621 



illuiiiina^ion, tluil llie strongest uniltiteral arier-illimiiiiiilioiis no lungei' 

 bring about negative curvatures. While at 5.5 C. M. the positive 

 curvatures only occur after a fore-illuniinalion of 20 niinules, we 

 see that with more intense fore-illuniination strong positive curva- 

 tures already occur in j)lants which had only 5 minutes fore-illu- 

 minatioji. 



Tables 11 and 111 show a furtiier feature to this extent, that 

 with more prolonged fore-illumination smaller quantities of energy 

 suffice • to give again a positive reaction, i. e. the plants become as 

 it were more sensitive. The four tables show gradual transition and 

 present a concordant picture. AH tables demonsti-ate the existence 

 side by side of at least two different processes. 



In the first place after any preliminary illumination a larger 

 amount of energy is required to bring about a positive reaction. 

 Secondly after a certain duration of the preliminary illumination the 

 capacity of giving negative curvatures is lost more or less completely ; 

 after more prolonged fore-illuminations only positive curvatures occur. 

 This second process, the adjustment phenomenon, recalls the phenomena 

 which are known to occur with unilateral illumination of greater 

 duration. In that case also the capacity of giving negative curvatures 

 is lost and after })rolonged illumination only positive curvatures appear. 

 Let us therefore first consider unilateral illuminations of great 

 duration. 



§ 4. Unilateral lllmni nations of (jrcat daration. 



A preliminary idea may be obtained from the following table of 

 intensities from 1 .4 to about 20000 candle metre power. 



TABLE V. 

 Unilateral illummation. 



Intensity in candle metre power. 



40 



Proceedings Royal Acad. Amsterdam. Vol. XVI. 



