522 RADIATION BIOLOGY 



seeds var. Grand Rapids germinate at 14°C with the same high per- 

 centage in both Ught and darkness (Evenari, 1952). As the temperature 

 rises above 14°C, the seeds get more and more positively photobhistic; 

 i.e., with rising temperatures, germination is more and more inhibited in 

 darkness. This inhibition is overcome by Ught. Lettuce seeds var. New 

 York are nonphotoblastic between 14° and 24°C. At higher temper- 

 atures they become negatively photoblastic (ibid.). 



Epilohium hirsutum and E. roseum react still differently. The lower 

 the temperature, the more positively photoblastic they are. At higher 

 temperatures they become indifferent to light (Gassner, 1915a). This is 

 contested for E. hirsutum by Fassbender (1925), who found that with 

 rising temperatures these seeds do not lose their photoblastism but only 

 become more photosensitive. 



Phacelia tanacetifolia is negatively photoblastic at all temperatures, 

 whereas five Oenothera species (Gassner, 1915a), Primula obconica, and 

 Eaymondea pyrenaica (Schroeder and Barton, 1938 -1939) were positively 

 photoblastic over the whole temperature range over which germination 

 occurred. 



Alternating temperatures mostly have a very pronounced influence 

 upon the germination of photoblastic seeds (e.g., Liebenberg, 1884; 

 Gassner, 1915a; Fassbender, 1925; Kincaid, 1935; Andersen, 1947). 

 Liebenberg (1884) even went so far as to identify the effect of light and 

 alternating temperature when he found that Poa pratensis did not germi- 

 nate at 20° and 30°C without fight but germinated well in darkness with 

 temperatures alternating daily between 20° and 30°C. The germination 

 of Ranunculus sceleratus is brought about by light only when the illumi- 

 nation is accompanied by intermittent temperatures. Light without 

 intermittent temperatures is more or less ineffective, and intermittent 

 temperatures without light are less effective than with added light (Gass- 

 ner, 1915c). When alternating temperatures of 12°C for 4 hr and 28°C 

 for 20 hr daily were used, the percentage of germination of Oenothera 

 biennis reached 90 per cent in the dark, whereas the dark germination 

 at 12° and 28°C was and 65 per cent, respectively. This means that 

 intermittent temperatures had exactly the same influence as light in over- 

 coming the germination inhibition observed at constant temperatures in 

 darkness, since at constant temperatures of 28°C in light the percentage 

 of germination reached only 91.5 per cent (ibid.). With Lactuca Thomp- 

 son (1938) observed that an alternation of temperature between 5°-10°C 

 and 22°-30°C brought about germination in the dark more effectively 

 than did low temperatures alone. But with Phacelia intermittent tem- 

 peratures are ineffective (Gassner, 1915a). 



An interesting feature of some seeds is the thermodormancy caused by 

 high temperatures. At temperatures of 30°-35°C lettuce seeds enter a 

 state of deep dormancy which cannot be overcome by light (Borthwick 



