ULTRA-VIOLET AND SEED PLANTS 869 



were used per pot and on what basis any of the plants in any pot could 

 be considered "representative." The photographs show that in any 

 pot there was extremely wide variation in size of plants. Such data 

 also are of limited value. There was no repetition of any of this work 

 reported. While there is a possibility in these experiments of the favor- 



o 



able effect of the region 2900 to 3100 A, the conditions under which the 

 experiments were conducted were such as to offer very legitimate reason 

 for doubt of the conclusions arrived at. 



INVESTIGATIONS DEALING WITH PLANTS GROWN UNDER SPECIAL 

 ULTRA-VIOLET TRANSMITTING GLASSES 



O 



Since the region 2900 to 3130 A — that region of the solar ultra-violet 

 which is not transmitted by ordinary window glass — has been emphasized 

 as necessary for the normal development and health of higher animals, 

 a series of special ultra-violet transmitting glasses has been developed 

 and put on the market. Various Corning glasses of this country and 

 vita glass of England are among these. There are also numerous German 

 glasses known by various trade names, such as Uviol glass made by 

 Schott and Genassen of Jena, U-glass of Dresden, ultra-violet glass of 

 Berlin, Ufau, Ultra, Brephos, Ultravit, Sendlinger, Bios, and Sanalux 

 glasses. Probably the Jena glassworks of Germany antedates all others 

 in manufacturing ultra-violet transmitting glasses, having made them 

 as early as 1903. Their Uviol glass in comparison with five other 

 German glasses was found to transmit farther down in the spectrum 

 than any of the others and to transmit a higher percentage throughout 

 in the ultra-violet than any of the others. 



These new glasses differ from each other considerably in ultra-violet 

 transmission and in addition some of them solarize, that is, lose a part 

 of their transmission in the ultra-violet upon exposure to light (Arthur 

 and Newell, 3; English, 26; Wood and Leathwood, 124, 125). This 

 solarization is far more marked when the glass is used under a mercury 

 arc than when used in ordinary sunlight. In a few cases glass substitutes 

 have been tried, but these usually have not been found so satisfactory 

 as glass screens. It is obvious that when one of these screens is used, 

 the actual quality and intensity of the ultra-violet reaching the plants can 

 be ascertained only by repeated definite measurements of transmission. 



Unfortunately, not only is the ultra-violet transmission of these glasses 

 different from that of ordinary glass, but so also is the visible, and in 

 great degree the infra-red transmission. These features have been 

 indicated by transmission curves of the glasses and by the temperature 

 differences occurring under the two types of glasses. Any effects, 

 therefore, obtained under one of these new glasses, cannot justifiably 

 be said to be ultra-violet effects, but must be attributed to the difference 

 in total transmission of the glasses used, other environmental conditions 



