92 GERMINATION OF SEEDS: SEEDLINGS 
Oxygen Requirement. — Although seeds are in the optimum 
temperature and properly supplied with moisture, they will usu- 
ally not germinate unless oxygen is supplied, as is often demon- 
strated in the laboratory by the use of some substance to absorb 
the oxygen in the germinator or by replacing the air in the ger- 
minator with hydrogen, nitrogen, or some other substance, so that 
oxygen is excluded. (Fig. 91.) However, since the air is about 
one-fifth oxygen, seeds receive enough oxygen to germinate well 
if only air is supplied, although germination is often hastened 
Fic. 91.— The two U-shaped tubes, which contain soaked seeds (s) on 
moist blotting paper at their stoppered ends, are alike except that in B the 
open end of the tube is in pyrogallate of potash, which absorbs the. oxygen 
from the air in the tube, while in A the open end of the tube is in pure water, 
in which case the oxygen still remains in the air of the tube. The seeds ger- 
minate well in A but not in B. 
when the amount of oxygen is increased artificially. For exam- 
ple, in an experiment Wheat, requiring 4 to 5 days to germinate 
in the air, germinated in 3 days in pure oxygen. There are a 
few seeds, however, which begin to germinate without oxygen, but 
they soon die unless oxygen is supplied. 
For lack of oxygen seeds germinate poorly when planted in the 
soil so deeply that not enough air is accessible, or when planted 
in soils with their pores so full of water that the circulation of the 
air is prevented, 
