Homosporous life histories
A homosporous life history occurs in nearly all bryophytes and in most pteridophytes (lower vascular plants). It is characterized by morphologically identical spores that germinate to produce bisexual (both male and female) gametophytes in pteridophytes but either bisexual or, more usually, unisexual (either male or female) gametophytes in bryophytes. Each mature gametophyte bears gametangia (sex organs) that produce gametes. Each antheridium (male gametangium) forms many motile flagellate sperm, and each archegonium (female gametangium) forms one nonmotile egg. Fusion of an egg and a sperm (syngamy) creates a zygote and restores the 2n ploidy level. Various mechanisms prevent the fusion of eggs and sperm from a bisexual gametophyte (inbreeding). For example, the sex organs may mature at different times (usually antheridia mature first), or inbreeding may be chemically or genetically inhibited. The zygote divides mitotically to form the embryo, which then develops into the sporophyte. This eventually produces sporangia, which bear meiocytes (sporocytes) that divide meiotically to form spores. The number of spores produced per sporangium ranges from 16 or 32 in some pteridophytes to more than 65 million in some mosses. The sporangia may be borne in specialized structures, such as sori in ferns or as cones (strobili) in many other pteridophytes. The leaflike structures that bear sporangia are called sporophylls.
In most homosporous life histories of pteridophytes, the spores are both morphologically and physiologically identical and produce bisexual gametophytes. In some species of horsetail (Equisetum), the spores may be physiologically different and produce male or female gametophytes. This uncommon situation is called functional heterospory and may represent the means by which the heterosporous condition in vascular plants evolved from the homosporous condition.
Anisosporous life histories
In anisosporous life histories, an unusual phenomenon in bryophytes, there is a size difference between spores produced in the same sporangium. Each meiotic division results in a tetrad of two small spores that produce male gametophytes and two larger spores that produce female gametophytes.
Heterosporous life histories
A heterosporous life history occurs in some pteridophytes and in all seed plants. It is characterized by morphologically dissimilar spores produced from two types of sporangia: microspores, or male spores, and megaspores (macrospores), or female spores. In pteridophytes, megaspores are typically larger than microspores, but the opposite is true in most seed plants.
The spores produce two types of gametophytes: each microspore develops into a microgametophyte (male gametophyte), which ultimately produces male gametes (sperm), and each megaspore produces a megagametophyte (female gametophyte), which ultimately produces female gametes (eggs). Fusion of an egg and a sperm creates a zygote and restores the 2n ploidy level. The zygote divides mitotically to form the embryo, which then develops into the sporophyte. Eventually the sporophyte produces sporangia, which bear sporocytes (meiocytes) that undergo meiosis to form spores. Microsporangia (male sporangia) produce microsporocytes (micromeiocytes) that yield microspores. Megasporangia (female sporangia) produce megasporocytes (megameiocytes) that yield megaspores. The sporangia may be borne in specialized structures such as sori in ferns, cones (strobili) in some pteridophytes and most gymnosperms, or flowers in angiosperms. The leaflike structures bearing microsporangia and megasporangia are called, respectively, microsporophylls and megasporophylls. In angiosperms these sporophylls represent, respectively, the stamens and the carpels of the flower; in gymnosperms these sporophylls may constitute parts of, respectively, microstrobili (male cones, or pollen cones) and megastrobili (female cones, ovule cones, or seed cones).
The essential difference between the homosporous and heterosporous life history is the presence in the latter of two spore types (microspores and megaspores) and their concomitant precursory structures (microsporocytes and megasporocytes; microsporangia and megasporangia; etc.) and subsequent structures (microgametophytes and megagametophytes).
Variations involving seed plants
The gymnosperms and angiosperms not only lack some reproductive structures found in the homosporous and heterosporous pteridophytes but also have certain reproductive structures peculiar to the seed plants. Heterosporous pteridophytes, like their homosporous counterparts, have archegonia, antheridia, and motile flagellate sperm. The seed plants completely lack antheridia, and of the extant groups only the ginkgo and the cycads have flagellate sperm. Archegonia occur in most gymnosperms except Gnetum and Welwitschia, but they are lacking in all angiosperms.
Pollen grains and pollen tubes (male reproductive structures), ovules and seeds (female reproductive structures), and seedlings are structures unique to all seed plants. The ovule is a single megasporangium (in seed plants, this is called the nucellus) surrounded by one or two integuments (in rare cases, none or three) and containing inside the nucellus a single megasporocyte (spore mother cell). The megasporocyte undergoes meiosis to form four megaspores, three of which typically degenerate, the remaining one developing into the megagametophyte (female gametophyte). Ovules never dehisce (split open) to release their megaspores, unlike the megasporangia of most pteridophytes. The pollen grain is the partly or completely developed microgametophyte (male gametophyte). It is usually multicellular, consisting of two or three cells in angiosperms and usually two to five cells in gymnosperms, although in conifers it is occasionally one cell (for example, the families Taxaceae and some Cupressaceae) or 6 to 43 cells (the families Araucariaceae and some Podocarpaceae).
During pollination, pollen is transferred from its source to a receptive surface: in gymnosperms from the microsporangium to the integument or, especially, the pollination droplet of the ovule (rarely to the cone scale); in angiosperms from the microsporangium (pollen sac) of the anther to the stigma of the carpel. Once pollen has reached the appropriate receptive source, it germinates to form the pollen tube, a structure that grows toward the megagametophyte and in so doing conveys the sperm directly to the egg. All angiosperms and most gymnosperms, except ginkgo, cycads, and some fossil seed plants, lack swimming sperm. The presence of swimming sperm apparently represents a more primitive transitional evolutionary condition. After fertilization, the ovule transforms into a seed. The integument or integuments become modified into the seed coat. The seed typically becomes dormant for a period of time before it germinates to produce a seedling.
Double fertilization is a phenomenon unique to angiosperms. Each pollen grain produces two sperm; one fuses with an egg to form the zygote, and the other fuses with one or more polar nuclei in the female gametophyte (megagametophyte, or also “embryo sac”) to form an endosperm, which has a ploidy level that varies from 2n to 15n. In approximately 70 percent of the known cases, the second sperm fuses with two endosperm nuclei to produce a 3n (triploid) endosperm. The endosperm is a special nutritive tissue for the embryo and, after seed germination, for the seedling. In contrast, the megagametophyte is the comparable nutritive tissue in the gymnosperms.