426 PLANT PHYSIOLOGY 



6 hr. and less, a depression of growth may be observed. The 

 relationship between the total size of plants grown at different 

 lengths of day is clearly demonstrated in Figs. 133 and 134. 



A very profound influence is exerted by the length of day 

 on the development of tubers, bulbs, and other underground 

 storage organs. In many plants, e.g., in the garden onion, a long 

 day promotes the formation of underground organs, the bulbs; 

 while a short day on the contrary stimulates the formation of 

 leaves. Many tropical plants, as for instance wild species of 

 potatoes, are characterized in their native habitat in the equa- 

 torial regions of South America by an abundant tuber formation. 

 They produce no tubers at all when grown in Central Europe 

 with its longer summer day. But it is sufficient to shorten the 

 day artificially to 12 hr. to obtain numerous tubers. In the 

 cultivated potato, however, a long day does not impede tuber 

 formation, and it gives excellent yields even beyond the Arctic 

 Circle, where the sun does not set in midsummer. Here the 

 influence of the soil temperature is most important. Potatoes 

 native to high altitudes develop best at low temperatures, and 

 an elevation of the soil temperature 22 to 25 per cent suppresses 

 tuberization. 



It is not necessary to subject the plant to a photoperiodic 

 influence during the whole time of its vegetation. It is sufficient 

 to give it the corresponding photoperiod during only its first 

 period of development, from 10 to 15 or 20 days, to obtain the 

 photoperiodic effect. Even when transferred to the reverse light 

 conditions, it continues to develop in accordance with the photo- 

 periodic effect of the first period (Garner and Allard, Egiz, 

 Rasumov). This phenomenon has been termed the ''photo- 

 periodic after-effect," or ''photoperiodic induction." Besides 

 its great theoretical significance, this phenomenon has an impor- 

 tant bearing on agricultural practice because it allows the 

 limiting of the time of action of the light period, which may be 

 more easily accompHshed in practice with plants grown in 

 hotbeds. 



The relative length of day and night exerts a considerable 

 influence upon the growth of trees. Those of different geo- 

 graphical origin react differently. Southern species are retarded 

 in their growth by the influence of the long northern days. The 

 shoots attain under these conditions a considerably larger size 



