SUPPLEMENT 153 



it might be either the releasing stimulus itself or some other factor that brought 

 about the insertion ; internal agencies might, for instance, operate so that, in 

 accordance with the construction of the mechanism, the voltameter might first 

 of all be in circuit, and that when a certain amount of silver had been deposited 

 the bell would then begin to ring ; after a certain period of action of the bell 

 or of the voltameter, the lamp would finally begin to glow, &c. 



Even assuming all this, we are yet far from having reached the degree 

 of complication seen in the organism. More especially we must remember that 

 the same external factor may operate on the plant in very diverse ways. Thus 

 light, when affecting the plant generally, with constant intensity, may act as 

 a stimulant, and the response on the part of the plant shows itself in the rate 

 of growth ; a quite different stimulatory effect follows when the light falls 

 on one side only, for the plant then responds generally with heliotropic curva- 

 tures. In contrast to these regional differences we have the periodic varia- 

 tions in light intensity which lead to nyctinastic movement. The preliminary 

 phenomena of stimulation in heliotropism, nyctinasty, and etiolation doubt- 

 less differ from each other, however, not merely in the fact that one and the 

 same perceptive organ is associated with different reactions ; in addition there 

 must be differences both at the beginning of the stimulation process and in 

 the perceptive process. We will attempt to show this at least for etiolation 

 and heliotropism. We may perhaps express the stimulus action of light in- 

 tensity by saying that certain materials arise in each cell proportional in 

 amount to the light intensity. Should each cell react independently, darken- 

 ing of the plant would result in similar elongation in all organs, but experience 

 teaches us that leaves behave quite differently from stems. In spite of, 

 apparently, the same kind of perception in both organs, the reactions 

 are quite distinct and are conditioned by correlations between the parts 

 concerned. 



The matter is different, and more complex in the case of helio- 

 tropism. 



1. 30 P. 525, 1. 22, for Although . . . response, read That heliotropic 

 curvature is not induced by unequal-sided etiolation may be concluded from 

 the behaviour of the hypocotyls of the Paniceae. These structures certainly 

 react with etiolation phenomena to diminution of light (FITTING, MS.), but 

 they are able to carry out heliotropic curvature only after excitations which 

 proceed from the apex. Other experiments carried out on grass seedlings 

 show clearly that unequal illumination of opposite halves is by no means 

 a necessary condition of heliotropic stimulation. According to FITTING we 

 must assume that the heliotropic action of light is to induce a ' polarized ' 

 state in the sensitive organ. The plant must then have the power of appre- 

 ciating the direction of the polarity induced by the unilateral light impact ; it 

 must obviously perceive the degree of divergence of this newly induced polarity' 

 from that already existing in its complete structure, that is to say, we ascribe 

 to it the power of comparing two conditions one with the other. We are 

 forced to admit this power of comparison from other heliotropic phenomena. 

 Thus if we allow light to fall on a plant from two sides, we find that there 

 must exist a quite definite relationship in the intensities of the light, if a helio- 

 tropic movement is to result. There can be no doubt that, in general, the 

 same relations repeat themselves in all tropisms that, in other words, a ' differ- 

 ential sensitivity ' exists in the plant in all cases. This phrase, as also the term 

 ' comparison ', might suggest that we here have to deal with psychical capaci- 

 ties on the part of the plant. Although psychical capacity suggests con- 

 sciousness, still we must dismiss such an idea at once from our minds, for there 

 is nothing in the plant in the nature of even the most primitive form of con- 

 sciousness. One would rather think of comparing such movements in plants 



