554 RADIATION BIOLOGY 



ence of centrifugation could be observed. According to Beams (1949), 

 the majority of the Elodea cells survive an iiltracentrif ligation of even 

 350,000 times gravity for 30 min. 



Only the apical part (20-30 cm at the top) of the shoot was used in 

 the experiments. The time required for the chloroplast movements was 

 found to be the same for all the leaves of the shoot at the same time, 

 but the value changed from hour to hour. When, however, the material 

 was kept in the dark for 2-3 days, the fluctuations ceased, and the time 

 of centrifugation was constant or almost constant from hour to hour and 

 during a shorter period from day to day. 



Objects subjected to such pretreatment were examined in the light; 

 ordinary electric-light bulbs were used as the source of light, which passed 

 through two glass cuvettes with running water to a depth of 15 cm. 



The plasmatic equilibrium, gradually reached in darkness, was immedi- 

 ately upset when the leaves were exposed to light of 5 48,000 m-c inten- 

 sity. After as short a time as 15 min, the first effect was noticeable. 

 Subsequent increases and decreases in the mobility of the chloroplasts 

 followed after varying periods. An exception to this rule was noted in 

 the experiments with strong light (32,000 and 48,000 m-c). During the 

 first few minutes of illumination the time of centrifugation dropped to a 

 low value that became nearly constant during the following hours. This 

 means that strong light has a paralyzing influence upon the reactions due 

 to the viscosity. 



Many changes in the state of the protoplasm were also found when 

 the light was limited to the first hours or minutes, thus indicating the 

 occurrence of aftereffects. Illumination of 100 m-c for 15 sec suflSced to 

 induce the alterations, which persisted for several hours. 



Owing to the aftereffects of light, the plasmatic system never reaches 

 photic equilibrium under natural conditions. In the morning and during 

 the day the leaves are exposed to fresh light inductions, whose effects do 

 not become exhausted during the following night. Centrifugation values 

 with hourly variations are always found when test objects are taken from 

 plants growing under natural conditions or in ponds. Weber (1925a, b) 

 also found that the viscosity of the objects studied by him {Elodea, 

 Spirogyra, succulents) varied in the course of the day and with the 

 seasons. 



The photic effect seems to be strictly localized to the illuminated part 

 of the object and even to a part of a cell (see Fig. 12-1). 



Virgin (1948) made centrifugation experiments, but with a somewhat 

 different method. Instead of using slides, he inserted the Elodea leaves 

 in small glass tubes, thus reducing the pressure on the cell wall to a 

 minimum. His experiments showed that the changes in the protoplasm 

 caused by light occurred in more regular periods if the plants were allowed 

 to grow in a constant light and temperature (e.g., 450 m-c and 20°C) for 



