Igit] SHULL—OXYGEN MINIMUM AND GERMINATION 455 
ing pollen, the nutrition of the parent plant, accidents of sun and 
shade, moisture and dryness of soil, high and low altitude, weather 
conditions during ripening, the time of harvesting, and subsequent 
handling of seeds are suggested as influential factors in determining 
germination behavior. 
The germination of Xanthium seeds was first investigated by 
ARTHUR (I), who noted the dimorphic character of the two seeds 
in the bur, and found that the lower seed germinated in the spring 
after ripening, while the upper seed germinated the second year or 
later. Believing that the testa was too nearly alike in both seeds 
to cause this difference, and finding much more reducing sugar in 
the lower seed than in the upper after exposure to germinative 
conditions, he suggested that enzymes were produced in the lower 
seeds more readily than in the upper, and that the delayed nutri- 
tion of the embryo of the upper seed was the probable cause of the 
delay in germination. 
MAsTERMAN (19) tested the germination of Xanthium in some 
field experiments during several years, and showed that over 90 
per cent of the burs grew both seeds the same year, thus contra- 
dicting the statement of ARTHUR that the upper seed was usually 
delayed till the second year or later. MAsTERMAN’S experiments 
were not critical, but the results that he and ARTHuR obtained are 
readily explained and harmonized in the light of more recent 
investigation. 
PamMMEL and Lummis (24) found that weed seeds germinated a 
much higher percentage of seeds after having been frozen than 
before. In the case of X. canadense, they found that none of the 
fresh seeds germinated during the fall, but after freezing and thaw- 
ing during the winter, over 50 per cent of them germinated. Faw- 
cETr (7) obtained similar results with many kinds of weed seeds, 
the percentage of germination being increased, and the “period of 
dormancy” shortened by freezing again and again. 
If the upper cocklebur seed is delayed till the second spring or 
later, it is because it has resisted the forces of disintegration which 
finally make possible the entry of oxygen in sufficient quantity for 
germination, or has not experienced the high temperatures which 
Crocker has shown will cause the germination of uppers with 
