AUTONOMOUS MOVEMENTS 527 



exhibit dissimilar degrees of dependence on external formal conditions, and 

 this fact enables us in certain cases to distinguish between perception, con- 

 duction, and reaction (compare p. 440) in cases where other criteria are 

 not available. For example, geotropic response is inhibited by chloroform 

 although its perception is not ; in other cases the same anaesthetic may con- 

 versely destroy the power of perception of a stimulus and yet not interfere 

 with the power of movement to any great extent (compare ROTHERT, 1903). 



In conclusion, let us inquire how these formal conditions really operate. 

 Do they operate in virtue of their own inherent energy or do they act as releasing 

 agents only ? In by far the majority of cases an exact answer cannot be given 

 to this question ; there can be little doubt, however, that certain essential sub- 

 stances, by bringing to the plant a store of energy or of constructive material, 

 act as energizing bodies, while temperature and, generally speaking, the 

 majority of the formal conditions are doubtless to be regarded as releasing 

 factors only. How then are we to distinguish between formal conditions and 

 specific stimuli ? In many cases the distinction is undoubtedly possible, as, for 

 instance, if the formal condition, e.g. temperature, is to be considered as 

 a general stimulus for many or all of the vital processes, and the factor, regarded 

 merely as a stimulus, may be proved to induce a single change or movement. In 

 other cases no such distinction can be drawn, for when several external con- 

 ditions are operative at the same time it is exceedingly difficult to say which 

 of them are to be considered as formal conditions and which as specific stimuli. 

 An example will make this clear. A lateral root in darkness assumes a different 

 geotropic rest position from what it does in light. If the roots be first of all 

 grown on a klinostat and then allowed to carry out a geotropic curvature, we 

 must look on gravity as the stimulus and say that the result of this stimulus is 

 different according as it operates on an illuminated or a darkened root. If we 

 allow the root, however, to attain its geotropic rest position in the dark and then 

 illuminate it, we must conclude that the light is the stimulus which brings about 

 the ensuing curvature. Similar cases are of very frequent occurrence. 



If we regard all the formal conditions as operating with optimum intensity, 

 and if care be taken that they remain constant for a long time, and that other 

 external influences are entirely excluded, the movements in response to stimuli 

 hitherto studied cannot take place ; but it would be quite incorrect to assume 

 that the plant under these conditions was incapable of movement. In the first 

 place, it is obvious that the conditions which we have assumed are operating 

 are just those which are favourable to growth, and all growth is necessarily 

 accompanied by movement. Although many plant organs when exposed to 

 uniform external conditions grow more or less in a straight line, there are other 

 organs also which exhibit growth curvatures without any specific external stimuli 

 being applied, which are very similar in character to curvatures in response 

 to stimuli already studied. Variation curvatures, however, do not, in general, 

 cease when specific stimuli leading to curvature are absent. Growth and varia- 

 tion movements, which cannot be referred to definite external stimuli, but 

 which are dependent on formal conditions just as are the reflexes, are spoken 

 of as autonomous or spontaneous movements. Each movement must naturally, 

 all the same, have its own specific cause and the term 'spontaneous' must not 

 be taken as synonymous with ' causeless '. If external factors be excluded 

 from consideration as the agents to which these movements are due, internal 

 factors must be assumed in their place. When we further investigate what 

 these internal causes in turn really are, it would appear in the highest degree 

 probable that we are dealing here also not with energizing but with releasing 

 agents ; in other words, spontaneous movements are to be considered also as 

 stimulus-movements, although the stimuli inducing them are not external but 

 internal and unknown. Although we have contrasted spontaneous movements 



