PHOTOPERIODISM 



499 



of definite responses to those with indeterminate ones are shown by 

 tobacco and strawberry. 



A way in which these various responses can be correlated is schemati- 

 cally shown in Fig. 10-4. It is based on the idea of limited effective 

 concentration ranges for some material that at low values is inadequate 

 for action and is inhibitory above an upper concentration limit. 



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SHORT DARK PERIOD (LONG DAY ) 

 CRITICAL LENGTH 



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LIGHT INTERRUPTION 



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DARK PERIOD, hr 



Fig. 10-4. A scheme illustrative of photoperiodic response. 



PHOTOPERIODIC AFTEREFFECTS 



Aftereffects of an inductive period consisting of one or more cycles of 

 light and darkness, followed by return to noninductive flowering con- 

 ditions, have been studied. Several questions are involved. How does 

 subsequent development depend on the degree of induction? Is the leaf 

 modified so as to continue its inductive action under adverse dark periods? 

 Is there storage of an effective material that gradually undergoes dilution? 



Information on the first question has been classified as "photoperiodic 

 induction" (Maximov, 1929) and "photoperiodic aftereffect" (Moskov, 



