874 BIOLOGICAL EFFECTS OF RADIATION 



elsewhere in this monograph and hence have been omitted here. The 

 relation of ultra-violet radiation to transpiration, sun scald, winter 

 hardiness, tropisms, electric potentials, and currents in plants and other 

 relations are not discussed because the evidence is too fragmentary. 

 There remain to be considered briefly, absorption and reflection of 

 ultra-violet by plant tissues and fluorescence of plant parts caused by 

 ultra-violet. Although the effect of radiation on the histology of plants 

 and its influence on plant pigments are considered elsewhere in this 

 monograph, a few statements are made regarding the specific relation of 

 ultra-violet radiation to these features. 



ABSORPTION AND REFLECTION OF ULTRA-VIOLET RADIATION BY PLANT 



TISSUES 



Of great importance in any consideration of the effect of ultra-violet 

 radiation upon plants is the question of the degree of penetration of 

 these rays into the various plant tissues, since only those rays which 

 penetrate can be effective in producing results. Undoubtedly the facts 

 that certain plants are more resistant than others to the harmful action 

 of short rays, certain parts of a given plant more resistant than other 

 parts of the same plant, and the same parts of a given plant more resistant 

 at one stage of development than at another, have their explanation in 

 part at least in differences in degree of penetrability and absorption of 

 these injurious rays. Different degrees of penetrability may also explain 

 some of the conflicting results of past experiments. Thus the shorter 

 lethal ultra-violet regions are ineffective if they do not reach the plant 

 parts under consideration. Furthermore, any beneficial rays would 

 have to enter the cells before they could be effective. Different media 

 vary considerably in their capacity to transmit radiation of various 

 wave-lengths. 



In earlier investigations the penetration of injurious rays only was 

 considered, and the degree of penetration was known only indirectly 

 by the degree of injury produced on the plant cell as a result of irradiation. 

 Maquenne and Demoussy (51), Kluyver (43, 44), and Ursprung and 

 Blum (118) noted the superficial action of ultra-violet radiation of short 

 wave-lengths as indicated by the fact that only epidermal cells or those 

 immediately beneath them were destroyed. They assumed that the 

 harmful rays were absorbed by these external layers and failed to pene- 

 trate more deeply. Stoklasa (108) found that flowers were much more 

 sensitive than leaves to short-wave ultra-violet, and that both leaves and 

 flowers of hot-house plants were more sensitive than those of outdoor 

 plants. Dangeard (16, 17) by noting various degrees of injury in differ- 

 ent plants which had been irradiated assumed differences in degree of 

 penetration of the harmful rays. He also thought that hairy leaves 

 retarded penetration more than glaucous or smooth ones. Schroeter (90) 

 thought that the thick cuticle of alpine plants protected them from the 



