SEED GERMINATION 529 



positively photoblastic seeds. Nicotiana and Verbascum do not germi- 

 nate in darkness, whatever the oxygen content of the germination atmos- 

 phere. With Daucus, Oenothera, and Rumex the dark germination 

 increases with increasing oxygen content (Gardner, 1921). The same 

 is reported for lettuce (Borthwick and Robbins, 1928). Maier (1933b) 

 reports for Phleum pratense an increase in the germination in darkness 

 and light alike with increasing oxygen content. When Phacelia is germi- 

 nated under reduced partial pressures of air in darkness, the intact seeds 

 no longer germinate below a vacuum of 220 mm Hg. If the chalaza end 

 of the seed coat is removed, a vacuum of 120 mm Hg is required to 

 prevent germination (Bohmer, 1928). 



A very interesting fact was reported for Lactuca. If lettuce seeds are 

 germinated at 26°C in darkness in an atmosphere of high carbon dioxide 

 content (5-20 per cent), the seeds germinate up to 90 per cent. Even 

 thermodormancy does not occur at 35°C either in light or darkness when 

 the seeds are exposed to very high carbon dioxide contents [40-80 per 

 cent (Thornton, 1936)]. Leggatt (1948) reports that in an atmosphere 

 of pure carbon dioxide no germination of lettuce seed occurs in light. 



3. PHOTOBLASTISM AS CONDITIONED BY PRETREATMENT 

 3-1. PHOTO- AND SKOTODORMANCY 



When lettuce seeds in a wet state are retained in complete darkness 

 for several days and then transferred to light, many do not germinate 

 (Shuck, 1936; Nutile, 1943-1944). They entered a "deeper state of 

 dormancy" (Shuck, 1936) or became ''skotodormant" ("dunkelhart" of 

 the German authors); i.e., for these seeds light lost its germination- 

 promoting influence. 



The same has been reported for many other photoblastic seeds (e.g., 

 Gassner, 1910, 1911a,b; Fassbender, 1925; Maier, 1933a). Negatively 

 photoblastic seeds such as Phacelia and Amaranthus become photodor- 

 mant ("lichthart") through prolonged illumination; i.e., they lose their 

 abihty to germinate in darkness (Baar, 1912; NicoHc, 1924; Resiihr, 

 1939a). The intensity of this photodormancy is proportional to the 

 light intensities applied (NicoHc, 1924). In the same way the deepness 

 of the skotodormancy of positively photoblastic seeds is dependent upon 

 the duration of the dark period (Gassner, 191 la, b). 



But photo- or skotodormancy cannot be induced in all photoblastic 

 seeds. The negatively photoblastic seeds of Blitiim and Celosia, for 

 instance, germinate well when put into darkness after having been 

 illuminated (Baar, 1912). 



Maier (1933b) brought out a very interesting fact for Phleum pratense. 

 Whereas the light sensitivity of these positively photoblastic seeds reaches 

 its peak after 12 hr in darkness, it afterward decreases steadily, and after 



