520 RADIATION BIOLOGY 



Mosheov, 1938; Chrismar and Fernando, 1942). On the other hand, 

 various investigators have tried to reveal the mechanism of the Hght 

 effect. Joensson (1893), Lehmann and associates (various papers, 1909 to 

 1931), Gassner (various papers, 1910 to 1930; Gassner and Franke, 1934- 

 1935), and their numerous coworkers have tried to find out how light acts 

 upon germination. This causal research received a new stimulus when, 

 through the development of a better experimental technique, detailed 

 action spectra of light germination became possible (Kommerell, 1927; 

 Fhnt, 1934, 1935, 1936; Fhnt and McAlister, 1935, 1937; Meischke, 

 1936; Reslihr, 1939a,b). 



But, though a mass of interesting experimental data has been collected, 

 we are still far from a real understanding of the influence of light on 

 germination [for detailed historical surveys see Lehmann (1915), Gardner 

 (1921), Lehmann and Aichele (1931), Crocker (1936)]. 



1-2. DEFINITIONS OF TERMS USED 



A clear-cut terminology is, just as a material tool, an instrument of 

 research. Since different authors use different terms in describing the 

 same fact and since sometimes the same term is used in a completely 

 different sense by different authors, we give first of all a definition of 

 all the terms used in this chapter. 



Dormancy: Any condition of perfectly viable seeds which makes them 

 resistant to germination under environmental conditions that are ordi- 

 narily favorable for quick germination (Toole, 1939). When dormancy 

 is caused by the presence or absence of light, we speak about photo- or 

 skotodormancy (from the Greek skotos = darkness) ; when caused by tem- 

 perature conditions, about thermodormancy; and when caused by chemi- 

 cal agents, about chemodormancy. 



Afterripening: Naturally occurring changes which take place in seeds 

 after harvest and which bring about changes in their dormancy (ibid.). 



Photoblastism: The influence of the presence or absence of light on the 

 germination of certain seeds. Photoblastism may be positive or negative, 

 depending on whether the light stimulates or inhibits germination. 



Photorequirement: The need for light of positively photoblastic seeds. 



Photosensitivity: The relation between the quantity of light and the 

 percentage of germination of photoblastic seeds. 



Inhibiting light: Light of a certain wave length which depresses the 

 germination percentage below that of darkness. When given after 

 stimulating light, it depresses the percentage of germination below that 

 obtained w^ith stimulating light alone. 



Stimulating light: Light of a certain wave length which increases the 

 percentage of germination above that of darkness. When given after 

 inhibiting light, it increases the percentage of germination above that 

 obtained with inhibiting light alone. 



