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phototropic change of sensitiveness after preliminary illumination 
(Arisz 1914 le. “fading of the excitation’). 
CrarK |. ¢. has described experiments in which a unilateral geo- 
tropic stimulation was followed by omnilateral illumination. Under 
these conditions also there occurs a curvature in contrary direction. 
With regard to the nature of the curvature which arises, it is difficult 
to form a definite judgment by this method of experimentation, 
because the geotropic stimulation induces a dorsiventrality (as vet 
not outwardly visible). A dorsiventral organ can, however, quite 
easily react to an omnilaterally symmetrical stimulus with a curva- 
ture of definite direction. In connection with the experiments 
described above it is however indeed probable that the curvatures 
mentioned by Crark correspond to those observed in the present 
investigation. 
May we now regard these curvatures as positively geotropic? Before 
answering this question, we may briefly examine the curvatures of 
opposite direction which arise in other cases and consider whether 
it is possible to form a simple conception of the way in which they 
arise. I have chosen Avena coleoptiles for a further analysis, because 
in their case inverse phototropic curvatures are very easily obtainable. 
In Arisz’ experiments (I. e. 1914), in which the seedlings were 
given an omnilateral preliminary illumination of varying duration, 
it was found that the sensitiveness rapidly diminished at the beginning, 
and after more prolonged illumination increased again somewhat. 
If the intensity of illumination was also varied, then the initial 
decrease in sensitiveness was seen to take place more rapidly accord- 
ing as the seedlings were exposed to stronger light, whilst the 
return of sensitiveness was thereby slightly delayed. Since when 
illumination is unilateral the front absorbs part of the light, the 
back receives less light. The consequence of this is that there the sen- 
sitiveness during illumination declines less markedly than in the front. 
When therefore after some time the sensitiveness of the front has 
more or less disappeared, the reaction of the posterior side can pre- 
dominate. The result must then be a curvature away from the 
source of light. 
In order to find out whether the here postulated differences in 
sensitiveness of the anterior and posterior sides can actually be 
observed, I have made a series of experiments in which three groups 
of boxes were always compared. The first group consisted of one 
box, the other two of from four to eight. The experiment began 
with an equally long and equally strong unilateral illumination of 
all the boxes. Afterwards the box of the first group was placed in 
