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The horned oak gall forms on twigs of Quercus palustris Muench. and is initiated when the wasp Callirhytis corrigera O.S. oviposits into the periderm or cortex of the twigs. Injury to phellogen as a consequence of oviposition results in dedifferentiation of phelloderm and underlying phloem tissue to form a wound‐response phellogen. The wound‐response phellogen encircles and compartmentalizes the wasp ovum and is continuous with the normal stem phellogen. This tissue arrangement forms the framework for the ensuing stages of gall development. Development of the insect larva coincides with the formation of a gall‐matrix phellogen. This additional meristem is derived from locally hyperplastic stem phellogen where adjacent to the wound‐response phellogen. The essentially parenchymatous, roughly spherical body of the gall, the gall matrix, is formed chiefly from derivatives of the gall‐matrix and wound‐response phellogens. In the immediate area of the wasp larva, the wound‐response phellogen produces cells toward the larva that differentiate to form a vascularized, externally sclerified, horn‐shaped chamber containing in its base the developing insect larva. The involvement of the vascular cambium is apparent during intermediate gall development stages. Vascular strands, produced transverse to the stem axis, penetrate the gall‐matrix tissue and terminate near the base of the horn‐like larval chamber. Vessel elements in the region of the gall that are not included in this tissue alteration often become heavily tylosed. Disruption of the vascular tissue through the galled area appears to be a primary cause of twig death.

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