Mar. 1. 1920 Effect of Length of Day on Plant Growth 597 



remains to be seen whether those individuals of a given species which 

 grow successfully in high latitudes have the same physiological require- 

 ments with respect to length of day as those growing quite as successfully 

 near the equator. In any study of the behavior of plants introduced 

 from other regions, with a view to determining certain economic qualities, 

 it is evident that the factor of length of day must be taken into considera- 

 tion as a matter likely to have great significance. 



LENGTH OF DAY AS A FACTOR IN CROP YIELDS 



From the facts which have been developed in this paper it would seem 

 that the seasonal change in length of day is a hitherto unrecognized 

 factor of the environment which must be taken into account when deal- 

 ing with the problems in crop production. So far as is now known, the 

 length of the day is the most potent factor in determining the relative 

 proportions between the vegetative and the fruiting parts of many crop 

 plants; and, in fact, as already pointed out, fruiting may be completely 

 suppressed by a length of day either too long or too short. In some 

 crop plants the vegetative parts alone are chiefly sought, while in others 

 the fruit or seed only are wanted, and in still others maximum yields of 

 both vegetative and feproductive parts are desired. It is apparent that 

 the merits of different varieties or strains may depend largely on the 

 relative length of day in which they are grown, and, therefore, the date 

 of planting may easily become the decisive factor. These are matters 

 of vital importance to the plant breeder and the agronomist. Obviously, 

 a delay of even two or three weeks in seeding certain crops because of 

 inclement weather conditions or other considerations may bring about 

 misleading results. It is to be remembered, furthermore, that planting 

 too early may be equally inadvisable, for crops requiring relatively short 

 days for blossoming may thus come under the influence of short days in 

 early spring, resulting in "premature" flowering and a restricted amount 

 of growth. An impressive lesson as to the influence of length of day 

 on the size attained by the plant before blossoming is seen in the 

 relative heights of consecutive plantings of the Biloxi soybean, as shown 

 in figure 3. For maximum yields of many crops it is essential that 

 the date of planting be so regulated as to insure exposure of the plant 

 to the proper length of day, due regard being had for the specific 

 light requirements of each crop as well as for the relative values of the 

 vegetative and fruiting portions of the plant. 



RESULTS OBTAINED WITH ARTIFICIAL LIGHT USED TO INCREASE 

 THE LENGTH OF THE DAILY ILLUMINATION PERIOD DURING THE 

 vSHORT DAYS OF WINTER 



These results are of particular significance, since increasing the dura- 

 tion of the illumination period of the short winter day by the use of 

 electric light of comparatively low intensity has consistently resulted in 



