SEED GERMINATION 521 



Indifferent light: Light of a certain wave length which acts hke dark- 

 ness. When it is given after stimulating or inhibiting light, the per- 

 centage of germination remains unchanged. 



Sensibilization and desensibilization: All processes that lead to a higher 

 or lower photosensitivity. We speak about photo-, thermo-, and chemo- 

 sensibilization, according to the causative agent. 



2. PHOTOBLASTISM AS A FUNCTION OF EXTERNAL CONDITIONS 



2-1. TEMPERATURE 



Photoblastism is greatly influenced by temperature, as has been pointed 

 out by Pauchon (1880) and Cieslar (1883). This was confirmed by many 

 others (Gassner, 1911a,b, 1915a; Baar, 1912; Lehmann and Aichele, 1931; 

 Kincaid, 1935; Thompson, 1938; Resuhr, 1939a;- Evenari, 1952). 



Chloris ciliata, a South American pampas grass extensively studied by 

 Gassner (1911a,b) is positively photoblastic at higher temperatures, non- 

 photoblastic at 20°C, and negatively photoblastic at temperatures below 

 20°C. 



Three species of Amaranthus and Physalis franchetti behave in the same 

 way (Baar, 1912). For A. caiidatus, for example, the percentages of 

 germination at different temperatures in light and darkness are as follows : 



Percentage of Germination 



This example shows clearly that the terms "positive photoblastism," 

 "negative photoblastism," and "photo-indifferent" cannot be used for 

 classifying seeds, since the photoblastism of one and the same species 

 and of one and the same sample is dependent upon external and internal 

 conditions. These terms designate only a certain physiological state. 

 As additional proof may be cited the fact that the same sample of seeds 

 is stimulated or inhibited by Ught at different stages of afterripening (see 

 Sect. 4-2). 



Whereas for most photoblastic seeds temperature is an important factor 

 in determining their germination behavior toward light, the relation may 

 be completely different from that found for Chloris, Amaranthus, and 

 Physalis. Thompson (1935, 1938) reported that lettuce seeds are posi- 

 tively photoblastic only through a temperature range of 18°-25°C. At 

 temperatures below 18°C they become indifferent to fight. This is too 

 general a statement, since different varieties behave differently. Lettuce 



