49 



most potent factor in determining the relative proportions 

 between the vegetative and fruiting parts of many crop plants. 

 Indeed, fruiting may be completely suppressed by a day either 

 too long or too short. The advance in agricultural practice 

 which may come through this new discovery will have to be 

 brought about largely by plant breeders and other crop specialists. 

 For instance, it will prove of material significance in the future 

 planning of cropping systems for different regions, especially 

 where consideration of new crops from difterent latitudes is 

 necessary. 



This new principle undoubtedly explains the erratic behavior 

 which has been observed with many crops when they are shifted 

 to different latitudes, and may also clear up the conflicting 

 results of variety tests and field tests conducted with the same 

 crops but in different regions. The experiments have shown, for 

 instance, that ragweed requires for flowering a stimulus that is 

 afforded by the shortening of the days and lengthening of the 

 nights. It does not come into flower until the period of daylight 

 falls below 15 hours. In the latitude of Washington, that comes 

 about July i. But if ragweed seed should be taken to northern 

 Maine and planted, the plants would not experience a length of 

 day below 15 hours until about August i. Therefore, they 

 could not come into flower until after August i and, though the 

 vegetative growth might be very rank, they could not mature 

 seed before killing frosts intervened. The long days, therefore, 

 make it impossible for rag%veed to perpetuate itself in that 

 latitude. On the other hand, plants that get their flowering 

 stimulus from a long day could not perpetuate themselves 

 through seed formation at the equator, where the day never 

 exceeds 12 hours. 



Explains Luxuriant GrovvTH in Northern 

 Latitudes 

 This principle affords the clue to the fact that many plants 

 grow most luxuriantly near the northern limit of their range. 

 The long northern day allows them to attain their maximum 

 growth before the shorter day intervenes to check vegetative 

 growth and start the reproductive process. 



