838 BIOLOGICAL EFFECTS OF RADIATION 



arbitrary "lithopone units" (L. U.), and they demonstrated for Pul- 

 monaria officinalis the relation which exists between the number of L. U. 

 and the time which must elapse (latent period) before the death of the 

 epidermal cells. Younger leaves were found to be more sensitive than 

 the older ones. They present a curve showing the relationship between 

 the intensity of the treatment and the latent period and point out that 

 there is also a relationship between the thickness of the cutin and the 

 sensitiveness of the leaf. Among the histological details given of the 

 effect of ultra-violet light on leaves, they show not only that there are the 

 changes within the epidermal cells, but also that the vacuolization and 

 destruction of the chloroplasts and the collapse of the palisade cells take 

 place with increased dosages. 



Martin and Westbrook also tested the effects of temperature during 

 the period of treatment, and for the latent period they found that differ- 

 ences of temperature between 5° and 25°C. have little effect upon the rate 

 of browming of the epidermis. These authors also discuss the relation- 

 ship between the erythema dose of the human skin and the dose causing 

 browning of the leaves, and point out that whereas many have attempted 

 to draw a parallel between these superficial effects, the skin recovers 

 and the pigmentation is produced within the cytoplasm without killing 

 cells, while the plant epidermis is killed and does not recover. 



There is a structural basis for the claim that the cutin of plants is 

 relatively opaque to ultra-violet rays. Kohler (17, 18), who used the 

 cadmium line (2750 A) in the photomicrography of plant tissues, shows 

 figures in which woody cell walls and cork cells are nearly impenetrable, 

 whereas the cuticle of leaves and stems, even in very thin places, is 

 impenetrable to these very short rays. Metzner (22) shows that there is a 

 similar opaqueness of cuticular and other plant-cell walls to the longer 

 ultra-violet rays (3500 to 4000 A) which may play a role in the influence 

 of the sun's radiation, especially at high altitudes; thus the ultra- 

 violet rays usually do not enter the plant and probably do not play an 

 important formative role in nature except in the structure of alpine plants. 



REFERENCES 



1. Bailey, L. H. .p:iectricity and plant growing. Mass. Hort. Soc. Trans. 1894: 

 54-79. 1894. 



2. Beauverie, J., and P. Cornet. Action des rayons ultra-violets sur la struc- 

 ture cellulaire dans la feuille et le bourgeon d'Elodea canadensis. Compt. Rend. 

 Soc. Biol. [Paris] 102: 775-777. 1929. 



3. BuRGERSTEiN, ALFRED. Die Transpiration der Pflanzen. Jena, 1904. 



4. BuRGERSTEiN, ALFRED. Transpiration der Pflanze. Zweiter Teil (Erganzungs- 

 band). Jena, 1920. 



5. Dane, H. Rebecca. The effect of ultra-violet radiations upon soybeans. 

 Science 66 : 80. 1927. 



6. Davenport, C. B. Experimental morphology. New York, 1908. 



7. Deh^rain, p. p. Experiences sur I'influence qu'exerce la lumi^re 61ectrique sur 

 le d6veloppement des v6gdtaux. Annales Agron. 7: 551-575. 1881. 



