526 



RADIATION BIOLOGY 



4000 



8000 



nates the region between 5000 and 6000 A as indifferent for Amaranthus 

 and as indifferent or slightly stimulating for Phacclia. Meischke (1936) 

 reports around 24,000 A a stimulating zone for positively photoblastic 

 seeds and an indifferent zone for negatively photoblastic seeds. In sharp 

 contrast to these authors, Kommerell (1927) found a direct proportion- 

 ality at equal energies between percentage of germination and wave length 

 for Lythruin salicaria and Nicotiana (abac am. In the light of the other 



authors' research, this does not seem very 

 probable. 



If for a positively photoblastic seed like 

 Lactuca and a negatively photoblastic one 

 like Amaranthus there are only minor differ- 

 ences between the wave lengths that stimu- 

 late or inhibit both, what is the reason for 

 the fact that with one white light stimulates 

 and with the other inhibits germination? 

 The answer is, according to Meischke (1936), 

 to be found in his experiments with differen- 

 tial filters and antagonistic light. These 

 have shown that for positively photoblastic 

 seeds the influence of the stimulating regions 

 is greater than the influence of the inhibiting 

 ones. For negatively photoblastic seeds the 

 reverse is true (Meischke, 1936; see Fig. 11-2). 

 Another difference between the two types 

 lies in their reaction to indifferent light, 

 which in both cases acts like darkness; i.e., 

 negatively photoblastic seeds germinate well 

 in indifferent light, but positively photo- 

 blastic seeds do not (Resiihr, 1939a). 

 We would like to point out here the similarity of the action spectrum 

 for photoblastism and the action spectra of other light-conditioned physi- 

 ological processes. The action spectra for leaf elongation in etiolated pea 

 seedlings and for floral initiation of barley, cocklebur, and soybean show 

 maxima between 6000 and 7000 A, i.e., in about the same region where 

 the stimulating-wave-length band for germination is situated ; a minimum 

 occurs between 4200 and 5200 A, i.e., in the same region where our inhibit- 

 ing sector is to be found (Parker et at., 1949). The maximum in the 

 action spectra of phototropism lies in about the same inhibiting blue 

 region (Burkholder, 1936; Galston, 1950). Whether this is just a simi- 

 larity or points to a common basic reaction remains to be seen. Cer- 

 tainly research in this direction seems to be promising. 



Since only light that is absorbed can act on photochemical processes, there 

 arises the question of the photoreceptors. As the action spectra seem 



5000 6000 7000 

 WAVE LENGTH, A 



Fig. 11-2. Schematic diagram 

 representing the influence of 

 light of different wave lengths 

 upon the germination of posi- 

 tively (curve a) and nega- 

 tively (curve b) photoblastic 

 seeds. With positively pho- 

 toblastic seeds the stimulating 

 influence is more pronounced 

 than the inhibiting; with nega- 

 tively photoblastic seeds it 

 is the reverse. Therefore 

 mixed (white) light inhibits 

 germination of negatively pho- 

 toblastic seeds and stimulates 

 germination of positively pho- 

 «toblastic seeds. 



