Seed development and function inArtabotrys hexapetalus (Annonaceae)

Until now seeds ofAnnonaceae were characterized as mesotestal only. The seed ofArtabotrys hexapetalus, however, is meso- and endotestal. An outer mechanical layer which surrounds the seed as a lignified fibrous tissue is derived from the mesotesta. A complex inner mechanical layer develops from a partially multi-layered endotesta built up by crystal-containing stone cells. The multi-layered endotesta forms a prominent seed plug in the micropylar region which is prolonged along both sides of the perichalaza as inner walls. The endotesta is one-layered on the sides of the seed and participates in rumination. In addition the endotesta may serve for deposition of end-products of metabolism. The complex growing process of the perichalaza and its surrounding tissues is described in detail. The perichalazal pad of tanniferous cells forms an U-shaped septum, in conjunction with the endotesta, dividing the seed into two chambers. During seed development it functions as transmitter of nutrients from the outer chamber filled with starch grains to the nucellus, endosperm and embryo contained in the inner one. During germination this pad probably serves for the uptake of water. — At the initial phases of germination the seed dehisces into two valves along the raphal and antiraphal side. Later on an additional parenchymatous operculum covering the seed apex disintegrates and the endotestal plug fixed by its two prolongations splits along a preformed fracture line into two parts to release the seedling. Rudiments of an aril are recognizable in young seeds only. — The data obtained fromArtabotrys hexapetalus are discussed and compared with published information on seeds in other annonaceous taxa. For systematic considerations the necessity to define the origin of the annonaceous seed plug from one or the other integument is emphasized as it may prove to be an important differential character withinAnnonaceae.