SEED GERMINATION 527 



to be similar, Resiihr (1939a) thinks the light-absorbing material inside 

 the seeds must be similar too. Only Flint and McAlister (1937) tried to 

 extract the photoreceptor (or photoreceptors) of Lactuca and found good 

 agreement between action spectrum and absorption spectrum of the ace- 

 tone extract of Lactuca seeds, which in turn is nearly identical with the 

 absorption spectrum of chlorophjdl. But the authors think that besides 

 chlorophyll there must be another yellow pigment present (perhaps 

 carotenes?) which absorbs the blue inhibiting light. In this connection 

 it is important that Seybold (cited in Resiihr, 1939a) found chlorophyll 

 in the fruit coat of Lactuca but not in the seed itself, and that Flint and 

 Moreland (1943) are of the opinion that there is evidence of the persist- 

 ence of chlorophyllous tissue in many seeds. 



2-3. CHEMICALS 



Interaction of Photohlastism and Certain Chemicals. Lehmann reported 

 in 1909 that positively photoblastic seeds of Ranunculus sceleratus germi- 

 nated well in darkness when the germination medium was wetted with 

 the nutrient solution of Knop. Gassner (1915b) found that the active 

 part of the nutrient solution was the nitrogen salts it contained, especi- 

 ally potassium nitrate, which since then has been used extensively for 

 stimulating the germination of photoblastic seeds (see Rules for Testing 

 Seeds, 1949). But there are quite a number of positively photoblastic 

 seeds that do not react to potassium nitrate [e.g., Gentiana andrewsii 

 (Giersbach, 1937-1938), Verbascum thapsus (Gassner, 1915b, c)]. 



The germination-promoting influence of soil upon Chloris (Gassner, 

 1911b) is explained by the presence of nitrogen compounds in it (Gassner, 

 1915a; Shuck, 1936). This seems doubtful after Nelson (1927) found 

 that the same nitrogen salts that stimulated the germination of Poa when 

 appUed in solution inhibited it when added to the soil. 



The nitrate effect in bringing about germination of positively photo- 

 blastic seeds in darkness is not identical with the light effect, since for 

 C. ciliata the light effect is dependent upon temperature, the nitrate 

 effect independent of it (Gassner, 1915a). Leggatt (1946) comes to the 

 same conclusion for Agrostis and sums up: "Nitrate while serving in 

 germination tests as partial substitute for light appears to have an entirely 

 different physiological effect." Nitrogen compounds increase the photo- 

 sensitivity and decrease the photorequirement of Poa (Maier, 1933a). 



For lettuce seed, thiourea and a number of other compounds contain- 

 ing carbon, nitrogen, and sulfur in a special linkage have the same effect 

 as potassium nitrate (Thompson and Kosar, 1938, 1939). They are also 

 able to break thermodormancy (Thompson and Horn, 1944). Weak 

 acids, too, promote germination of positively photoblastic seeds in cer- 

 tain cases (Lehmann, 1913; Lehmann and Ottenwalder, 1913; Kerbosch, 

 1920; Gardner, 1921; Fassbender, 1925), but apparently only in combi- 



