90 



PHOTOCONTROL OF GROWTH 



501 



PROMOTION 



LactucQ sotiva 

 (scale X 0.04) 



_L 



_L 



40 



E 

 u 



^2 30 





>. 



<v 



c 



20- 



INHIBITION 



Lactuca- 



sativa 

 (scale X 10) 



L.virginicuin 



5500 6000 6500 7000 7000 7500 



Wove Length in Angstrom Units 



8000 



Fig. 1. Action spectra for promotion and inhibition of germination of 

 seeds of Lepidium virginicum and of Lactuca sativa to 50% (Toole et 

 al, 1956). 



brium altissimum L., Sisymbrium officinale (L.) Scop., Thlaspi 

 arvense L., and Ubnus americana L. 



The response of seeds to a given light exposure varies with the time 

 of imbibition preceding the exposure (Borthwick et al., 1954). After 

 lettuce seeds are placed on the moist substratum and imbibition starts, 

 the amount of promotion with a fixed irradiance with red increases for 

 8 to 10 hr and then remains constant until about 20 hr (Fig. 2). After 

 20 hr the amount of stimulation decreases rapidly. The amount of 

 inhibition with a fixed irradiance with far red also remains constant 

 for imbibition times of 10 to 20 hr but increases after 20 hr. The fact 

 that as one sensitivity increases, the other decreases is a strong argu- 

 ment that the photocontrol of seed germination is a coupled reaction. 



That the promoting effect of red and the inhibiting effect of far- 



