524 TRANSFORMATION OF ENERGY 



stimulus, but which is in connexion with different kinds of apparatus. This 

 would correspond to the case where the electric wire, closed by the key, was 

 connected up, in one case with a bell, in another case with a glow-lamp, and in 

 a third with, let us say, a voltameter ; the work done as a consequence of the same 

 releasing stimulus would be quite different in each case. 2. It is also possible, 

 however, that the difference in the reaction depends upon the varying struc- 

 ture of the perceptive organ. NOLL (1892) makes an assumption somewhat 

 like this, and has accounted for the different forms of geo tropic response in this 

 way. There are many arguments against such a hypothesis, however, and we 

 are inclined to think the first view is the more probable one. 



Let us now inquire whether there is or is not only one kind of perceptive 

 apparatus for each and every stimulating agent. The same stimulus can 

 indeed induce a response in quite different ways. Light, for example, gives 

 a stimulus to the plant when it affects the plant equally in every part, and 

 the response given by the plant is indicated by alteration in its rate of growth. 

 The response is totally different when the light falls with unequal intensity 

 on opposite sides, say, of the shoot, for then the plant responds by exhibiting 

 heliotropic curvatures. In contrast with these regional light stimuli we have 

 the periodic variations in light intensity which lead to nyctitropic movements. 

 The preliminary phenomena of stimulation in the case of heliotropism and 

 nyctitropism are doubtless different from those which lead to etiolation. 

 We have been compelled, for good reasons, to discard the ' etiolation theory ' 

 of heliotropism, and on similar grounds we are entitled to look with scepticism 

 on any hypothesis which would seek to explain thermotropic or chemotropic 

 curvatures by asserting that the organ in question exhibited growth in each 

 longitudinal area with a rapidity proportional to the temperature to which it was 

 exposed or to any definite concentration of the chemical medium employed. The 

 facts are in striking opposition to such a hypothesis, for under certain circum- 

 stances the side that is more remote from the influence of the optimum tem- 

 perature grows more rapidly than the others (compare p. 480). Although 

 the phenomena precedent to stimulation in etiolated growth are doubtless 

 different from those of heliotropism the first effect of light may be the same in 

 both cases ; we may assume, for instance, that in each cell a quantity of 

 a certain material, proportional in amount to the intensity of the light, dis- 

 appears with the discontinuance of illumination. This change, consequent on 

 the action of light, may be compared with the closure of the key as a result of 

 pressure, it indicates a preliminary chemical (or physical) action of the stimu- 

 lant, and may represent what we term perception. If thereafter an acceleration 

 or retardation of growth takes place in each sensitive cell, we must look upon 

 that as the released movement. We have already especially noted that, in the 

 case of etiolation, not all cells which perceive the stimulus proceed to react to 

 it, otherwise all organs must elongate in darkness ; experience teaches us that 

 organs behave differently in this respect ; correlations between the individual 

 units prevent a similar reaction of all. All the same we may assume in this case 

 that perception and reaction striven for are similar throughout ; secondary 

 influences, however, which we need not consider here, but which may be of 

 a relatively simple character, may interfere with the reaction in certain regions. 



The phenomena of heliotropism are different and more complicated. When 

 perception, variable in its intensity, is brought about in different cells in con- 

 sequence of unequal intensity of light, reaction does not follow it directly. On 

 the other hand, the varying degree of sensation on opposite sides operates as 

 a new stimulus and it is this that induces movement. Assuming that unequal 

 sensitiveness in different regions acts as a new stimulus, we must grant to the 

 plant the power of comparing the primary light effects in different situations. 

 The term comparison suggests that we have here to deal with a psychical capacity 

 in the plant. Although psychical capacity suggests consciousness, still we 



