( 34J ) 



The specific change which ihe plant nudergoes under tiie intluence 

 of illumination, we will call phototo/ius. The phototonus is then seen 

 to bring about the ditference in sensitiveness between the aj)ex and 

 base and by reason of this in the last resoi't, the polarity of the 

 transmission of stimulus also. 



In the foregoing section emj)hasis has been laid on the fact that 

 this phototonus can al)soluteh' not be compared to the "attuning 

 phenomeiion". This phototonus expresses itself as a wave of sensi- 

 tiveness which in a short time runs through the plant organ and 

 must, on the ground of experiment, t)e regarded as distinct from the 

 phototropic curvature eifect. From the experiments on the sensitisation 

 of the base when the apex is illuminated, it is clear that the process 

 of sensitisation is primary, and the curvature effect secondary. The 

 rapidity with which the phototonus spreads through the coleoptile, 

 taken in connection with the very remarkable polarity in sensitiveness 

 of apex and base, gives rise to the surmise that we have here to 

 deal with a polar displacement followed by a polar massing of ions; 

 in the apex an accumulation of ions specially sensitive to light, in 

 the base of those less sensitive to light, and it is this process which 

 tirst shows itself oji illumination; this phototonus would therefore 

 be the actual j)rocess of perception; experiments prove that this 

 process is an independent one. We then get as a secondary process 

 the specific intluence of direct light on the two accumulations of ions; 

 this also is ah independent process and is very probably of a chemical 

 nature, a })hotocheinical process therefore ^). This photochemical process 

 causes the curvature effect; to what extent the actual curvature is 

 directly caused by any change of permeability which might be set 

 up photochemically is left undecided. The ions in the apex wdiicli 

 are very sensitive to light cause there also a very intense pholo- 

 cheniical process, which in its turn brings about an equally powerful 

 transmission towards the basal portion, while in the base the ions 

 which are slightly sensitive to light cause a weak basal photochemical 

 process with a corresponding!}^ weak transmission of negligible 

 magnitude; it is thus that the phenomenon of irreversibility of the 

 transmission of phototropic stimulus arises. 



If we take the general standpoint of the j)hysiology of stimulus, 

 that where there is absence of or little sensitiveness, absence of or 

 slight transmission of stimulus is to be expected, then the well-known 

 slowness of transmission of vegetable phototropic slimuli is explained 

 by the tact that the base is so slightly sunsiti\e. 



1) A. H. Blaauw. Die Perzeption des Lichtes. (Recucil ties Travaux Bolaniciucs 



Néei'landais. Vol. 5). 



