802 



BIOLOGICAL EFFECTS OF RADIATION 



ripening on the light requirement of Poa achenes. Maier maintained, 

 however, that although fully after-ripened achenes could germinate 

 completely without light, they were even more sensitive to Ught than 

 non-after-ripened achenes, for a dosage of hght too small to favor the 

 germination of non-after-ripened achenes hastened the germination of 

 after-ripened achenes. 



According to Gassner (28) germination of Poa achenes was favored 

 by daily intermittent temperatures whether the hulls were intact or 

 removed. Maier (71) found that light favored germination of achenes 

 with hulls intact or removed, but that removal of the hulls increased 

 germination and light sensitivity of the achenes. Andersen (1) got 

 considerably higher germination both in light and darkness when the 

 hulls were removed and the achenes were moistened with water and 

 germinated at the intermittent temperature 20° to 30°C. While the 

 hulls of Poa modify somewhat the light sensitiveness and germination 

 of the achenes, they play no such an important role as do the hulls of 

 Chloris. 



Nitrogen compounds have been shown to favor the germination of 

 Poa achenes to a marked degree. With many samples of Poa corrvpressa, 

 Toole (95, 96) obtained complete germination of the achenes only when 

 intermittent temperatures were supplemented by exposures to light and 

 to a 0.2 per cent KNO3 solution. Nelson (73) found that next to inter- 

 mittent temperatures KNO3 was most effective in forcing germination of 

 Poa achenes. Potassium nitrate, NaNOa, and Ca(N03)2 were effective 

 in the order named. Germination was depressed by 0.1 and 0.01 per 

 cent Pb(N03)2, also by NaN02 and KNO2, but NH4NO3 was as effective 

 as KNO3. The nitrogen compounds most effective on filter paper were 

 least effective in the soil. Pb(N03) and Pb(N02) were very effective 

 in soil. Hite (43) and Maier (71) also found nitrates favorable to the 

 germination of Poa achenes. Andersen (1) germinated achenes of 

 Poa pratensis in light and darkness at daily intermittent tempei-atures 

 (18 hr. at 20°C. and 6 hr. at 30°C.), wetting some achenes with water 

 and others with N/50 KNO3. Her results were as follows: 



Table 4 



Condition 



Light .... 

 Darkness . 



Germination percentages 



With water 



60 to 70 

 20 to 30 



With KNO, 



90 to 95 

 80 to 85 



Increased partial oxygen pressures (28) did not increase germination 

 of Poa. Germisan hastened and increased germination (77), especially 



