Mar. 1, 1920 Effect of Length of Day on Plant Growth 559 



In the preliminary tests of 191 8 no special means were provided for 

 moving the boxes and pots containing the plants in and out of the dark 

 chamber. In the spring of the present year a much larger dark house 

 was constructed, and suitable facilities were installed for easily moving 

 the test plants in or out of the house as often as desired. The dark house 

 consisted of a rectangular frame structure 30 feet by 1 8 feet and 6 feet in 

 height to the eaves and 9 feet to the ridgepole. All crevices by which 

 light could enter were covered, tight-fitting doors were provided, and the 

 interior was painted black. Means were provided at the bottom and 

 top of the house for free circulation of air without the admission of light. 

 A series of four steel tracks, each entering through a separate door, was 

 provided ; and on these tracks were mounted a number of trucks carry- 

 ing the test plants in their containers. This equipment proved very 

 satisfactory. A general view of the dark house, the trucks, and the test 

 plants is shown in Plate 64, B. 



It has been rather generally assumed that the pronounced changes in 

 plant activities which come on with the approach of fall are due in some 

 way to the lower mean daily temperatures or the wider daily range in 

 temperature caused by cool nights. It seemed desirable, therefore, to 

 compare the temperatures inside and outside the dark house, and for this 

 purpose thermographs were installed. It was found that there were only 

 slight differences in temperature. The temperature inside the dark house 

 tended to run 2° or 3° F. higher than the temperature outside, particu- 

 larly at night. Hence, any responses on the part of the plants resembling 

 those appearing in the fall of the year could not be attributed to lower 

 temperatures. To guard further against possible temperature effects, as 

 soon as the above-mentioned temperature difference was discovered all 

 doors of the dark house were opened as darkness came on each day. 



In the various tests the length of the exposure to light was varied 

 from a minimum of 5 hours per day to a maximum of 1 2 hours, 7 hours 

 and 1 2 hours being the exposures chiefly used. For the shortest exposure 

 the plants were placed in the dark house at 3 o'clock p. m. and returned 

 to the light at 10 a. m. ; for the 7-hour exposure the plants were darkened 

 at 4 p.m. and returned to the light at 9 a.m.; and for the 12-hour 

 exposure they were in the dark house from 6 p. m. till 6 a.m. A further 

 modification in exposure consisted in placing the plants in the dark 

 house at 10 a.m. and returning them to the light at 2 p.m. In most 

 instances the daily treatment began with the germination of the seed or 

 in the earlier stages of growth and continued until maturity, but in 

 some cases the plants were permanently restored to the open as soon as 

 blossoming occurred, and in other cases the artificial shortening of the 

 day was not begun until after blossoming had occurred. To facilitate 

 discussion it will be convenient to use the expressions "long day" as 

 meaning exposure to light for more than 12 hours and "short day" as 

 referring to an exposure of 12 hours or less. The term "length of day" 



