454 PLANT PHOTOPERIODISM 



of threshold energy requirement, the dark-adapted eye is a consider- 

 ably more sensitive biological detector. 



An intensity of 10"^^ /xw/cm" of red light represents approximately 

 3X10^ quanta per cmVsec. If, for estimation purposes, one considers 

 a cell as having an average cross-sectional area of 0.1 X 0.1 mm, the 

 incident quantum intensity per cell is 3 per sec. Since it has not been 

 possible to extract a pigment with absorption characteristics of the 

 action spectrum, that is, a blue substance with a single strong absorp- 

 tion band in the red, and in vivo spectrophotometry has not revealed 

 any strong red absorption band other than those of the chlorophylls 

 and cytochromes, it is evident that the pigment must be present in an 

 extremely low concentration. Therefore, only a very small portion of 

 the incident quanta could be captured by the photomorphogenic pig- 

 ment system. This means that the cellular quantum-absorption rate 

 cannot be more than a few quanta per hour; yet the seedling is still 

 capable of responding. These results clearly indicate that one must 

 postulate some form of amplification in which the effect of the initial 

 photoproduct is amplified many orders of magnitude by a secondary 

 system. 



The threshold energy requirements for the various red and blue light 

 responses are given in Table II. The energy values in the last column 

 are those obtained by extrapolating the log-response curve back to 

 zero. This yields an arbitrary threshold; it is not the lowest value that 

 can be detected, but it represents a convenient criterion of minimal 

 excitation. It will be noted that the inhibition of the first internode in 

 the oat seedling, the stimulation of leaf in pea, and inhibition of the 

 hypocotyl in bean, all have approximately the same threshold energy 

 requirement of 0.01 /^j/cnr, which is 0.1 erg/cnr. Curvatures of the 

 Avena coleoptile and the sporangiophore of the fungus Phycomyces 

 are of the same general order of magnitude. The other responses have 

 relatively higher threshold values. 



RED LIGHT BIOLOGICAL RESPONSES 



The biological consequences of the red, far-red photoreactions in 

 higher plants are extremely varied. An analysis of the kinetics of the 

 various responses reveals two broad classes of phenomena in which 



