Physical Forces in Dormancy and Germination of Xanthium Seeds

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Abstract
The germination of seeds of Xanthium pensylvanicum Wallr. occurs in 2 phases, an initial passive phase of water uptake followed by an active phase of growth. These 2 phases have been separated experimentally, and shown to occur similarly in isolated cotyledons and embryonic axes. Measurements of the physical thrust generated by the entire seed and its separate components of cotyledon and axis reveal that nondormant Xanthium seeds develop more than twice the thrust of dormant seeds, and that this difference develops principally in the 2nd phase of enlargementof the axis. Measurement of the forces required for piercing the testa of these seeds establishes that whereas the thrust developed by nondormant seed is adequate to cause testa rupture, that developed by dormant seeds is not. It is concluded that the dormancy of Xanthium involves an inadequacy in the embryo for rupture of the testa.