876 BIOLOGICAL EFFECTS OF RADIATION 



Metzner could not determine from his results whether radiation 

 which did not penetrate the sections examined was absorbed or reflected. 

 Hence part of what he regarded as absorption may in reality have been 

 reflected radiation. 



Of significance in relation to the effect of ultra-violet radiation upon 

 seed germination is the paper by ShuU and Lemon (103), which deals 

 with the penetration of various seed coats by the radiation of an 

 unscreened quartz mercury arc. Results were determined by spectro- 

 grams. They found that, with a maximum duration of irradiation of 

 1 hr., only the longer ultra-violet rays penetrated seed coats, the lowest 



o 



limit being indicated by a feeble line at 3120 A and penetration of rays 

 shorter than 3630 A always being feeble. There was some variability in 

 penetration shown by different species. Even in the same seed coat 

 there was variation. Thus in the case of corn-grain membranes penetra- 

 tion was greatest on the embryo side. Wet coats differed little from dry 

 ones as far as penetrability was concerned. 



In the paper just referred to there is an actual demonstration of the 

 reason, mentioned as a probability by Popp in 1921, for the failure of the 

 short-wave ultra-violet to injure ungerminated seeds, namely, the failure 

 of these rays to penetrate the seed coat. This paper also gives evidence 

 which suggests that the region of ultra-violet used in Masure's inves- 

 tigation did actually penetrate the seed coats of his seeds. 



It should be noted that no determinations of penetration to date have 

 given percentage transmissions of various wave-lengths through seed 

 coats or any other plant parts. In other words, we have only qualitative 

 and no quantitative data. 



ULTRA-VIOLET RADIATION AND FLUORESCENCE OF PLANT PARTS 



The capacity of various parts of numerous different kinds of plants 

 to exhibit fluorescence phenomena when exposed to ultra-violet radiation 

 has been noted in several studies. Some suggestions have been made 

 concerning the nature of the fluorescing substances and the significance 

 of their distribution and capacity to fluoresce. No facts have been 

 established to date, however. 



Gentner (31), MezzadroU and Vareton (61), Foy (27), and Masure 

 (55) all noted the fluorescence of germinating seeds in the presence of 

 ultra-violet radiation of 3000 to 4000 A. Gentner (31) has attempted 

 to find out whether the nature of the fluorescence is specific enough that 

 it may be used practically for seed testing, particularly for the differentia- 

 tion between seed varieties and races. Foy (27) has used this method 

 successfully in diagnosing various types of rye-grass in New Zealand. 

 Tile annual Italian rye grass and some or all of the false perennial types 

 exhibit a brilliant blue fluorescence while the true, normal, perennial 

 rye grass reacts absolutely negatively. 



