Hypostase development in Ornithogalum caudatum (Liliaceae) and notes on other types of modifications in the chalaza of angiosperm ovules

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Abstract
Seven types of chalazal modifications have been reported to occur as normal developmental changes in angiosperm ovules. Among them is the hypostase, a group of usually lignified cells. In Ornithogalum, hypostase differentiation becomes evident during the meiotic–mitotic interphase. Differentiation proceeds toward the micropylar end of the ovule at first but later becomes bidirectional. Differentiation is usually completed after the third mitotic division of the megagametophyte has occurred. One to three hypostase cells per ovule have what appears to be a nonstarch carbohydrate storage product. The greatest proportion of starch reserves in the mature ovule occurs in the chalazal end of the nucellus and, although the hypostase does not seem to be an important storage tissue, it probably has an integral function in the translocation of nutrients into the megagametophyte and, after fertilization, into the embryo sac.