ULTRA-VIOLET AND SEED PLANTS 855 



From 1921 to 1927 the reports of Sibilia (104), Russell and Russell 

 (84), Dane (14), and Ritson (Delf, Ritson, and Westbrook, 21) all indi- 

 cated only injurious effects of the unscreened mercury-vapor arc. 



MORE RECENT INVESTIGATIONS 



Since 1927, in addition to reports indicating only injurious or indiffer- 

 ent effects of ultra-violet on seeds and seedlings, numerous papers have 

 appeared in which an apparent effort has been made to demonstrate 

 beneficial or "stimulating" effects of the ultra-violet. 



Sheard with Higgins and Foster published a number of papers on 

 the effect of "general" and "selective" irradiation upon plants. Four 

 of these (Sheard and Higgins, 95, 96; Sheard, Higgins, and Foster, 97; 

 Higgins and Sheard, 39) deal with germination of seeds and early growth 

 of seedlings. Although the conclusions of these authors have been 

 repeatedly referred to as though they were established facts, a careful 

 examination of the experimental procedure, the results obtained, and the 

 complete lack of data in some instances, reveals the unsoundness of these 

 conckisions. For a more detailed criticism of this work the reader is 

 referred to Popp and Brown (74, pages 163-165). 



Another good illustration of the type of work that has been reported 

 so often since possible stimulating effects of ultra-violet on plants have 

 been sought is seen in Valentin's experiments (119) with Ultravit glass, 

 a German product, which transmits all wave-lengths of daylight ultra- 

 violet. He compared school children, various chemicals, and the germi- 

 nation and growth of plants in two schoolrooms having Ultravit glass 

 windows, with those in two schoolrooms having ordinary glass windows. 

 Corn, oats, beans, and peas were put in each schoolroom, eight seeds per 

 room. These were planted in flower pots equally deep in the soil. 

 (" Die Samen wurden gleichmassig tief in die Erde gebracht".) Because 

 the seedlings in the pots behind Ultravit glass appeared above ground 

 sooner than those behind window glass, the conclusion of the author was 

 that the earlier appearance of the one set of seedlings was due to stimula- 

 tion of germination by the ultra-violet transmitted by Ultravit glass and 

 not by window glass. It is difficult to understand how any ultra-violet 

 could possibly have reached seeds buried in soil. Further growth of the 

 seedlings was somewhat better behind Ultravit glass than behind window 

 glass. 



While it is obvious that results of this type cannot legitimately be 

 attributed to ultra-violet, Valentin's work, like that of Sheard and 

 Higgms, has been quoted by later investigators in support of the thesis 

 that ultra-violet radiation is "stimulating" to plants. It cannot be 

 overemphasized that in the reading of reports of this type it is extremely 

 important to examine carefully the methods of the investigators and not 

 merely their summaries and conclusions. 



