134 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



This interesting research definitely removes thermotro- 

 pism from the class of mysterious phenomena which are 

 said to be due to the action of stimuli. 



THE EFFECT OF VARIATIONS IN THE INTENSITY OF 

 ILLUMINATION UPON PERMEABILITY. 



A very elaborate research upon the alterations in per- 

 meability brought about by variations in illumination 

 was carried out by Trondle (1910). He concluded that the 

 alteration in permeability which takes place under the 

 influence of light is a typical stimulus action, for in the 

 narcotized condition no change in permeability takes 

 place. In view, however, of the fact that Osterhout has 

 shown that the effect of small quantities of anesthetics 

 is to decrease the permeability, it is possible that Trondle 's 

 verdict on this point may need revision, as a decrease due 

 to illumination might just balance an increase due to 

 a narcotic. 



Trondle found that prolonged illumination (about twenty- 

 four hours) led to a decrease in permeability with strong 

 intensities. With medium intensities an increase was 

 observed, whereas with weak intensities a decrease again 

 resulted. Exposure to a strong light for a short time was, 

 on the contrary, seen to produce an increase. Storage 

 in the dark leads to a return of normal permeability. 



Under natural conditions of growth the permeability 

 of plant cells increases with the intensity of illumination. 

 The physiological importance of this change appears to be 

 that it permits of the translocation of assimilates from the 

 leaves. It does not, however, appear probable that all 

 phototropic curvatures are directly attributable to altera- 

 tions in permeability due to the action of light, as cases 

 are known in which a stimulus is transmitted to a region 

 shielded from light. 



