694 BIOLOGICAL EFFECTS OF RADIATION 



lengths for these responses. In flax the thickness of the stem and the 

 development of xylem and phloem decrease, while the diameter of the 

 pith increases with decrease in the daily light period. 



THE PHOTOPERIODIC AFTEREFFECT 



{Photoperiodic Induction) 



Observations were made by Garner and Allard on the minimum 

 number of days in an initial treatment with a light period favorable to 

 flowering which is required to obtain definite response when the plant is 

 subsequently transferred to a day length distinctly unfavorable to 

 flowering. With typical short-day plants it was found that initial 

 exposure to a short day for at least 10 successive days was necessary to 

 obtain flowering when the plants were subsequently exposed to a long 

 day. A longer pretreatment with the short day was required to obtain 

 abundant flowering and the minimum pretreatment period for successful 

 fruiting was about 21 days. In this connection, in view of the fact that 

 in nature plants usually are subjected to a changing rather than a fixed 

 length of day, Russian investigators have recently emphasized the 

 importance of this phase of photoperiodism. The carry-over effect of a 

 given light period after the plant comes under the influence of another 

 light period has been designated as "photoperiodic induction" and as 

 "the photoperiodic aftereffect." 



Rasumov (57) studied the aftereffects on growth and development in 

 representative short-day and long-day types of an initial exposure to a 

 long- or a short-day length for various periods, and the relation of these 

 effects to those dependent on time of seeding. The day length experi- 

 enced by the plant at the beginning of development is found to be of 

 great significance in its later development, particularly as to the trans- 

 formation from the vegetative to the reproductive stage. Pretreatment 

 with the shortened daylight period for some days markedly hastens 

 flowering in short-day plants and delays it in long-day plants. Similar 

 exposure to a long day produces reverse effects in the two types. The 

 degree of acceleration or retardation in the onset of flowering lies between 

 the extreme effects of continued long-day or short-day exposures so 

 that the alternating treatment may be employed to regulate the time of 

 flowering. In the short-day type of plant the strongest aftereffect is 

 produced by a short day and in the long-day type the reverse is true. 



In similar experiments with species of the short-day and long-day 

 types Lubimenko and Szeglova (43) likewise found that the initial 

 exposure to a long or a short day serves to displace the time of flowering, 

 an effect which is called photoperiodic induction. The retarding or 

 accelerating effects are manifested in both the vegetative and the repro- 

 ductive phases of development. Pretreatment with total darkness 

 accelerates subsequent development of the short-day type and retards 



