The Oxygen Minimum and the Germination of Xanthium Seeds

Abstract

1. The naked embryos of the dimorphic seeds of Xanthium exhibit a marked difference in their demand for oxygen for germination. 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 7 mm. 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 experiments 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 atmospheric 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. 6. A very slow progressive deterioration of the seeds takes place, which after a few years causes entire loss of power to germinate. 7. The general conclusion that the organs of the seeds of higher plants can grow in entire absence of free oxygen is not supported by the results obtained with Xanthium seeds. They cannot grow without comparatively large amounts of free oxygen. 8. The oxygen pressures required for germination of Xanthium seeds are very much higher than those reported by Lehmann for the epicotyls of such plants as Helianthus perennis, Zinnia elegans, and Glyceria fluitans. 9. Since the coats cause delay by excluding oxygen, we might expect to find the oxygen demand for growth high. Xanthium seeds stand at the opposite end of the series from the seeds of certain aquatic plants, as water plantain and rice, in demands for oxygen for germination. 10. The high oxygen demand, and the difference in this demand in the two seeds, act with the coats to secure delay, and a difference in delay, in the two seeds. But if the coat has been removed, the demand for oxygen by the embryo is too low to be significant in securing delay in germination.