PHOTOPERIODISM 695 



that of the long-day type when the plants are transferred to a long day. 

 The pretreatment with long or short days affects not only the rate of 

 development of the plant as a whole but influences differently the various 

 organs. The persisting effects of the initial day length treatment may be 

 explained by assuming that a rather stable structure of the protoplasm 

 is created which determines from the outset the rate of development of 

 the whole plant and its various organs. 



Rasumov (58) has shown that the photoperiodic afteraffect is of 

 importance in the process of tuber formation. Though different species 

 vary widely in their sensitivity to day length with respect to tuber forma- 

 tion, generally speaking a short day affords optimum conditions for 

 formation of tubers and produces the maximum yield in relation to 

 general development of the plant. An initial exposure to a long day so 

 as to favor extensive top development, followed by exposure to a short 

 day to induce rapid tuberization usually produces maximum absolute 

 yield of tubers. The reverse treatment, due to the afteraffect of the 

 initial short-day exposure, may produce a small crop of tubers, but 

 frequently the subsequent long-day effect causes the tubers to revert to 

 stolons and give rise to new shoots. Kondo (38) found that a short-day 

 exposure throughout the seed-bed stage produced two crops of panicles 

 in the rice plant, but shorter treatments produced no persisting effects. 



SUPPLEMENTARY ARTIFICIAL ILLUMINATION AND CONTINUOUS LIGHT 



As was pointed out in the introductory paragraphs, early workers 

 found that with certain species night illumination with artificial light 

 from various sources could be used successfully as a supplement to day- 

 light to increase the rate of growth and development. Some of these 

 investigators arrived at the general conclusion that plants do not require 

 a daily rest period. In most cases not much attention was given to the 

 intensity of the artificial illumination used. Subsequent research has 

 show^n that while the rate of development can be increased by use of 

 continuous light in many plants of the long-day or indeterminate types, 

 some are more or less severely injured, or even killed, by uninterrupted 

 illumination. The amount of injury undoubtedly is influenced by the 

 intensity of the light and by other factors. It was observed by Garner 

 and Allard that in many cases supplementary illumination of an intensity 

 of less than 5 foot-candles when used to prolong the daily light period 

 may function much the same as the natural long day so far as concerns 

 formative effects. In particular the supplementary light may promote 

 flowering in long-day plants and retard it in short-day plants. 



Similar observations have been made by several other investigators. 

 Adams (2) observed that with a daylight period of more than 12 hr. 

 weak supplementary illumination for 5 to 6 hr. prevented flowering in 

 soy beans, did not hasten flowering in tomato or buckwheat, retarded 



