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After penetrating the degenerated synergid, thepollen tube releases the twosperm into the embryo sac, where one fuses with the egg and forms azygote and the other fuses with the two polar nuclei of the centralcell and forms atriple fusion, orendosperm, nucleus. This is calleddouble fertilization because the true fertilization (fusion of a sperm with an egg) is accompanied by another fusion process (that of a sperm with the polar nuclei) thatresembles fertilization. Double fertilization of this type is unique to angiosperms. The zygote now has a full complement of chromosomes (i.e., it is diploid), and the endosperm nucleus has three chromosomes (triploid). The endosperm nucleus divides mitotically to form the endosperm of the seed, which is a food-storagetissue utilized by the developing embryo and the subsequent germinating seed. It has been shown that some of the most basal angiosperms actually form diploid endosperm, although they still experience double fertilization.

The three principal types of endosperm formation found in angiosperms—nuclear, cellular, and helobial—are classified on the basis of when thecell wall forms. Innuclear endosperm formation, repeated free-nuclear divisions take place; if a cell wall is formed, it will form after free-nuclear division. Incellular endosperm formation, cell-wall formation is coincident with nuclear divisions. Inhelobial endosperm formation, a cell wall is laid down between the first two nuclei, after which one half develops endosperm along the cellular pattern and the other half along the nuclear pattern. Helobial endosperm is most commonly found in theAlismatales (monocotyledons). In many plants, however, the endospermdegenerates, and food is stored by the embryo (e.g.,peanut [groundnut],Arachis hypogaea), the remaining nucellus (known as perisperm; e.g., beet), or even the seed coat (mature integuments). Cellular endosperm is the least specialized type of endosperm with nuclear and helobial types derived from it.

The zygote undergoes a series of mitotic divisions to form a multicellular, undifferentiatedembryo. At the micropylar end there develops a basal stalk or suspensor, which disappears after a very short time and has no obvious function in angiosperms. At the chalazal end (the region opposite the micropyle) is the embryo proper. Differentiation of the embryo—e.g., the development of cells and organs with specific functions—involves the development of a primaryrootapical meristem (or radicle)adjacent to the suspensor from which the root will develop and the development of onecotyledon (in monocotyledons) or two cotyledons (in eudicotyledons) at the opposite end from the suspensor. A shoot apical meristem thendifferentiates between the two cotyledons or next to the single cotyledon and is the site ofstem differentiation.

The mature embryo is a miniatureplant consisting of a short axis with one or two attached cotyledons. Anepicotyl, which extends above the cotyledon(s), is composed of the shoot apex andleaf primordia; a hypocotyl, which is the transition zone between the shoot and root; and the radicle. Angiosperm seed development spans three distinct generations, plus a new entity: the parentsporophyte, thegametophyte, the new sporophyte, and the new innovation—namely, the endosperm.

Seedlings

Mature seeds of most angiosperms pass through a dormant period before eventually developing into a plant. Thelife span of angiosperm seeds varies from just a few days (e.g.,sugar maple,Acer saccharum) to over a thousand years (e.g.,sacred lotus,Nelumbo nucifera). Successful germination requires the right conditions of light,water, and temperature and usually begins with imbibition of water and the subsequent release from dormancy. During its early growth stages and before it has become totally independent of the food stored in theseed or cotyledons, the new plant is called a seedling.

Two patterns of seedgermination occur in angiosperms, depending on whether the cotyledons emerge from the seed: hypogeal (belowground germination) and epigeal (aboveground germination). Inhypogeous germination, the hypocotyl remains short and the cotyledons do not emerge from the seed but rather force the radicle and epicotylaxis to elongate out of the seed coat. The seed, with the enclosed cotyledons, remains underground, and the epicotyl grows up through the soil. When the cotyledons contain seed-storage products, these products are transferred directly to the developing radicle and epicotyl (e.g., garden pea). When the endosperm or perisperm contains the storage products, the cotyledons penetrate the storage tissues and transfer the storage products to the developing radicle and epicotyl (e.g.,garlic,Allium sativum).

Inepigeous germination, the radicle emerges from the seed and the hypocotyl elongates, raising the cotyledons, epicotyl, and remains of the seed coat aboveground. The cotyledons may then expand and function photosynthetically as normal leaves (e.g.,castor bean,Ricinus communis). When the cotyledons contain seed-storage products, they transfer them to the rest of the seedling anddegenerate without becoming significantly photosynthetic (e.g., garden beans,Phaseolus). Eventually, the seedling becomes independent of the seed-storage products and grows into a mature plant capable ofreproduction. Although the dispersal of seeds is essential in the reproduction and spread of angiospermspecies, it is equally important for successful germination and seedling establishment to take place in an appropriate habitat.