48 



the plants that were left in the light all day grew to an average 

 height of 57 or 58 inches. 



These plants required a short day and a long night for flowering 

 and seed-bearing. In tests with other plants, just t-he opposite 

 was found to be true. The plants that were left in the light all 

 day did not grow luxuriantly, but produced flowers and seed, 

 while those that were kept in the dark part of the day made 

 abundant growth, but produced no seed or else were greatly 

 retarded in producing seed. 



Temperature Has Little Influence 

 Temperature appeared to exert no influence in these tests. 

 The results were the same, even when the temperature was 

 higher in the dark house than on the outside. Another striking 

 illustration of the relative unimportance of temperature is the 

 fact that plants kept in the dark for a part of the day underwent, 

 in midsummer, the changes that in nature come in the fall and 

 have always been attributed to lower temperatures. This, also, 

 was true even when the dark houses registered a higher tempera- 

 ture than that of the outside summer atmosphere. 



The results obtained by artificially extending the period of 

 light are just as interesting as those obtained by artificially 

 shortening it. The artificial illumination, in a test with iris, 

 was so arranged as to give 18 hours of continuous light in a 

 greenhouse during the winter. Control plants were kept in a 

 similar greenhouse with no artificial light. The test was begun 

 on October 20, 1919. In the greenhouse where daylight was 

 supplemented with electric light the plants made rapid growth, 

 soon attained normal size and produced blossoms on December 

 24. The plants in the greenhouse where no artificial light was 

 used, though it was kept at the same temperature, remained 

 practically dormant and showed no tendency to blossom as late 

 as February 12, 1920. 



Important Influence on Crop Yields 

 The influence of this discovery on crop yields is likely to be of 

 no little importance. The length of day is proved to be the 



