Leaves in Photoperiodism • 17 



THE ROLE OF LEAVES IN PHOTOPERIODISM. 

 PHOTOPERIODIC INDUCTION 



Neither of these topics will be considered in detail until 

 Chapter Five where the discussion is on the nature of the flower- 

 ing stimulus, since both are more germane to that question than 

 to photoperiodism proper. Brief summaries are given here simply 

 to render the rest of this chapter intelligible. 



In almost every plant studied, it is the leaf blades that perceive 

 the photoperiodic treatment. This has been shown in several ways. 

 Photoperiodic treatments given to all, or in some cases one or a 

 few, leaf blades on a plant will have the same effects as though 

 the entire plant had been treated. Defoliated plants, with rare 

 exceptions, are photoperiodically unresponsive. Photoperiodic 

 treatment of the apices or other meristematic areas is usually in- 

 effective, although the meristems are the actual sites of the change 

 from vegetative to reproductive growth. One can conclude that 

 the primary photoperiodic effect occurs in the leaves and that the 

 leaves somehow communicate its results to the meristems. 



Certain plants require more or less constant exposure to 

 appropriate photoperiodic cycles, at least until flower primordia 

 can be easily detected, in order to flower successfully. In many 

 others, however, exposure to only a few such cycles will cause 

 flowering even when the plants are returned to unfavorable photo- 

 periodic conditions. Such plants are said to be induced by the 

 photoperiodic treatment; photoperiodic induction is an aftereffect 

 of favorable photoperiods which will result in flowering or at least 

 considerable primordium development, even on unfavorable photo- 

 periods. An induced plant indicates clearly by this behavior that 

 some change has taken place and persists, but no anatomical or 

 morphological changes can usually be detected after the few induc- 

 tive cycles required in such plants. Naturally, not only is induction 

 of great theoretical interest but it is also experimentally useful. 

 One of the major reasons for the widespread use of Xanthium 

 in photoperiodic studies is that, under favorable conditions, a 

 single short-day cycle (even given to a single leaf) will lead to 

 flowering in plants kept the rest of the time on noninductive long 

 days. This sensitivity to a single cycle is unusual, but is not unique 



