IRRADIATION OF LIVING PROTOPLASM 631 



times as much material (undefined) had diffused from the illuminated 

 cells as from the controls, a figure agreeing well with the results obtained 

 by the method of isotonic coefficients. 



Blackman and Paine (28) did similar work with pulvini of Mimosa, 

 determining by electrical conductance measurements the amount of 

 effusing salt. Using a 50-candle Nernst lamp as a source of illumination, 

 they observed an immediate increase followed by a dechne in the rate of 

 increase of conductivity. Dillewijn (73; cited after Zycha, 464) made 

 similar measurements with tissue of Avena and obtained results contra- 

 dicting those of Blackman and Paine. It should be pointed out that 

 exosmosis may depend on other factors than permeability, notably the 

 concentration of diffusible electrolytes in the cytoplasm, and, in experi- 

 ments with whole tissues, the intercellular materials. 



Direct measurements of tissue conductivity were used by Brauner 

 (43) to determine permeability changes in the coleoptile of Avena; his 

 illumination is very accurately defined. This author was able to detect 

 a conductivity increase upon illumination superimposed upon that caused 

 by the implantation of platinum electrodes (made fast with hot paraffin!). 

 The inadequacies of such a method are obvious and, indeed, are well 

 indicated by Brauner himself. 



Direct chemical methods also have been applied to the study of the 

 effects of visible light on cell permeability. Hoagland and Davis (150) 

 and Hoagland, Hibbard, and Davis (151) measured directly the amount 

 of anion accumulating in the sap of Nitella cells in the darkness and in 

 white light. In the early experiments, daylight was the source of 

 illumination, and CI, NO 3, and Br were determined in the sap. Chloride 

 absorption in the light against a concentration gradient sometimes left 

 the surroundings totally free of CI in a few days; in 3 days twice as 

 much NO3 was absorbed by the illuminated cells as by the darkened 

 ones, and in 4 to 5 days three times as much Br. In the later work arti- 

 ficial sources of illumination were used, the amount of light being defined 

 only in terms of the wattage of the lamps. Bromide, not found naturally 

 in the sap of these cells in appreciable concentration, was determined. 

 Light accelerated accumulation of this ion, and even cells exposed to Br 

 in the dark showed increased absorption of Br if they had been just 

 previously illuminated. Accumulation did not vary directly as the 

 amount of light; intensity was the more weighty factor, contrary to the 

 conclusion of Trondle. These authors present their data as indicative 

 of changes in a metabolic process which they (and Zycha) contrast with 

 ordinary permeability phenomena, which last depend on the laws of 

 physical diffusion. The difference of opinion depends, of course, on the 

 chosen definition of permeability. Hoagland, Hibbard, and Davis found 

 that the presence of Ca inhibited penetration, and reported a temperature 

 coefficient of accumulation, Qk, = 2.0 to 3.0. Both of these facts charac- 



