860 BIOLOGICAL EFFECTS OF RADIATION 



Tinker (112) obtained indifferent results on rate of germination of vege- 

 table seeds in frames with ordinary glass and special ultra-violet trans- 

 mitting glasses when daylight was the source of radiation. 



SUMMARY 



Viewing collectively the experiments thus far carried out on the effect 

 of ultra-violet radiation on seed germination and the early growth of 

 seedlings, one observes that the only fact clearly demonstrated is the 

 injurious effect of short-wave radiation, that below 2900 A. While 

 there is some indication that certain ranges of ultra-violet might be 

 beneficial in one way or another, the results so far reported are far from 

 conclusive, and the evidence from more carefully controlled experiments 

 would indicate little or no effect of the longer wave-lengths, those from 

 2900 to 4000 A. 



GENERAL STUDIES ON THE EFFECT OF ULTRA-VIOLET RADIATION 



UPON MORE MATURE PLANTS 



INTRODUCTION 



The preceding paragraphs have been concerned with investigations 

 dealing exclusively with the effects of ultra-violet radiation upon seed 

 germination and the first stages of seedling growth. Numerous investiga- 

 tions dealing with effects on plants in more advanced stages of growth 

 and on plants under observation for considerable periods of time have also 

 been reported. In these, evidence for effects of ultra-violet has been given 

 in terms of general appearance of the plant, necrosis, height measurements, 

 number and size of leaves, time and degree of flowering and fruiting, fresh 

 weight, dry weight, and ash content. In addition, stem diameter, 

 pigment development, development of spines, hairs, etc., anatomical 

 features, chemical composition, enzyme activity, and various other 

 criteria have been used. 



The experiments have been carried out in various ways. In some, 

 the effects of single exposures or several exposures to a mercury-vapor 

 arc have been noted. In others, plants have been grown for considerable 

 periods in daylight from which all ultra-violet rays have been eliminated 

 by appropriate screens. In others, plants have been grown for a number 

 of weeks under ordinary greenhouse conditions with additional short 

 daily irradiations from a mercury arc in quartz either unscreened or 

 covered with one of several filters. In others, artificial illumination only 

 has been used, in combination with various filters. In still others, plants 

 grown in houses, frames or boxes with ordinary glass panes have been 

 compared with plants under similar conditions except that the ordinary 

 glass panes were replaced by one of a number of special ultra-violet trans- 

 mitting glasses. The wide differences in the methods of procedure make 

 it necessary to consider the results obtained under the various conditions 

 separately. 



