Post Fertilization Structure and Events

The events that occur after fertilization - development of the endosperm and embryo, and the formation of seed and fruit - are collectively called post-fertilization changes. As these proceed, essentially every part of the flower is transformed into a corresponding part of the mature fruit and seed: the sepals, petals, stamens, style and stigma usually wither and fall away; the ovary develops into the fruit, and the ovule into the seed; the egg becomes the zygote; the funicle becomes the stalk of the seed; the ovule's micropyle becomes the seed's micropyle (through which oxygen and water are later absorbed during germination); the nucellus, if any of it survives digestion by the growing embryo sac and embryo, persists as the perisperm (as in black pepper and beet root); the outer integument becomes the testa (outer seed coat) and the inner integument becomes the tegmen (inner seed coat); the synergid cells degenerate; the secondary nucleus develops into the endosperm; and the antipodal cells likewise degenerate. A number of unusual, species-specific transformations are also recorded: the receptacle becomes fleshy and edible around the enclosed seeds in Pyrus malus (apple); the calyx may persist and enlarge (Solanum melongena) or cover the fruit entirely (Physalis minima); the floral axis below the gynoecium swells into an edible, pear-shaped body in Anacardium occidentale (cashew apple), while in jackfruit it is the perianth itself that becomes fleshy; the cells at the tip of the outer integument around the micropyle develop into a fleshy caruncle in Ricinus communis (castor); and the funiculus develops into a fleshy, often brightly coloured aril in Myristica and Pithecellobium. Embryogenesis - the development of the embryo itself - is illustrated by the Onagrad/crucifer-type development of the dicot embryo in Capsella bursa-pastoris, which develops at the micropylar end of the embryo sac. The zygote first divides transversely into an upper (terminal) cell and a lower (basal) cell; the basal cell then divides transversely while the terminal cell divides vertically, producing a four-celled proembryo, after which a second vertical division at right angles to the first, in the terminal cell, produces a four-celled quadrant, and a further transverse division of the quadrant produces an eight-celled octant arranged in two tiers of four - an upper (epibasal/anterior) octant and a lower (hypobasal/posterior) octant. A periclinal division within these octants then produces a sixteen-celled stage with eight outer and eight inner cells: the outer eight form the dermatogen, which divides anticlinally to build the epidermis, while the inner eight divide vertically and transversely to form an outer periblem (which gives rise to the cortex) and a central plerome (which gives rise to the stele). Meanwhile the two cells derived from the basal cell divide repeatedly (transversely) to build a six-to-ten-celled suspensor, which pushes the now-globular embryo deep into the developing endosperm; the uppermost suspensor cell enlarges into a haustorium and the lowermost, called the hypophysis, itself divides (one transverse plus two vertical divisions at right angles) into eight cells arranged in two tiers, whose upper tier gives rise to the root cap and epidermis. At this stage the embryo proper is heart-shaped; further cell division in the hypocotyl and cotyledon regions elongates it into a curved, horseshoe-shaped embryo, and the mature embryo that finally results has a radicle, two cotyledons and a plumule. The mature, fertilized ovule is now a seed, comprising an embryo, an endosperm (in endospermous seeds such as wheat, maize, barley, sunflower - or none, in non-endospermous seeds such as bean, mango, orchids, cucurbits) and a protective seed coat; seed size itself is hugely variable, from an orchid seed weighing around 20.33 micrograms fresh to the double coconut (Lodoicea maldivica) at roughly 6 kg. As a worked example of a dicot seed, Cicer (gram) shows a funiculus that leaves behind a scar called the hilum once it disappears, a micropyle just below the hilum (through which oxygen and water enter during germination), a thick outer seed coat differentiated into a hard outer testa and a thin, membranous inner tegmen (fused together in Pea), two laterally attached, food-storing cotyledons, a radicle (embryonic root) projecting beyond the cotyledons at one end of the embryonal axis and a plumule (embryonic shoot) at the other, with the axis above cotyledon level called the epicotyl (terminat …