VISCOSITY, PERMEABILITY, PROTOPLASMIC STREAMING 573 



According to Brauner and Brauner, the effect of light is due, among 

 other factors, to the changes caused by it in certain details of the plas- 

 matic structure, thus leading to changes in the cellular permeability. 

 The importance of the plasmatic structure for permeability has been 

 particularly stressed by Seifriz (1945). He stated: 



The selective permeability of cells is generally handled as a subject apart from 

 structure, but several of the classical theories of cellular permeability involve 

 mechanisms, and mechanisms involve structure. Thus the sieve hypothesis of 

 selective permeability is a pureh' mechanical interpretation; indeed, chemical 

 hypotheses such as those involving the solution of fats are, in the last analysis, 

 structural interpretations. 



Thus the analysis of the influence of light on the permeability of the 

 protoplasm appears to point to the plasmatic structure and to properties 

 of the protoplasm associated with it. According to the analysis of the 

 influence of light on the viscosity of the protoplasm and to the account 

 already given of this question, there is also in this case reason to seek the 

 explanation in the plasmatic structure. 



Association between Ejfects of Light on Permeability and Viscosity of 

 Protoplasm. As mentioned earlier, the influence of hght on the perme- 

 ability resembles, in some cases, the alterations in viscosity caused b}^ 

 hght. Lepeschkin (1930, p. 964), working with Elodea canadensis and 

 E. densa, found that a periodic variation in permeability occurred in 

 plants that had been kept in darkness, and that these variations con- 

 tinued for 3 days, after which the value remained almost constant. With 

 increasing light the permeability also increased, but not proportionately. 

 At a light intensity of about 10 per cent of sunlight, the permeability 

 reached its maximum, a subsequent increase in the intensity having no 

 effect. Similar changes in the permeability following the action of light 

 were also found by Dillewijn (1927). 



Indeed, these observations on the permeability correspond to the 

 changes in viscosity caused by light, as described in the foregoing. This 

 seems to suggest a correspondence between viscosity and permeability, 

 and may be interpreted as supporting the hypothesis of a causal con- 

 nection between the alterations in permeability and viscosity or of both 

 phenomena as consequences of a third alternating factor. 



It is necessary for studies of such a connection between permeability 

 and viscosity to use methods that permit determinations during short 

 periods of time. Most of the methods used hitherto require considerable 

 time and are therefore unable to allow the details of the influence of light 

 to be observed. 



Physiological Consequences of Effect of Light on Permeability. It can 

 be expected that hght, by influencing the permeability, will cause impor- 



