( 330 ) 



of Arisz^) have made us somewhat sceptical with regard to the 

 existence of a threshold of stimulation ; fairly conclusive experiments 

 have rendered its existence very doubtful. On the other hand there 

 certainly exists a definite maximal curvature which appears after a 

 definite time and at any definite temperature corresponds to a definite 

 quantity of energy. 



The more sensitive a plant is, the less light is necessary to produce 

 one and the same definite maximal curvature. Now in order to be 

 able to compare plants of different sensitiveness with one another 

 it is necessary to have a standard to indicate that the different 

 quantities of energy all correspond to one and the same definite 

 maximal curvature. 



B'or practical considerations the angle of curvature cannot be taken 

 as this standard, seeing that its determination presents great difficulties 

 and would demand much time. Further, just as little can we accept 

 the maximal curvature which is still just visible to the naked eye, 

 because we can observe exceedingly small curvatures with the naked 

 eye; and this would be a source of individual errors; but this 

 method is moreover especially objectionable, because these curvatures 

 only arise after a long time and the longer an experiment lasts, the 

 more unfavourable it is for the object and for the result, especially 

 in the case of seedlings which are operated upon by one or another 

 method. 



We must indeed above all use a method in which the duration 

 of the experiment shall be as short as possible. And seeing that the 

 above described maximal curvature occurs after a definite time and 

 it being so remarkable that, Avhen two seedlings of different sensi- 

 tiveness get such a quantity of light, that they produce the same 

 maximal curvature, this maximal curvature is attained in the same 

 time, this is also an indication, to express the measure of maximal 

 curvature in terms of the time in which it arises. We are then able 

 to make this time as small as possible, in which endeavour we are 

 helped by carrying out the experiments at higher temperatures. 



In this investigation I have worked with maximal curvatures which 

 occur at 25 — 27° C. after half an hour, an interval which might 

 even be somewhat shortened in the favourable condition of high 

 temperatures. In order to obtain this maximal curvature after half 

 an hour, a definite quantity of energy is necessary. If now a plant 

 is more sensitive than the object with which it is being compared 



1) W. H. Arisz. On the connection between stimulus and effect in the photo- 

 tropic curvatures of seedlings of Avena saliva. (Proceedings Kon. Akad. v. Wetensch. 

 March 1911. 



