1911] SHULL—OXYGEN MINIMUM AND GERMINATION 475 
and anaerobic respiration. This question can be answered only 
by further investigation. 
A detailed study of the permeability of the seed coats of Xan- 
thium to oxygen and other gases and reagents will be carried on, 
with the purpose of determining whether BECQUEREL’s conclusions 
regarding the réle of dry seed coats have general applicability. 
The necessity for more exact quantitative studies of the factors 
which cause delayed germination is emphasized by this work. 
The factors which cause specific behavior of the seed are sometimes 
very minute and may escape detection entirely unless the methods 
employed in investigation are adapted to that end. The most 
refined methods of quantitative study are best suited to this 
purpose, and for a further advance with the problems of delayed 
germination it will be necessary to adopt the most exact and 
rigorous methods of analysis. 
Summary 
1. The naked embryos of the dimorphic seeds of Xanthium 
exhibit a marked difference in their demand for oxygen for germina- 
tion. 
2. The oxygen minimum for the germination of decorticated 
Xanthium seeds at 21° C. is approximately 12 mm. for the upper 
seeds, and about 9.5 mm. for the lowers. 
3. Increasing the temperature decreases the minima, a rise of 
10° from 21° lowering the necessary minimum of oxygen from 12 mm. 
to approximately 7mm. for the uppers, and from 9.5 mm. to 
approximately 3 mm. for the lowers. 
4. Variation of the total atmospheric pressure probably does 
not influence the oxygen minimum for germination. The experi- 
ments indicate that equal partial pressures of oxygen produce 
approximately the same effect on the seeds, regardless of the total 
pressure of which it forms a part. 
5. There is very little after-ripening, or at least the after- 
ripening is not visible in an altered germination behavior at atmos- 
pheric pressure and ordinary temperatures. There is evidence either 
of a decrease in the oxygen need, or an increase in the permeability 
of the coats to oxygen, or both, as ripening progresses. 
