386 S. O. MAST 



in the sense of orientation, consequently, can be induced by a 

 number of different factors, and this seems to indicate, as con- 

 cluded in the preceding paper in this series ('18, p. 518) that it 

 is due to some specific physiological change which can be pro- 

 duced by any one of the factors referred to. 



The fact that the region of maximum efficiency for reversion 

 probably coincides with that for stimulation indicates that the 

 physiological phenomena associated with these two processes 

 may be the same. Stimulation, however, is usually if not al- 

 ways accompanied with an increase in permeability and a de- 

 crease in electrical potential. Reversion, then, if our contention 

 is correct, should also be accompanied with changes in permea- 

 bihty and all of the factors which produce positive orientation 

 should induce changes in one direction while all those which 

 produce negative orientation should induce changes in the oppo- 

 site direction. As previously stated, decrease in illumination, 

 decrease in alkalinity, increase in anesthetics, increase in tem- 

 perature, and increase in age all tend to produce positive orienta- 

 tion. What effect have these factors on permeability? 



Krabbe ('96) found in stems of Helianthus annuus that a rise 

 in temperature greatly increases permeability, and Rysselberghe 

 found the same in experiments on Tradescantia and Spirogyra 

 (Trondle, '10, p. 173). He maintains that the permeability for 

 water, potassium nitrate, glycerine and urea increases slowly from 

 to 6 degrees, more rapidly from 6 to 20 degrees, and again more 

 slowly from 20 to 30 degrees, and that it is eight times as rapid 

 at 30 degrees as it is at zero. This shows clearly that increase 

 in temperature produces increase in permeability. We have 

 demonstrated, as stated above, that increase in temperature 

 causes positive photic orientation. Therefore, if reversion is due 

 to change in permeability, positive orientation must be associated 

 with increase in permeability, and if this is true then all of the 

 factors which produce positive orientation should cause increase 

 in permeability. 



The work of Lilhe ('09, p. 248) indicates that this is true for 

 decrease in alkalinity; Trondle maintains that moderate illumi- 

 nation, such as favors positive orientation, causes increase in 



