VISCOSITY, PERMEABILITY, PROTOPLASMIC STREAMING 569 



siderable variations in the experimental conditions are taken into con- 

 sideration. Such variations occurred in the following respects, among 

 others : 



1. The objects varied not only in species but also with regard to the 

 tissues and organs, age, and degree of development. The importance of 

 these variations has been particularly stressed by Arisz (1947). He 

 therefore warned against drawing conclusions from the data concerning 

 one object as to the behavior of others. It was also evident from Wahry's 

 (1936) experiments that leaves on the same plant may react differently 

 under apparently identical external conditions. 



2. The pretreatment of the objects varied, thereby causing a lack of 

 uniformity in the initial conditions. The fact that pretreatment influ- 

 ences the manner of reaction of the test object has been shown by the 

 experiments of Trondle (1910), Lepeschkin (1930), and Jarvenkyla (1937), 

 among others. 



3. The quality and intensity of the light and the length of illumination 

 varied in the different experiments. The results are influenced by vari- 

 ations in these factors (cf., for example, Brauner and Brauner. 1936). 



4. The permeating substances studied varied. The influence of light 

 on their permeation was therefore subject to variations (cf. Jarvenkyla, 

 1937). 



5. The duration of the experiments varied between a few minutes and 

 several days. If, as shown by Dillewijn (1927) and Lepeschkin (1930), 

 the influence of light is increased and decreased in wavelike periods, the 

 choice of the duration of the experiment will be decisive for the final 

 results. 



Despite the afore-mentioned variations in the experimental conditions, 

 the results showed good agreement. It may therefore be considered as 

 an established fact that light changes the permeability of the cells, that 

 this change usually consists in an increase in the permeability, and that 

 the effect increases with a decrease in the wave length. With regard to 

 the last-mentioned conclusion, there has been agreement between all the 

 experiments made with a view to analyzing this relation (Packard, 1925; 

 Brooks, 1927; Lepeschkin, 1930, 1932a,b,c; Jarvenkyla, 1937). 



Thus the experiments of Lepeschkin (1908a,b, 1909) and of Trondle 

 (1910), which were subjected to many objections and considerable criti- 

 cism in the years following their publication, have been confirmed after 

 forty years' research. 



Mechanism of Influence of Light on Permeability. An attempt to ana- 

 lyze the nature of the photic mechanism presupposes, in the first place, 

 a distinction between the direct and indirect influence of light on the 

 intake and output of the substances. The experiments first described 

 here form examples of the indirect influences. They can presumably be 

 explained on the grounds of, among other factors, an increased consump- 



