction ; when the in- 

 tensity of stimulus is increased, the effect cul- 

 minates in an actual contraction — 

 a result exactly parallel to the 

 contraction of the pulvinus under 

 direct stimulus. This would ex- 

 plain the similarity of tropic move- 

 ments in pulvinated and growing 

 organs. 



Indirect Effect of Stimuhis. — A 

 novel result was discovered under 

 indirect stimulation — that is to say, 

 when the stimulus was applied at 

 some distance from the responding 

 area, i.e. the pulvinus or the grow- 

 ing region. This caused an increase 

 of turgor, an expansion, an en- 

 hancement of the rate of growth, 

 and an erectile movement of the 

 leaf of Mimosa, and an electro- 

 motive variation of galvanometric 

 positivity. This effect is specially 

 exhibited in tissues which are semi- 

 conductors of excitation. 2 The 

 contrasted effects of direct and 

 indirect stimulus are given in the 

 following tabular statement : — 



Table 1.— Direct and Indirect Effects of Stimulus. 



Direct stimulus induces contraction; indirect 

 stimulus causes the opposite effect of expansion. 

 The same law applies when stimulus acts on 

 o 



( 



Diminution of turgor, 



contraction. 

 Fall of leaf of Mimosa. 

 Diminution of the rate of 



growth. 

 Galvanometric n e g a- 



tivity. 



Indirect 



^ 



Increase of turgor, ex- 

 pansion. 



Erection of the leaf. 



Enhancement of the rate 

 of growth. 



Galvanometric p o s i- 



tivitv. 



In Fig. 4 is given a record which shows in 

 the same specimen (i) the acceleration of growth 

 under indirect, and (2) a retardation of growth 

 under direct, stimulation. 



Fig. 4. — Effect of indirect and direct stimulation on growth : (f) shows applicaiion of 

 indirect stimuUis with consequent acceleration of growth ; application of direct 

 stimulus at ( X ) induces contraction and subsequent retardation of growth. ' 



We thus arrive at the law of effects of direct 

 and indirect stimulus : — 



* " Pl.-int Response," p. 52 >. 



NO. 2647, VOL. 105] 



. — Effects of direct and indirect stimulus: a, Stimulus applied directly at the growing re^iion 

 inducing retardation of growth or contraction as represented by dotted line (stimulated 

 area in this and in following represented as shaded) ; i, stimulus applied indirectly (at 

 some distance from growing region) gives lise to acceleration of growth and expansion; 

 c, stimulus applied at right side of organ caiiies contraction of that side and expansion of 

 the opposite side, thus giving rise to positive curvature towards stimulus ; tf, excitation 

 transmitted to the opposite side cau&es neutralisation ; e, excitation caused by intense 

 stimulation is transmitted ac! OSS and thus reverses the normal curvature to negative,!.^, 

 away from stimulus. 



one side of the organ. When stimulus of any 

 kind acts on the right side (Fig. 5c) the directly 

 stimulated right side contracts, and the indirectly 

 stimulated opposite, or left side, expands, the 

 result being a positive tropic-curvature towards 

 the stimulus. This explains the twining of 

 tendrils and positive heliotropism. 



Negative Heliotropism. — When the light is very 

 strong and long continued, the over-excited plant- 

 organs may begin to turn away. How is this 

 effected? My experiments show that the strong 

 excitation percolates into and traverses the organ 

 and provokes contraction on the further side, thus 

 neutralising their former bending (Fig. 5^). The 

 organ now places itself at right angles to the 

 light, and this particular reaction has been 

 termed dia-heliotropism. In certain cases 

 the transverse conductivity of the organ is 

 considerable. The result of this is an 

 enhanced excitation and contraction of the 

 further side, while the contraction of the 

 near side is reduced on account of fatigue 

 caused by over-excitation. The organ thus 

 bends away from light or exhibits so-called 

 negative heliotropism (Fig. 50). These 

 effects are accentuated when one side of the 

 organ is more excitable than the other. 

 Thus under the continued action of light the 

 response record shows first a movement 

 towards light, then neutralisation, and 

 finallv a movement away from light. In 

 this way a continuity of reaction is demon- 

 strated proving that the assumption of 

 specific positive and negative heliotropic sensi- 

 bility is unjustified. 



That the application of stimulus on the near 



