DORMANCY IN SEEDS 



83 



percentages of O2, determined the percentage germination of naked embryos 

 and intact seeds of both seeds of the cocklebur at 21° and 30° C (70° and 

 86° F) at a range of O2 percentages. Upper naked embryos (Fig. 24) at 

 21° C (70° F) require 1.5 per cent and at 30° C (86° F) 0.9 per cent O2 for 

 100 per cent germination; lowers under like conditions require 0.7 and 0.5 

 per cent O2. Note that complete germination of the lower embryos occurs 

 at percentages of O2 that give no germination in the upper embryos. 

 Higher temperature in both lowers and uppers reduced the O2 percentage 

 needed for germination. Fig. 25 shows percentage germination of both 



100 



eo 



z 

 o 



z 



2 

 (C 



LOWER SEED 

 AT 2i' C 



UPPER SEED 

 K ZI'C 



3 6 



t 



Z 



UJ 100 



a 

 a 



so 



60 



40 



20 



20 



60 



70 



eo 



90 



100 



LOWER SEED 



AT ao* c 



UPPER SEED 

 AT 30° C 



3 6 10 



30 



40 50 60 70 



OXYGEN, PERCENTAGE BY VOLUME 



eo 



Figure 25. Minimum oxygen required for germination of the intact cocklebur seed 

 during six days at 21° and 30° C (70° and 86° F). 



upper and lower intact seeds at like temperatures and range of O2 per- 

 centage. Intact seeds require higher percentage of O2 for germination than 

 do naked embryos. The naked embryo of the upper seeds gives 100 per 

 cent germination in 1.5 per cent O2, the uppers ^dth coats intact require 

 100 per cent, or 66 times the O2 pressure. Exact quantitative comparisons 

 between ShuU's and Thornton's data are impossible since they worked 

 with different species and used somewhat different temperatures and dura- 

 tion of experiments. They do, however, agree on all essential points: 

 naked embryos of both upper and lower seeds need definite, easily measur- 

 able minimum Go pressures for germination at various temperatures; 

 minimum pressure needed for the upper embryo is always higher than that 



