Gynoecium and Development of Embryo Sac, Endosperm, and Seed in Pholistoma (Hydrophyllaceae) Relative to Taxonomy

Abstract

The objective of this research was to provide data for a taxonomic evaluation of Pholistoma and for a later survey of evolutionary morphology within the tribe Hydrophylleae. The unilocular ovary bears specialized unicellular prickles. Ovary and capsule wall consist of five or seven cell layers, of which three develop thickened walls for protection and dehiscence. Two parietal placentae enlarge to fill the entire locule. The four to eight ovules per ovary, some of which degenerate, are irregularly ventral and half-epitropous to half-hypotropous. Each ovule is tenuinucellate, unitegmic, and anatropous with a feeble vascular strand. A large amount of ovular parenchyma is produced from two meristems, one of which is the integumentary tapetum. Cells of the ovular epidermis grow into giant cells, each with a tubular portion penetrating deep into the seed interior, thereby producing a most unusual seed coat. The embryo sac is of the Polygonum type, the embryo of the Onagrad type. The endosperm is cellular with a micropylar and a chalazal endosperm haustorium. Both haustoria produce lateral branches. The mature endosperm is porously pitted, due to the giant cells of the testa. Great similarity between the three Pholistoma species, and in particular the presence in all species of two lateral haustorium branches, of gigantic, funnel-shaped epidermal cells, and of a porously pitted endosperm, strongly supports the notion that these three species constitute a natural entity.