The Biochemistry of Induction • 93 



reported to inhibit photoperiodic induction in Xanthium in a 

 manner possibly suggestive of an effect on the synthesis or effective- 

 ness of the flowering hormone (Salisbury and Bonner, 1960). But 

 2-thiouracil also causes a strong inhibition of induction in another 

 SDP, hemp (Cannabis sativa); careful histological observations sug- 

 gest that this action and, by inference, those above are due to a 

 general effect on the differentiation capacities of the meristem 

 rather than to a specific effect on flowering (Heslop-Harrison, 

 1960). 



A question of fundamental importance concerning photo- 

 periodic induction was recently raised by R. M. Sachs on the basis 

 of his and other work with LSDP (see Sachs, 1959). It has been 

 widely assumed that the basic induction process in both LDP and 

 SDP is alike, there being at least two grounds for this assumption. 

 One is the participation of the red, far-red system in both types 

 and the other is the apparent equivalence of florigen in both types, 

 at least among many closely related plants. But Sachs points out 

 that in the LSDP Cestrum nocturnum (night-blooming jasmine) 

 long- and short-day induction appear to differ considerably. The 

 product of long-day induction ■ is not translocated from the treated 

 leaves; short-day induction following long-day induction, however, 

 gives rise to a translocatable flowering hormone. Further, the se- 

 quence of long- and short-day induction is not reversible for any 

 plants requiring both— in LSDP the former must precede the latter, 

 whereas in SLDP the reverse is true. Thus if one assumes that long- 

 day induction in both LSDP and SLDP (as well as in simple LDP) 

 controls the same step in a series of reactions, one then suspects 

 that the short-day induction step in LSDP is not equivalent to that 

 in SLDP. Similarly, assuming that short-day induction in both types 

 (as well as in SDP) is the same, then the long-day induction in the 

 two types must differ. In addition to indicating that short- and long- 

 day induction may affect different processes, Sachs suggests that "we 

 should be wary of the assumption that LD induction affects the 

 same stage of synthesis of the floral stimulus in every LDP (the 

 same doubt exists with regard to SD induction in all SDP)." The 

 question will be finally answered only by a complete understanding 

 of the biochemistry involved, which may take many years. The logic 

 of Sachs's analysis warns that the answer will not be simple, and 

 may also be different for different plants. 



