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coleoptile and in it we therefore see the perception retlecteil. Hut 

 this current of sensitiveness in itself never gives rise to curvature 

 effect; for tliat, direct illumination is necessary, a second physiological 

 principle, as it were of light which calls into existence a distinct 

 process, namely the curvature elfect, which thus manifests itself to 

 some extent as independent of the perception process. When we 

 seek to explain the facts described above we are thus justified in 

 expressing in our hypothesis, the independent course of these two 

 photo-physiological processes, and so the results of the present 

 investigation have perhaps brought us a step nearer to the solution 

 of the process of phototropic stimulation. I shall now attempt to 

 construct with the facts mentioned a hypothesis which may perhaps 

 lead to further investigations in the field of the physiology of stimulus. 



Let ns first of all take the really striking fact that the base is so 

 very much less sensitive than the apex. We must duly consider 

 that we are dealing with the following phenomenon : in one and 

 the same small organ two parts are in proximity, only separated 

 from each other by about 1 cm. and differing in age only by about 

 one day, and these parts show a difference of sensitiveness to light 

 to the extent of the apex being J 500 times more sensitive than 

 the base. The question is whether such a difference of sensitiveness 

 corresponds to a normal condition ; can it l)e regarded as true under 

 all circumstances? is it a predisposition, independent of any illumination, 

 a difference in the constitution of the pi-otoplasm existing apart from 

 any effect of light? 



Is it probable that the plasma of one aiul the same young tissue 

 growing vigorously and healthily in every part, can in the course of 

 one day decrease so enornu^usly in sensitiveness that the apex possesses 

 an almost proverbial sensitiveness, while with respect to the base 

 there has even been a time (I am thinking of Darwin) when the 

 latter was regarded as insensitive to light? 



Such a phenomenon must l)e based on something else than a 

 decrease of sensitiveness in the ordinary sense of the word. It points 

 to a certain definite influence which the illumination as such exercises 

 on the coleoptile, and suggests a certain change which the plant 

 undergoes by means of the illumination. If in this polarity of the 

 sensitiveness of apex and base, we were inclined to see a suggestion 

 of the polarity of the transmission of stimulus, the admission of a 

 change wliich is brought about by the light itself corresponds with 

 the fact that the polarity of phototropic transmission of stimulus is 

 also not influenced by external circumstances which lie outside the 

 actual illumination. 



