74 PHOTOCONTROL OF GROWTH 



photoperiodic effects, and this was found to be so (Black and Wareing, 

 1954, 1955), The behavior of the seeds is markedly affected by tem- 

 perature. At 15 °C, the seeds show definite photoperiodic responses, 

 so that a high germination is obtained under long days, whereas under 

 short days germination is low. It is found that 8 long-day cycles are 

 required to give maximum germination. At temperatures of 20-25 °C, 

 the response is radically modified and 50% germination will now 

 occur in response to a single light exposure of 8 to 12 hr. At the higher 

 temperature, therefore, the photoperiodic behavior is lost and germina- 

 tion occurs in response to a single fight-exposure, as with lettuce seed. 

 It is found that the most effective spectral region for stimulation of 

 germination of birch seed lies in the red, and the effect of a single ex- 

 posure to red radiation can be completely nullified if it is followed by 

 infrared irradiation. 



Photoperiodic effects in seeds have also been reported by Isikawa 

 (1954) and Bunnino et al. (1955), who found not only seeds in which 

 germination is promoted by long days, but others which give a higher 

 germination under short days than under long days. We have investi- 

 gated (Black, 1957; Black and Wareing, 1957) the responses in such 

 a "short-day" seed, as a corollary to our studies in birch seed, and for 

 this purpose the seed of Nemophila insignis was selected, since it has 

 long been known to be a light-inhibited seed (Lehmann, 1909). This 

 seed is also known to be very temperature-sensitive, and germinates 

 equally in fight and dark at temperatures of 19 °C or lower, whereas it 

 is completely inhibited in both light and darkness at temperatures of 

 26 °C and above. At 21-22°C, however, the germination is affected 

 by the light conditions, and with white fluorescent tubes the response 

 is markedly photoperiodic, a high germination percentage being ob- 

 tained under short days, whereas germination is strongly inhibited un- 

 der long days. The response is affected by the duration of both the 

 light and the dark periods, but particularly by the latter. We have in- 

 vestigated the response of this seed to various spectral regions. 



Earlier workers reported that the seed of Phacelia tanacetifolia, a 

 species related to Nemophila, is inhibited by both blue and red light 

 (Meischke, 1936; Resiihr, 1939). For this purpose we used a set of 

 10 Schott interference filters (having a band pass of 10-14 mix at half- 

 maximum transmission) so chosen as to cover the range 405 to 760 



