532 



RADIATION BIOLOGY 



Shuck, 1936) ; this proves that light, in the presence of a certain mini- 

 mum amount of imbibition water, initiates certain changes in the seeds 

 which are not reversed when the amount of water needed is removed 

 later on. The same holds true for low-temperature pretreatment of 

 lettuce seed with subsequent drying (Borthwick and Robbins, 1928). 



10 20 30 



SOAKING TIME, min 



Fig. 11-4. Germination index (see Fig. 11-3) of Grand Rapids lettuce seed soaked in 

 v/ater in darkness for different lengths of time (4-60 min), illuminated with 2 min X 250 

 ft-c of white light, and returned to darkness at 20°C. 



3-4. CLIMATIC CONDITIONS DURING RIPENING OF SEEDS 



Since temperature and light pretreatment of the seeds influences the 

 photoblastic reaction, it is not surprising that the chmatic conditions 

 under which the seeds ripen when still attached to their mother plants 

 are also important. The seeds of Drosera and Pingiiicula react differ- 

 ently toward light when they are ripened under high- or low-temperature 

 conditions (Kinzel, 1908a). Likewise the grains of Poa pratensis react 

 differently toward light when ripened in rainy or sunny weather (Kinzel, 

 1913-1926). Maier (1933b) reports for Phleum pratense a great differ- 

 ence in the photorequirement of different samples. Those ripened 

 quickly during sunny weather have a less pronounced photorequirement 

 than those ripened slowly during a rainy season. Seeds from sunny 

 habitats require less light or lose their photorequirement faster than seeds 

 from less sunny habitats (Niethammer, 1928). 



It may be mentioned here that Lehmann (1912) found for Verbascum 

 a relation between photoblastism and topographic position of the seed 

 capsules on their mother plant. Seeds from capsules of low position on 

 the inflorescence do not need light for their germination. Seeds from 

 capsules higher up are positively photoblastic. 



