SEED GERMINATION 523 



and Robbins, 1928; Thornton, 1936; Thompson, 1938). This thermo- 

 dormancy can be broken only by low-temperature treatment (Davis, 

 1924; Borthwick and Robbins, 1928; Thornton, 1936) or by removing 

 or pricking the endosperm together with the integumentary membrane, 

 whicii is inseparable from it (Borthwick and Robbins, 1928; Leggatt, 

 1948). 



Summing up, we may say that temperature very largely determines 

 the photoblastic reaction of seeds, but different plants react differently. 

 The only common feature may perhaps be found in the fact that, at the 

 thermal optimum of germination, photoblastism is at its minimum. The 

 farther away we get from this optimum, the more pronounced becomes 

 the photoblastic reaction (Resiihr, 1939a,b). If this is so, many seeds 

 considered as indifferent to Hght will in the future be found to be photo- 

 blastic when experimented with at nonoptimal temperatures (ibid.). 



2-2. QUANTITY AND QUALITY OF LIGHT 



Photosensitivity. Photoblastic seeds differ very much in their photo- 

 sensitivity. Lythrum salicaria still reacts to 730 HK (Hefner Kerzen; 

 730 HK = 657 ft-c) appHed for 0.1 sec at 30°C (Lehmann, 1918). 

 Tobacco seeds show a significant increase in germination when illumi- 

 nated for 0.01 sec with direct sunlight. They are so photosensitive that 

 strong moonlight applied for 15 min stimulates germination (Kincaid, 

 1935). The germination of Phleum pratense is stimulated by 200 MK 

 (Meter-Kerzen; 200 MK = 18.5 ft-c) of white light applied for 1 sec 

 (Maier, 1933b). 



For Lactuca var. Grand Rapids 0.2 sec of 250 ft-c at 26°C is the mini- 

 mum quantity of light which produces a statistically significant stimu- 

 lation of germination (Evenari, unpublished observations). In contrast 

 to these very photosensitive seeds, there are other seeds that possess a 

 much lower photosensitivity, although their photorequirement may be 

 equally great. Chloris ciliata needs 1-4 hr of daylight in order to show a 

 stimulation of germination (Gassner, 1910). The use of temperature 

 alternations greatly increases the photosensitivity, so that 10 min of day- 

 light is sufficient to stimulate germination (Gassner, 1911a). The mini- 

 mum amount of light needed to stimulate germination of Poa nemoralis 

 is 0.5 hr of daylight (Joensson, 1893). The photoblastic seeds of Mimulus 

 ringens need 14 days of sunlight in order to be able to germinate (Hutch- 

 ings, 1932). 



Photosensitivity is a function of temperature, as already mentioned for 

 C. ciliata. Whereas for Lactuca at 26°C the minimum quantity of light 

 needed to bring about a minimum photoblastic reaction is 0.2 sec X 

 250 ft-c, at 30°C about 300 sec X 250 ft-c is needed to bring about the 

 same minimal effect; i.e., with increasing temperatures the photosensi- 

 tivity decreases (Evenari, unpublished observation). But for C. ciliata 



