KINETIC ANALYSIS OF PHOTOPERIODISM 



457 



function of energy. In the case of the opening of the hypocotyl hook 

 in bean and in the inhibition of hypocotyl growth, the first part of 

 the curve is proportional to incident energy, after which the rate be- 

 comes proportional to the logarithm of the incident energy. In the 

 ungraded or threshold type, no response is obtained until the intensity 

 of the stimulus reaches a certain threshold value, Eo. Then the magni- 

 tude increases very rapidly to a saturating level, Es, after which a 

 further increase in stimulus elicits no further increase or appreciable 

 change in response. This is frequently referred to as the all-or-none 

 type. If the stimulus is less than Eo, nothing is obtained, and above 

 E», the full magnitude of the reaction is induced. The responses to 

 photoperiod are of the ungraded or threshold class. 



Table III. Extrapolated Threshold Energy Requirements 

 Plant Response 



Growth 

 Effect" 



Exposure 

 Time 



Energy, 

 yuj cm- 



Red Spectral Region 



Photomorphogenic systems 

 Avena first internode 

 Pea leaf 

 Bean hypocotyl 

 Bean epicotyl 

 Bean leaf 



Bean hook (hypocotyl) 

 Bean hook (hypocot3'l) 



Photoperiodic systems 



Seed germ, (lettuce) 

 Soybean, S. D. 

 Cocklebur, S. D. 

 Barley, L. D. 

 China aster, L. D. 

 Spinach, L. D. 



Avoid coleoptile 

 Phycomyces, sporangiophore 



" Growth or flowering: stimulation -j-; inhibition — . 



Table III shows a marked difference in energy requirement between 

 the photomorphogenic and photoperiodic classes. If in photomor- 

 phogenesis, the organism is exposed to the energy for many days, the 



