Double Fertilization and Triple Fusion

Double fertilization is the phenomenon, discovered independently by S.G. Nawaschin and L. Guignard in Lilium and Fritillaria (1898-99) and unique to angiosperms, in which both male gametes carried by a single pollen tube are used in fertilization, each fusing with a different component of the embryo sac. One male gamete fuses with the egg nucleus - a fusion called syngamy - to form the diploid zygote. The second male gamete instead migrates to the central cell, where it fuses with the two polar nuclei (or, if they have already fused, with their combined secondary nucleus) to form the primary endosperm nucleus (PEN); because three nuclei (two polar plus one sperm nucleus) are involved in this second fusion, it is called triple fusion, and the PEN it produces is triploid (3n). The PEN divides repeatedly - immediately after fertilization and before the zygote itself begins dividing - to build up the endosperm, the triploid nutritive tissue that both feeds and, through its own developmental timing, regulates the precise mode of embryo development. Three principal patterns of endosperm development are recognised, based on whether and when wall formation follows nuclear division: nuclear endosperm, in which the PEN divides repeatedly with no wall formation at all, so the endosperm consists purely of free nuclei suspended in a shared cytoplasm (walls may appear only later, if at all - examples: Coccinia, Capsella, Arachis); cellular endosperm, in which the very first division of the PEN is immediately followed by wall formation, and every subsequent division likewise forms a wall straight away (examples: Adoxa, Helianthus, Scoparia); and helobial endosperm, an intermediate pattern in which the PEN first moves to the base of the embryo sac and divides into two nuclei that are separated by a single wall into a large micropylar chamber (whose nucleus divides further, freely, without walls) and a smaller chalazal chamber (whose nucleus may or may not divide further) (examples: Hydrilla, Vallisneria). A fourth, less fundamental variant - ruminate endosperm - describes an endosperm whose surface becomes irregular and uneven, either because the seed coat's layers elongate unevenly (as in Passiflora) or fold/ingrow in a defined pattern (Annonaceae, Aristolochiaceae, and the irregular seed-coat surface of Myristica); Areca catechu is the standard textbook example. Seeds whose endosperm is entirely consumed by the developing embryo before the seed matures are called non-endospermous (ex-albuminous) seeds (pea, groundnut, beans), while seeds that retain endosperm at maturity, using it to nourish the embryo during germination, are called endospermous (albuminous) seeds (paddy, coconut, castor). A further specialised feature seen in some endosperms is the presence of haustoria - absorptive projections that draw nutrients from surrounding ovary/ovule tissue and pass them on to the growing embryo; in helobial endosperm the chalazal chamber itself performs this haustorial role, while in cellular and nuclear endosperm de …