EFFECTS UPON GERMINATION 805 



It is interesting to compare Chloris and Poa as to the effect of other 

 factors on the favoring action of hght. In both, after-ripening, inter- 

 mittent temperatures, and N-compounds are effective Hght substitutes. 

 In both, no proportionaUty exists between the dosage of light and 

 the extent of its influence upon germination. In Chloris the hulls play 

 a major part in the light need, and in Poa a rather minor part. High 

 partial oxygen pressures are very effective in forcing Chloris achenes 

 in darkness with the hulls intact, but they have no effect on Poa. 

 Chloris becomes "dunkelhart" in a dark germinator furnishing poor 

 conditions for germination. This apparently is not the case with Poa. 

 The coats of Chloris achenes are impermeable to KNO3 in water solution, 

 although it is an effective substitute for light. Adequate data are 

 lacking on the permeability of Poa coats. 



Although no other grasses have been studied as thoroughly as Poa 

 and Chloris in respect to the effect of light on germination, many other 

 grasses have received some attention in this regard. Except for the 

 cultivated cereals (Avena, Hordeum, Secale, Triticum, and Zea), which 

 are indifferent to light, the germination of most grasses is favored by 

 light under some condition or other. The effect of light on germination 

 of grass achenes is reviewed by Lehmann and Aichele (66, pages 432 to 

 461). Of 56 species reported by them, 36 species were favored by light, 

 6 species were favored by darkness, mainly after long dry storage, and 

 22 species, including the cultivated cereals, were indifferent to light. 

 The disagreement between the total number of species mentioned above 

 and the sum of the figures for the three categories is due to the fact that 

 certain species were favored by light under some conditions and indiffer- 

 ent to light or favored by darkness under other conditions. 



Ranunculus sceleratus. — Many investigators have shown that the 

 germination of Ranunculus sceleratus achenes is greatly favored by light. 

 According to Niethammer (76) 4 months' dry storage modified germina- 

 tion in both light and darkness. Lehmann (60), however, indicated that 

 10 months' after-ripening was necessary for germination in darkness. 

 Pricking the coats (81) with a pin did not increase germination. The 

 coat effects, however, need a more critical study. 



Contrary to Lehmann's early results, Gassner (26) found that light 

 did not force Ranunculus sceleratus seeds unless accompanied by inter- 

 mittent temperatures, and, conversely, that intermittent temperatures 

 were only moderately effective without light. He considered the best 

 daily intermittent temperatures those with large differences between the 

 low and high temperatures and with the long period at the low tempera- 

 ture. There was little germination in darkness at any constant tempera- 

 ture (27), but the intermittent temperatures of 28°C. (4 hr. daily) and 

 12°C. (20 hr. daily) gave 50.3 per cent germination in darkness and 87 

 per cent in light; the intermittent temperatures of 28°C. (4 hr. daily) 



