( 329 ) 



Naturally the first tliijig that strikes one is the great rapidity 

 with which, to the naked eye, the reaction sets in; the next thing 

 worthy of attention is the strongly marked parallelism between the 

 amount of energy applied and the time of curvature, which hitherto 

 has not really shown itself in the physiology of any stimulation effect 

 at one and the same temperature of 18° C. Further we see that the 

 softer the object is the longer must it be rubbed in order to obtain 

 corresponding times of curvature. 



It is also certainly worthy of note that one can quite well obtain 

 a contact curvature with soft gelatine, although after rubbing longer 

 and waiting longer; we here recall the studies of Pfeffer on tendrils. 

 In short, although these facts give only a cursory and very imperfect 

 representation of the sensitiveness to contact-stimulation of the 

 coleoptiles of Avena, they are doubtless remarkable enough to be 

 worked out in a further investigation. 



Now this sensitiveness to contact-stimulation plays a very important 

 part in experiments with the coleoptiles of Avena and when it 

 is neglected may lead to all kinds of erroneous and unexpected 

 results. 



A knowledge of the sensitiveness to contact-stimulation and of 

 the typical contact curvatures was of the greatest importance in those 

 of my experiments where it was necessary that the coleoptiles should 

 be touched in various ways, either by cutting them, or by placing 

 on them little caps or by covering them with closely fitting little 

 cylinders; every investigator in this field must above all make the 

 necessary preliminary studies of this point. For a few striking cases 

 this paper may be referred to. 



At the same time the investigation of the susceptibilitj' of the 

 coleoptiles of Avena to stimuli has been enriched by a new field of 

 work, which in certain respects raised the importance of this plant 

 for the physiology of stimulation, yet, on the other hand, experi- 

 mentation with so extremely sensitive an object thus becomes extra- 

 ordinarily difficult. 



In the second place a few words must be said in this introductory 

 section with regard to the determination of phototropic sensitiveness, 

 as it has been carried out in the present investigation and as it might 

 perhaps be carried out with advantage in all future phototropic 

 experiments. The earlier view adopts as measure of sensitiveness, the 

 quantity of energy which is necessary for the crossing of the threshold 

 of stimulation; the smaller the amount of energy required for this, 

 the greater the sensitiveness of the plant. But the recent investigations 



