SUMMARY OF PAR ATONIC MOVEMENTS 525 



must dismiss such an idea at once from our minds, for we are acquainted with 

 plenty of examples of movement in our own bodies which take place without 

 our being conscious of them, i.e. reflex actions, which, obviously, strongly 

 resemble these responsive movements in plants. In the higher animals, 

 however, an afferent conductive apparatus to a central organ and an 

 efferent one to the motile organ are also required for the carrying out of the 

 reflex action, in addition to the organ which perceives the stimulus (sense 

 organ) ; the existence of such a central organ in plants has been suggested 

 by CzAPEK (1898), but no reliable evidence in favour of the idea has been 

 adduced, still less do we know as to its localization. 



At present it is not easy to illustrate by means of a mechanism those move- 

 ments at least which arise as a result of a comparison of differential primary 

 stimuli. This much is clear, however, viz. that, in principle, such a machine 

 must differ from that we have employed above, inasmuch as it must show 

 two or more consecutive releasing movements before the chief or final response 

 is observed. The electric current set up by closing the key must be made to release 

 another current, or even another energy altogether, which in its turn does the work. 



We may conclude, therefore, that the primary effects of light in etiolation 

 and in heliotropism may be the same, and hence that it is sufficient to 

 assume one type of perceiving organ for every stimulant, and this leads us to the 

 conception that very frequently quite a chain of releasing movements may inter- 

 vene between the first application of the stimulus and the final response. 



Simple machines, like our electric bell arrangement, may still serve to render 

 intelligible many phenomena of stimulation, such for instance as the relation 

 between the intensity of the stimulus and the amount of the response. Looking 

 at an ordinary electric key we see that a certain amount of releasing energy must 

 be expended before perception follows. Every pressure which fails to make 

 contact, fails also to reach the hminal value of the stimulus and induces no re- 

 sponse, no matter how long it may be continued. Every pressure, on the other 

 hand, which succeeds in making contact, induces a maximum reaction. The same 

 thing occurs in Mimosa. In other stimulation phenomena, such as geotropism 

 (pp. 439 and 457), we found that the response varied according as the releasing 

 energy increased or decreased. It would not be difficult to construct a key of such 

 a character that as pressure on it was increased, more and more electric elements 

 became involved and added their currents to the total, so that the activity of 

 the apparatus would thus bear a certain quantitative relation to the releasing 

 force. The primary positive curvature of a heliotropic plant, followed by 

 a negative curvature as the intensity of the light is increased, may be demon- 

 strated by means of an electric model, where the key is pressed upon with ever- 

 increasing force first on one side and then on the other. Such an apparatus also 

 shows that the regulating arrangements seen in all stimulation phenomena, and 

 which as a rule have a definite purpose to fulfil, are not characteristic of organic 

 nature only ; they apply to machines as well. We will not pursue these analogies 

 further, however, for they are apt to mislead beginners into the belief that plant 

 phenomena are simpler than they really are. We may, however, once more 

 draw attention to the long chain of phenomena which is as a rule interpolated 

 between the reception of the stimulus and the response (compare p. 440), and 

 which in the machine is reduced to a minimum. 



If we do not recognize, after all these remarks, at least one essential differ- 

 ence between the releasing stimulus in a machine and that in an organism, then 

 the idea of a stimulus is to all intents and purposes superfluous. We retain it, 

 in the first place, on historical grounds, but also because we are able, by its means, 

 to recognize the organism at once as the scene of certain phenomena, while, at 

 the same time, admitting that we are entirely in the dark with regard to the 

 releasing process, as indeed to the entire chain of events right up to the final 

 response (Pfeffer, 1893). 



