( 333 ) 



by the ciil ; lliis iievei- luippciis; ihc cut imder all cirriiiusUDices 

 heconies speedily a gaping wuiuid. If we now bring a coleoptile 

 wliieh has once more become erecl, iiilo the ordinary air of a room 

 which by contiiuions ventilation is ke|)t as pure as possible, then 

 we again see a strong curvature take place towards the wound. 



Evaporation doubtless |)lays a predominant part or, generally 

 speaki]ig, the intluence of the wound is very great in the ordinary 

 atmosphere of the I'oom. Is it therefore to be wondered at, that 

 with a cut on the posterior side and with unilateral illumination of 

 the apex in front, the base does not curve phototropically in the 

 ordinary atmosphere of the room ? 



As W'ill follow from further experiments, the oidy right interpretation 

 is that through evaporation from the wound the base has an inclination 

 to curve backwards, so that its phototropic curvature is annulled. 

 That the apex itself under these circumstances indeed curves photo- 

 ti'opically, is one })roof the more of its extraordinary sensitiveness 

 to light. p 



These experiments emphatically show that any experiments in 

 which incisions are made must per se take place in a space saturated 

 with water-vapour: this requirement has not always been fulfilled, so 

 tlial we have data in ])lant physiology on so-called traumatic stimuli 

 and their transmission, wdiich probably must be ascribed more to 

 an evaporation-effect than to the effect of a stimulus; this applies 

 for instance to the transmission of traumatic stimuli by dead elements. 

 When BoYSEN Jensen finds that the base, in a space saturated with 

 water-vapour, and with a posterior cut, curves phototropically- when 

 the apex is illuminated (I call the illuminated side the anterior), then 

 this is caused by the fact that -on account of the great decrease of 

 evaporation, the force directed backwards is also much smaller and 

 therefore the j)hototropical effect is hardly counteracted at all. But 

 we must at once add, that in the latter case the extent of the basal 

 phototropical curvature is dependent on the size of the wound but 

 above all on the time, during which the incised coleoptiles remained 

 in the atmosphere of the room before being brought into the space 

 saturated with water-vapour. A few miiuites, exposure to the air of 

 the room can indeed be sufficient to prevent the phototropic curvature 

 of the base in the atmosphere saturated with water vapour. Alike 

 in the experiments of Boysen Jensen and in my own the incision 

 was always made in the ordinary air of the room, because of the 

 very considerable difficulties that attend the performing of the various 

 operations and |)reparatiüns directly within a space saturated with 

 water vapour. 



23* 



