623 



certainty witli regard to the oeciirreiice of a negative photolropic 

 ciirxature. We iiave thus to consider the fact that at small intensities 

 no negative curvatures were observed, whereas at greater intensities, 

 as ijideed Clakk also found, after stimulation with a definite amount 

 of energy the plants curve negatively. Clark's observations were 

 entirely at variance with the energy law. The question now arises, 

 whether the facts, as above set forth, necessitate a limitation of the 

 energy law to smaller amounts of energy. It seems to me that from 

 the data obtained for negative cui-vatures we may not draw the 

 conclusion that the energy law is invalid for small intensities and a 

 long duration of the stimulus. There are so many facts in favour of 

 the general validity of this law that it is safer to assume tliat the 

 occurrence of negative curvature is not entirely dependent on a 

 definite quantity of energy. It is necessary that this quantity should 

 be supplied withiji a certain time, for otherwise, owing to piocesses 

 to be discussed below, the effect is so much diminished, that the 

 excitation, which is required for the negative curvature, is no longei- 

 reached. 



In place of the negative curvature there arises again at all inten- 

 sities employed a positive one, when the illumination is continued 

 for a longer period. For this second positi\-e curvature also there is 

 a striking discre})ancy between Clakk's figures and my own. My 

 figures (as indeed those of Clark) shov.' convincingly, that the occur- 

 rence of the second positive curvature is not dependent on a definite 

 quantity of energy. 



If we take into consideration the well-known fact, that it is not 

 even necessary to supply this energy unilaterally, but that the latter 

 as Pringsheim has shown, may be partially replaced by an illumi- 

 nation from the opposite side, then the hypothesis presents itself to 

 us that this second positive curvature arises through a process which 

 is independent of the direction of illumination. This process results 

 in a lowering of the excitation. In this train of thought there is 

 therefore no essential difference between the Jirst and the second 

 positive curvature. On further consideration of the tables an additional 

 conclusion may be drawn. We see that the duration of stimulus, 

 i.e. the time during which illumination was necessary to induce the 

 second positive curvature, decreases continuously at greater intensities, 

 that is to say, that tlie intensity of the process, throuqli. which the 

 excitation diminishes is greater according as the qaantiti/ of energu 

 supplied per unit of time increases. We see therefore in unilateral 

 illumination the same process which we have studied as adjustment 

 phenomenon with omnilateral fore-illumiuatiou. In that case also 



40* 



