Fertilization and embryogenesis

After penetrating the degenerated synergid, the pollen tube releases the two sperm into the embryo sac, where one fuses with the egg and forms a zygote and the other fuses with the two polar nuclei of the central cell and forms a triple fusion, or endosperm, nucleus. This is called double 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) that resembles 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-storage tissue 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 the cell wall forms. In nuclear endosperm formation, repeated free-nuclear divisions take place; if a cell wall is formed, it will form after free-nuclear division. In cellular endosperm formation, cell-wall formation is coincident with nuclear divisions. In helobial 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 the Alismatales (monocotyledons). In many plants, however, the endosperm degenerates, 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, undifferentiated embryo. 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 primary root apical meristem (or radicle) adjacent to the suspensor from which the root will develop and the development of one cotyledon (in monocotyledons) or two cotyledons (in eudicotyledons) at the opposite end from the suspensor. A shoot apical meristem then differentiates between the two cotyledons or next to the single cotyledon and is the site of stem differentiation.

The mature embryo is a miniature plant consisting of a short axis with one or two attached cotyledons. An epicotyl, which extends above the cotyledon(s), is composed of the shoot apex and leaf 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 parent sporophyte, the gametophyte, 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. The life 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 the seed or cotyledons, the new plant is called a seedling.

Two patterns of seed germination occur in angiosperms, depending on whether the cotyledons emerge from the seed: hypogeal (belowground germination) and epigeal (aboveground germination). In hypogeous germination, the hypocotyl remains short and the cotyledons do not emerge from the seed but rather force the radicle and epicotyl axis 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).

In epigeous 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 and degenerate 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 of reproduction. Although the dispersal of seeds is essential in the reproduction and spread of angiosperm species, it is equally important for successful germination and seedling establishment to take place in an appropriate habitat.