PHOTOPERIODISM 



495 



pigment. Sensitivity of the biological material, however, varies with 

 conditions other than the state of the pigment. Thus for lettuce-seed 

 germination the sensitivity varies with time of imbibition prior to irradi- 

 ation (Borthwick, Hendricks, Parker, et al., 1952). But the curves giving 

 germination response as a function of irradiance are parallel. Energy- 

 response curves for germination of tobacco and lettuce seed can be 

 brought into approximate superposition by translation by a given factor 

 in energy. This is the more remarkable when it is remembered that not 

 only are the plants of different families, but also the temperatures, times 



30 



20 - 



30 



20 E 



10 X 



100 

 ft-c-min 



100.0 



1.0 

 RELATIVE ENERGY, 



Fig. 10-3. Variation in inhibition in length of the second internode of dark-grown 

 Colsess I barley .seedlings with radiant energy. Population of albinos about 36; of 

 potentially green, about 110. Results from four experiments are shown by the various 

 symbols. 



of imbibition, and durations of germination are different for the two kinds 

 of seed. 



The energy-response curves for floral initiation are quite different from 

 those for seed germination {ibid.; Parker et al., 1946). There appears 

 to be a threshold energy for response, and an energy change of approxi- 

 mately fourfold is adequate to pass from a measurable response to satu- 

 ration. Variance in response at a given energy is very small. These 

 differences are compatible with a partial conversion of the pigment being 

 necessary for initiation of response and with saturation being reached at 

 some other intermediate value, such as 30 and 60 per cent conversion, 

 respectively. 



Variation of internode elongation of barley with energy (Borthwick 

 et al., 1951) is shown in Fig. 10-3. Essentially similar results were 

 obtained for lengthening of leaves of dark-grown pea seedlings (Parker 

 et al., 1949). In stem lengthening the response to radiation is, to a con- 

 siderable extent, inhibition of cell division (Avery et al., 1937). 



