880 BIOLOGICAL EFFECTS OF RADIATION 



These investigators have all attributed possible biological significance 

 to the development and presence of these anthocyanins and flavones. 

 One of the possible roles suggested for these substances was that they 

 absorb ultra-violet radiation and consequently protect deeper lying cells 

 from its injurious effects. Metzner (60) found that the region 3500 to 



o 



4000 A does not pass through cell sap which contains tannins and flavones. 

 However, this hypothesis loses weight when it is remembered that ultra- 

 violet radiation of the intensity and quality present in solar radiation 

 has never been conclusively demonstrated to be injurious to plants. 

 Such evidence as is available, as, for example, that of Arthur and Newell 

 (3), indicates rather that ultra-violet of the intensity and quality present 

 in daylight is not injurious. 



One of the possible demonstrations of the injurious effects of solar 

 ultra-violet is indicated by Schanz's experiment with the purple beech 

 (89). While the experiment was not carried out with proper controls, it 

 does indicate a possible destructive action of short-wave radiation on 

 plants containing epidermal anthocyanin when they are first grown under 

 conditions which prevent the development of this anthocyanin and later 

 exposed to radiation containing ultra-violet. Schanz, having raised this 

 purple beech under various screens, found that it lost its red anthocyanin 

 color the more the short waves were cut off. He then transplanted one 

 of the plants which had developed large green leaves lacking anthocyanin 

 to the open where it was exposed to the full radiation of daylight. At 

 the end of 14 days all of these green leaves were dead and the new ones 

 that had developed were red in color. Schanz concluded that possibly 

 the red anthocyanin acted as a screen here to protect the plants against 

 ultra-violet radiation. Unfortunately plants that had ah-eady developed 

 anthocyanin were not transplanted to the open in a similar manner. 

 Furthermore, other species of plants containing a similar anthocyanin 

 relationship were not injured when placed in the open. This situation 

 might be cleared up if such plants as the purple beech were first grown 

 under conditions in which anthocyanin fails to develop and then exposed 

 to an artificial source of ultra-violet by tlie same method as was used by 

 Arthur and Newell. Then at least we would be able to ascertain whether 

 such plants were more sensitive to long-wave-length ultra-violet than 

 ordinary plants. 



Popp (69, 70) found that mustard seedlings grown continuously in 

 the dark, or grown in the dark and exposed to the mercury arc from which 

 the ultra-violet portion had been eliminated, formed anthocyanin. The 

 fact that beet roots developed in the soil in the absence of all radiation 

 are rich in anthocyanin is well known. 



The experiments seem to indicate that anthocyanin formation is 

 possibly favored by ultra-violet radiation, but ultra-violet effects have 

 never been clearly separated from those of total radiation intensity or 

 from blue-visible effects. 



