534 



RADIATION BIOLOGY 



coumarin effect on lettuce seed (see Sect. 2-3) also is almost abolished by 

 decoating (Evenari, unpublished observations). 



Since the photoblastic influence of light is in some way bound to the 

 coats surrounding the seeds, the light absorption of these coats has been 

 studied. Meischke (1936) found for the seed coats of a number of posi- 

 tively and negatively photoblastic seeds a slow decrease of the light trans- 

 mission from the red to the violet part of the spectrum [same for Lactuca 

 (Evenari, unpubU.shed observations; see Fig. 11-5)]. This is in contrast 



7000 



6000 

 WAVE LENGTH, 



5000 



4000 



Fig. 11-5. Absorption spectrum of fruit and seed coat of Grand Rapids lettuce seed. 



to Kommerell (1927), who found for Lythrum and Nicotiana a maximum 

 in the absorption curves of the seed coats at the same wave length that 

 produced a minimum in the germination curves. 



4-2. AFTERRIPENING 



Joensson (1893) was the first to point out clearly that the stage of 

 afterripening of the seed is a decisive factor in photoblastism. Whereas 

 shortly after harvest the seeds of Poa pratensis germinated to 88 and 

 1 per cent in light and darkness, respectively, the germination percent- 

 ages were 80 and 78 per cent, respectively, 11 months later. Most posi- 

 tively photoblastic seeds behave in the same way. With increasing 

 length of afterripening the photosensitivity increases and the photo- 

 requirement decreases until the seeds become more or less indifferent to 

 hght (Laschke, 1907; Gassner, 1911a,b; Reihng, 1912; Kinzel, 1913-1926; 

 Kleine, 1924; Stechmann, 1925; Hite, 1923; Maier, 1933a,b; Thompson, 

 1935; Shuck, 1936, 1938; Resiihr, 1939a). The negatively photoblastic 

 Phacelia, too, lost about 50 per cent of its photoblastism after 6 years 

 (Kuhn, 1915). Sometimes the seeds change from positive to negative 

 photoblastism with increasing length of afterripening [Niethammer (1928) 

 for Stenophragma and Bupleurum]. 



