558 RADIATION BIOLOGY 



Alberda (1948) cultivated Zea mays in nutrient solutions of different 

 concentrations. In darkness the absorption came to a standstill with the 

 "high-salt" plants (grown in solutions of higher concentrations) and, in 

 most cases, even passed into an exit of ions. "Low-salt plants" (grown 

 in more dilute solutions) were, on the other hand, capable of absorbing 

 phosphate even in darkness. 



Alberda concluded from his experiments that the growth of the shoot 

 is determinative for the absorption of the high-salt plants. He explained 

 this correlation between shoot and root by the hypothesis that, with 

 high-salt plants, a part of the ions secreted by the root in the xylem are 

 carried back to the root by the phloem when the consumption by the 

 shoot is less than the supply by the root. 



Investigations of Andel et al. (1950) have shown that the influence of 

 light on the intake of salts can be altered by changing the balance between 

 the sugar production and salt content of the plant. They cultivated 

 maize in light and in darkness, glucose being administered to the roots 

 in some experiments. They stated that the intake of phosphate increased 

 when glucose was added to the nutrient solution. This connection 

 between sugar and phosphorus was explained by the authors as an 

 acceleration by sugar of the synthetic processes by which phosphorus is 

 fixated. Glucose also increased the intake of phosphate by plants con- 

 tinuously exposed to light, but the intake diminished when, after a long 

 dark period with glucose, plants were once more placed in the light, but 

 without glucose. From this fact it was concluded that under normal 

 conditions the sugar content of the root is too low to obtain a maximal 

 absorption of phosphate. 



It was also found that the intake of potassium was to a great extent 

 dependent on the exposure to light, but that it was not furthered to any 

 considerable degree by the addition of sugar to the nutrient solution. 

 In dark there was a distinct exit of potassium, but not of phosphate, 

 from the roots to the solution. In accordance with Alberda (1948), the 

 authors explained this exit 'of potassium as a consecjuence of the retarded 

 growth of the shoot in the dark. In these experiments the effect of light 

 on the exchange of solutes could be explained without the necessity of 

 assuming any change in the permeability. No proof that this had not 

 actually occurred was forthcoming, however, and therefore the possibihty 

 of a photic effect of this nature cannot be ruled out. 



DIRECT INFLUENCES 



In a number of other studies the experiments were arranged in such a 

 way that the light had a more direct effect on the permeability of the cells. 



Lepeschkin (1908a,b, 1909) was the first to report experiments of this 

 kind. He studied the movements of the leaves of Phaseolus, Mimosa, 



