724 BIOLOGICAL EFFECTS OF RADIATION 



various combinations of the two are considered. There is definite evi- 

 dence that many plants are injured by continuous artificial Hght and many 

 plants produce at their maximum on a 17- to 19-hr. day as contrasted 

 with the 24-hr. day under the intensity and quality so far used in the 

 artificial illumination of plants. An 18-hr. day, 12 hr. of which are 

 sunlight and 6 of which are high-intensity artificial light, is better than 

 an 18-hr. day of all artificial light. This indicates that sunlight is 

 better in quality for plant growth than many of our common artificial 

 light sources. In the experiments where plants were grown in ever- 

 increasing day lengths of sunlight up to and including continuous illu- 

 mination, the evidence is not so clear, since temperature and other 

 factors were not well controlled in these experiments. There is some 

 evidence, however, to indicate that plant production of dry weight 

 is eventually checked even under continuous sunlight conditions, so that 

 plants produce most abundantly on a slightly shorter day. That is, 

 while growth and dry-w^eight production proceed at first at a more rapid 

 rate on a 24-hr. day, as the plants age this point of maximum rate of 

 production shifts to a 17 or 19-hr. day. This indicates that plants need 

 either a period of rest of 5 to 7 hours in each 24-hr. period or that they are 

 attuned to the falling off of the length of day which normally obtains in 

 nature during the growing season. Further experiments with other 

 growth factors, such as temperature well controlled, are needed to 

 determine these responses. 



REFERENCES 



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2. Albright, W. D. Crop growth in high latitudes. Geograph. Rev. 23 : 608-620. 

 1933. 



3. Arthur, John M., John D. Guthrie, and John M. Newell. Some effects of 

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 Jour. Bot. 17: 416-482. 1930. {Also in Contr. Boyce Thompson Inst. 2: 445- 

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4. Bailey, L. H. Some preliminary studies of the influence of the electric arc lamp 

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5. Bailey, L. H. Second report upon electro-horticulture. New York [Cornell] 

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6. Bonnier, Gaston. Les plantes arctiques compar^es aux mfimes especes des 

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7. Bonnier, Gaston. Influence de la lumiere 61ectrique continue sur la forme et la 

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 1895. 



8. Darrow, G. M. Tomatoes, berries, and other crops under continuous light in 

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9. Evans, Morgan W. Relation of latitude to time of blooming of Timothy. 

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