The paracarpous gynoecium in Capsella bursa-pastoris is characterized by postgenital fusion of two carpels into a single structure representing a false two-loculated ovary. The ovular primordium is initiated by periclinal cell divisions of both the subdermal and third layers of the placenta. The ovule is ana-amphitropous, medionucellate, funicular and bitegmic, with the micropyle formed by both integuments. During development the cells of the micropylar and middle nucellar zones degenerate and the persisting chalazal zone assumes a column shape (the postamento-podium). The integuments develop according to Dermal type and Variation C (author’s term). In the mature ovule the inner integument consists of three layers, and the outer integument is two-layered except on the abaxial side of the ovule where the integuments are more massive. At the two-nucleate megagametophyte stage the inner epidermis cells of the inner integument begin to divide periclinally. These divisions are followed by differentiation, giving rise to cells that differ in their form, structure, substance accumulation and participation in seed coat organization. The inner layer, consisting of cytoplasm-rich cells with marked radial expansion, represents the endothelium. The cells of the other layer become vacuolate and extend tangentially. They form the middle layer. The cells of the outer epidermis and middle layer are destroyed (in the latter only partly) during seed development. The endothelium becomes the endotegmen (pigment layer), composed of thick-walled cells that contain tannins and possibly lipids. The outer integument gives rise to the testa, composed of an epidermal mucilaginous layer and a sclerotic (mechanical) layer consisting of cells with thickened radial and inner tangential walls and containing starch. The hypostase is differentiated at the base of the nucellus and integuments in contact with the chalaza. The vascular bundle of the ovule reaches the hypostase, which is preserved also in the mature seed and represented by 3-5 layers forming a cup. The cells of the hypostase accumulate proteins and dextrins during the late stages of ovule development, and starch after fertilization. Later, at the early globular embryo stage, the cell walls begin to lignify, the cell contents showing tannin-like substances.
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