ULTRA-VIOLET AND SEED PLANTS 865 



plants were larger and more vigorous than the controls. As there were 

 just 5 of these plants, the results were considered only "suggestive." 

 They were never repeated. Harmful effects were obtained in various 

 other experiments and were more marked on plants given shorter day- 

 light illumination per day than on those given longer periods in the 

 light. 



Eltinge (25) has made observations on rooted cuttings and young 

 seedlings of plants grown for a number of weeks under ordinary green- 

 house conditions with additional daily irradiations from a mercury-vapor 

 lamp at distances of 50 and 100 in. Plants were exposed to the unscreened 

 lamp, to the lamp screened by vita glass which was said to transmit 

 down to 2890 A, or to the lamp screened by quartzlite glass which was 

 said to transmit down to 3130 A. The irradiations were given by 

 the incremental method, that is, for a period of }/2 min. the first day 

 and for periods increasing by 3^ min. each day, on succeeding days. 



While a considerable total number of plants was used, there were 

 too many different species under too many different conditions so that 

 generally no more than 6 to 10 individuals of a given species were given 

 a similar treatment. Average figures recorded in tables, therefore, are 

 computed from data on 6 to 10 plants grown under variable environmental 

 conditions for from four to eight weeks. While all of her beneficial 

 results were attributed to ultra-violet radiation, she had no controls to 

 eliminate the possibility of visible, infra-red, temperature, and other 

 effects of the lamp, since her so-called controls received no irradiation 

 whatsoever from the lamp. Furthermore, she made no measurement of 

 the quality or intensity of ultra-violet reaching her plants under the vari- 

 ous conditions used. 



The only consistent result of treatments was injury by the unscreened 

 arc to all plants used. Individual groups of plants irradiated through 

 vita glass or quartzlite glass did prove superior to nonirradiated plants 

 in a number of cases, but the differences were often slight and no one 

 type of glass gave uniformly better results. The superiority expressed 

 itself differently in almost every set of plants in which it appeared. It 

 might be greater height, greater thickness of stem, larger leaves, greater 

 number of leaves, greater average rate of growth, greater rate of growth 

 during the last days or during the last weeks of the experiment, earlier 

 flowering, or better color of plant. There were various combinations of 

 these features. The same treatment caused stimulation in one respect 

 and retardation in another. For instance, no type of irradiation proved 

 superior to controls for Raphanus and Nicotiana. For Bryophyllum and 

 Phaseolus, vita glass and a distance of 50 in. from the lamp proved best. 

 For Coleus, vita glass and 100 in. was best. For Zea Mays, vita glass 

 and a distance of 100 in., and quartzlite glass and 50 in. distance from the 

 lamp gave equally superior results. For Laduca, quartzlite glass and 



