XXIX 



EFFECTS OF X-RAYS UPON GREEN PLANTS 



Edna L. Johnson 



Department of Biology, University of Colorado 



Introduction. Physiological effects of X-radiation: Effect upon seed germination and 

 early growth — Effect on root development — Condition affecting ray-sensitivity of plants — 

 Effect on respiration — Effect upon plant movements — Sui7imary. Morphological and 

 histological effects of X-radiation: Morphological effects upon leaves, aerial stems, and 

 underground stems — Morphological effects upon flower and fruit development — Histo- 

 logical effects — Summary. Cytological effects of X-radiation. General summary. 

 References. 



INTRODUCTION 



Since the discovery of X-rays by Rontgen in 1895, the majority 

 of pubHshed papers deahng with the influence of these rays on plant 

 life have considered their general effects on physiological processes 

 including rate of seed germination and growth, respiration, and move- 

 ments. The morphological and cytological aspects have attracted a 

 considerable number of investigators, while relatively few workers 

 have been interested in the histology of rayed tissues. 



The use of different units of measurement by the various investigators 

 has increased the difficulty of a comparative study of the results obtained 

 with X-rays upon plant and animal tissues. Many of the German and 

 French writers have used the Holzknecht (H) which is approximately 

 the equivalent of one-fifth skin-erythema dose. The erythema is 

 largely used by medical men and by some research workers; authorities 

 differ as to the exact definition. It is that amount of X-rays which 

 will produce a distinct reddening of the human skin at the end of a 

 development period, usually 10 days following the exposure. The 

 erythema dose is the equivalent of 5 H units. Unfortunately, the 

 erythema is difficult to determine accurately, owing to individual and 

 racial differences. It has been abandoned by the research worker 

 for a new arbitrary international unit (see Taylor, Paper II, p. 68) 

 called the "roentgen," designated by the small letter r. Dosimeters 

 have been designed which accurately measure the X-ray output in 

 r-units, 300 to 600 r-units constituting an erythema dose. 



Many investigators, irrespective of the unit of measurement employed, 

 have given the "set-up" of the machine, recording the voltage, amperage, 

 distance from target to area irradiated, time, and statement as to whether 



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