ULTRA-VIOLET AND SEED PLANTS 873 



possibility of a certain small significance in the production of dry weight, 

 but that definite establishment of this point could only come from much 

 more carefully conducted experiments. For the practical horticulturist 

 the author considered the glass of little value. 



A review of the facts brought out by these reports in regard to evidence 

 for ultra-violet effects by the use in greenhouses of special ultra-violet 

 transmitting glasses reveals that out of 31 reports, unqualifiedly favorable 

 results are reported in only 8 cases and the data for these are either not 

 given or are of questionable value. The other articles report either 

 conflicting results, results which could not be duplicated, very slightly 

 favorable, or indifferent results. Of these at least 10 report the greater 

 temperatures or heat effects under the special glasses. Several report 

 more favorable results under them early in the spring, but not in the 

 summer; a number report solarization effects of the glasses. The most 

 carefully conducted experiments show nothing or very little in favor of 

 the new glasses. Even were we to assume that all the beneficial effects 

 reported were attributable to this factor, we should still have to hesitate 

 to recommend the use of the special glasses for greenhouses because of 

 the slight differences that have been found in even the most favorable 

 reports. 



GENERAL SUMMARY 



In general, the evidence presented in this large group of papers on the 

 effects of ultra-violet on more mature plants reveals only one point which 

 seems to be clearly demonstrated, namely, that the short-wave ultra- 



o 



violet, that from 2890 to 2000 A is distinctly harmful. Even in very 

 slight doses it has never satisfactorily been demonstrated to be beneficial. 

 The degree of injury increases with decrease of wave-length, increase of 

 intensity, and with greater ease of penetration. While we would not 

 overlook the possibility of beneficial effects being demonstrated for the 

 longer wave-lengths of ultra-violet when accurately controlled experi- 

 ments are forthcoming, the evidence from the most accurately controlled 

 experiments to date shows little if any outstanding stimulation or increased 

 growth from this region. When we consider the low energy value of this 

 region in daylight, the universal source of radiation for plants, and when 

 in addition, it has clearly been demonstrated that many different kinds 

 of plants can be grown from seed to seed in the total absence of all ultra- 

 violet without exhibiting any very outstanding difference from plants 

 receiving solar ultra-violet, we may legitimately doubt whether any very 

 outstanding stimulation of growth will be demonstrated for this region 

 in the future. 



OTHER RELATIONS OF ULTRA-VIOLET RADIATION TO SEED PLANTS 



INTRODUCTION 



The effects of ultra-violet radiation on enzymes, vitamins, and other 

 cell constituents, and on photosynthesis and respiration are considered 



