458 PLANT PHOTOPERIODISM 



energy requirement ranges from 0.01 to 0.1 /xj/cm-, whereas a short- 

 term irradiation results in lower sensitivities and a higher energy 

 requirement which may extend to 50 /xj/cm-. On the other hand, the 

 photoperiodic responses have a requirement of at least one to four 

 orders of magnitude higher; the range is from 1000 to 5000 ^ij/cm^. 

 This is to be expected if one considers that the photomorphogenic 

 group is graded and a significant effect is obtained at a very low 

 energy level, and that there is a continuous range of effectiveness from 

 a very low to a relatively high energy level. In photoperiodism, how- 

 ever, the ungraded nature makes it necessary to attain some threshold 

 level which is capable of driving the system over an irreversible barrier, 

 so to speak. Some relatively high energy level is required for this. 

 Flowering behavior and the breaking of dormancy as well as seed 

 germination phenomena are of this type. Seed germination has been 

 included in the photoperiodic class for two reasons. The threshold 

 energy requirement is of the same order of magnitude as that for 

 photoperiodic flowering responses, and Wareing (p. 73) and others 

 have shown that there appears to be some internal periodicity which 

 must be phased with the irradiation to obtain the maximal response. 

 In the long-day plant, as the light intensity during the supple- 

 mentary light period is increased, the plant remains vegetative until a 

 critical level of intensity is reached at which flower bud initiation 

 occurs. The magnitude of floral initiation as measured by the number 

 of buds formed and rate of formation is then a function of intensity of 

 energy up to a certain critical level. At this point the plant has reached 

 its maximal capabilities of response and there is no further increase 

 over a 1000-fold range of intensity. A short-day plant such as Xanthiiim 

 also exhibits ungraded behavior. A light break during the dark period 

 produces no effect on inhibiting flower bud initiation until a certain 

 critical intensity is reached. Then flower bud initiation is progressively 

 decreased in magnitude until a relatively low intensity of saturation is 

 obtained, at which point the plant is vegetative; from then on, there is 

 no further increase in activity. Undoubtedly, the differences between 

 the graded and ungraded responses are not in any way correlated with 

 the initial photochemical processes themselves, but are concerned 

 wholly with the growth-response capability of the tissue systems in- 

 volved. 



