MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 18. SEED PLANTS

Conclusion

Seed plants are divided into gymnosperms and angiosperms. All of them produce seeds, and all are characterized by megaphylls. The Evolution of the seed required heterospory, the retention of a single megaspore within the megasporangium, The Development of the embryo (or young sporophyte) within the megagametophyte, and the presence of integuments. All seeds consist of a seed coat (derived from the integument), an embryo, and a nutrient reserve. In gymnosperms, this reserve is provided by the haploid female gametophyte, whereas in angiosperms it is typically a triploid endosperm. The oldest known seeds are found in Late Devonian deposits dating back about 360 million years. The probable ancestors of seed plants are progymnosperms, a Paleozoic group of vascular spore plants from which various gymnosperm lineages may have arisen independently.

Extant gymnosperms comprise four phyla: cycads, ginkgophytes, conifers, and gnetophytes. Angiosperms, or flowering plants, are subdivided into two major classes—monocots and dicots.

The life cycles of gymnosperms and angiosperms are largely similar, representing an alternation of heteromorphic generations with large, independent sporophytes and highly reduced gametophytes. In gymnosperms, ovules (megasporangia with integuments) lie exposed on

Within the ovule, the megaspore mother Cell (or megasporocyte) develops and ultimately undergoes Meiosis, giving rise to four megaspores, three of which degenerate. The fourth develops into the female gametophyte, which at maturity is a seven-celled, eight-nucleate embryo sac.

A pollen grain germinates on the stigma, producing a pollen tube that grows down through the style toward the Ovary and enters the ovule via the micropyle. The Nucleus of one sperm cell from the pollen tube fuses with the egg cell to form a zygote, while the Nucleus of the second sperm fuses with the two polar nuclei of the embryo sac to form the triploid primary endosperm nucleus. This process of double Fertilization is unique to angiosperms.

The embryo develops within the embryo sac, and the ovule integuments transform into the seed coat. Ultimately, seeds are dispersed from the surfaces of megasporophylls or analogous structures, whereas in flowering plants they are enclosed within folded megasporophylls (carpels) that form part of the characteristic reproductive Structure of angiosperms—the flower.

Gametophytes in flowering plants are more reduced than those in gymnosperms. At maturity, the female gametophyte of most gymnosperms is a multicellular structure bearing several archegonia. In angiosperms, it is most commonly a seven-celled, eight-nucleate structure (the embryo sac). Archegonia are absent here, and the egg cell lies adjacent to two synergids, together forming the so-called egg apparatus.

In both gymnosperms and angiosperms, male gametophytes are represented by pollen grains. Antheridia are absent in both groups. In gymnosperms, male Gametes (sperm Cells) arise directly from a spermatogenous cell, whereas in angiosperms they derive from a generative cell. A germinated pollen grain with a tube nucleus and two sperm cells constitutes the mature male gametophyte. In cycads and Ginkgo, the sperm are flagellated, whereas in all other seed plants they are non-motile and are delivered to the egg cell by a pollen tube following pollination. Water is not required for this process.

Pollination in gymnosperms involves The transfer of pollen from a microsporangium to a megasporangium, whereas in angiosperms it is the transfer from an anther to a stigma.

In gymnosperms, one sperm of the male gametophyte (the germinated pollen grain) fuses with the egg cell of the archegonium. The second sperm has no apparent function and degenerates. In angiosperms, both sperm are functional: one fuses with the egg cell (true fertilization, or syngamy), and the other fuses with the two polar nuclei. The former produces a diploid (2n) zygote, as usual, while the latter yields a triploid (3n) primary endosperm nucleus. This phenomenon—a unique hallmark of angiosperms—is termed double fertilization.

Following fertilization, ovules in both gymnosperms and angiosperms develop into seeds, with the Ovaries (sometimes along with other floral parts) in flowering plants maturing into fruits that enclose the seeds. The fruit is one of the defining features of this largest of plant phyla.