524 RADIATION BIOLOGY 



photosensitivity increases with increasing temperatures (Fassbender, 

 1925). 



Quantity of Light. Lehmann (1918) and Lehmann and Lakshmana 

 (1924) were the first to point out that for photoblastism the product law 

 (quantity-of-stimulus law) is valid exactly as for photo- and geotropism. 



This seems to be true, at least within certain limits. Rao (1925) 

 reports for Lythrum salicaria for 3000 MK-sec at 31°C the following 

 germination percentages: 42.8 (10 min X 5 MK), 37 (1 min X 50 MK), 

 37 (30 sec X 100 MK). For Laduca (Flint, 1934) at 20°C and 3840 

 ft-c-sec the germination percentages were 53.5 (64 ft-c X 1 min), 52.8 

 (32 ft-c X 2 min), 54.5 (16 ft-c X 4 min), 58.1 (8 ft-c X 8 min), 47.5 

 (4 ft-c X 16 min), 58.2 (2 ft-c X 32 min) (var. Arlington Fancy). Simi- 

 lar results for var. Grand Rapids were obtained by Evenari (unpubhshed 

 observations). 



The limitations of this "law" are very great. Here again temperature 

 is an important factor. For Lythrum it is valid only for the intensities 

 used between 30° and 35°C. At lower temperatures the intensity range, 

 inside which the product law is valid, is more and more narrowed down. 

 For higher light quantities the effect of light at these temperatures is 

 inversely proportional to the light intensity; i.e., for an equal Meter- 

 Kerzen-second the low intensities used for longer times bring about higher 

 percentages of germination than the high intensities used for shorter times 

 (Lehmann, 1924; Rao, 1925). Exactly the same, i.e., an inverse propor- 

 tionality of light effect and light intensity when an equal amount of light 

 is used, is reported for some negatively photoblastic seeds, e.g., Phacelia 

 (Nicolic, 1924) and three Bromus species (Zeiher, 1936), for which the 

 product law is not valid. 



When the relation between increasing light quantities and germination 

 for a given light intensity is considered, it stands out very clearly that 

 the curves obtained are logarithmic ones. This was first pointed out by 

 Kincaid (1935) for tobacco seeds and holds true for the figures published 

 by Lehmann and Lakshmana (1924) and by Wieser (1927) for Lythrum. 



When the seeds are intermittently illuminated, Talbot's law (summa- 

 tion-of-stimulus law) is valid as long as the dark periods between the light 

 flashes are not too long [L. salicaria, Epilobium hirsutum in Lehmann 

 (1924), Rao (1925), Fassbender (1925)]. If the dark periods are pro- 

 longed without a change in the total amount of light applied, the germi- 

 nation is highly stimulated {ibid). If this fact can be verified, it seems 

 to us to be of considerable importance in explaining the nature of the 

 light effect upon germination. 



Up to now we have considered the effect of different quantities and 

 intensities of fight upon germination when light was applied for short 

 periods only. What happens when different light intensities are used in 

 continuous illumination? For Lactuca (Evenari, 1952) var. Grand 



