562 RADIATION BIOLOGY 



that the permeabiUty to water is altered by light, the rate of exchange of 

 water being hastened. 



Lepeschkin (1934) objected to this conclusion on theoretical grounds. 

 He pointed out that the new state of equilibrium established by the move- 

 ments of the leaf implies a change in the permeability to the dissolved 

 constituents of the cell and that this cannot be caused only by a change 

 in the permeability to water. 



In order to throw light on the nature of the photopermeability reactions 

 involved, Brauner and Brauner (1947) made experiments with the pulvini 

 of Rohinia pseudoacacia. The pulvini were illuminated under water, and 

 their reactions compared with the effect of the same Ught stimulus on 

 pulvini in air. Whereas in air white light of all intensities caused posi- 

 tive reactions only, the sign of the response of the submerged pulvini 

 depended on the strength of the stimulus. A low-intensity light pro- 

 duced negative reactions, and a fivefold higher one, positive reactions. 

 It was concluded from the experiments that illumination with moderate 

 quantities of light only increases the permeability to Avater in the sensi- 

 tive cells. In air this change was considered to cause an increase in the 

 loss of water by transpiration, and in submerged organs, a transitory 

 increase in the water absorption from the medium. Stronger stimulation, 

 however, led to a partial breakdown of the semipermeability which, both 

 in air and under water, resulted in a loss of suction force and hence of 

 turgescence. 



In a new study of Phaseolus muUiflorus Brauner (1948) used colored 

 light of fixed quahty. It was found, among other things, that red and 

 blue light had the same effect as white light, whereas green hght was 

 inactive. 



Experiments loith the Method of Chemical Analysis. In a number of 

 investigations the interpretation of the results has been based on an 

 analysis of the solution from which the test object imbibed salts or other 

 substances, or an analysis of the content of such substances in the cells 

 or their sap. 



Brauner and Brauner (1936) also made use of this method, their test 

 object being the root parenchyma of Daucus carota. In order as far as 

 possible to obtain natural conditions, they used the cell substances of 

 the object, particularly mono- and disaccharides, as the test substance. 

 After illumination an analysis was made of the sugar content of the cells 

 and of the solution. 



The ratio between the quantity of sugar given out during a fixed period 

 and the initial sugar content of the cells was taken as a measure of the 

 permeability. The ratio between the amount of sugar given off in dark- 

 ness and in light was used as a measure of the effect of Ught. It was 

 found that light decreased the permeation of sugar at all the intensities 

 tested within the range 156-5000 m-c and that the maximal effect 



