The nucellar epidermis and micropyle of Ornithogalum caudatum (Liliaceae) with a review of these structures in other taxa

Abstract

The nucellar epidermis of Ornithogalum caudatum is modified in its micropylar region to form a nucellar cap. The cells become palisadelike with lignified walls and dense, RNA-rich cytoplasm and a large distal vacuole. High vesicular activity is associated with the greatly thickened proximal walls. The micropyle is formed by the inner integument and at anthesis, it is filled with an exudate secreted by the nucellar cap and inner integument. The exostomium is sealed by a hymen. The exudate and hymen probably function as surfaces for localized deposition of synergid-synthesized chemotropic agents. After fertilization the micropyle is occluded by a plug of flocculent material which appears to be more dense than the prefertilization material. This plug probably serves to preclude pathogen invasion and prevent desiccation of embryo and endosperm.