Plant reproductive morphology

''. In this species, gametes are produced on different plants on umbrella-shaped gametophores with different morphologies. The radiating arms of female gametophores (left) protect archegonia that produce eggs. Male gametophores (right) are topped with antheridia that produce sperm. Plants have complex lifecycles involving an alternation of generations. One generation, the sporophyte, produces spores which then grow to become the next generation, the gametophyte. These produce gametes, the eggs and sperm, which then unite and grow to become sporophytes, completing the cycle. Spores may be identical (isospores) or come in different sizes (microspores and megaspores), but strictly speaking, spores and sporophytes are neither male nor female because they do not produce gametes. The alternate generation, gametophytes, can be monoicous (bisexual), where an individual can produce both eggs and sperm, or dioicous (unisexual), where one produces only eggs and another produces only sperm. In the bryophytes (liverworts, mosses, and hornworts), the sexual gametophyte is the dominant generation. In ferns and seed plants (including cycads, conifers, flowering plants, etc.) the sporophyte is the dominant generation; the obvious visible plant, whether a small herb or a large tree, and the gametophyte is very small. In bryophytes and ferns, the gametophytes are independent, free-living plants, while in seed plants, each female megagametophyte, and the megaspore that gives rise to it, is hidden within the sporophyte and is entirely dependent on it for nutrition. Each male gametophyte typically consists of two to four cells enclosed within the protective wall of a pollen grain. The sporophyte of flowering plants is often described using sexual terms (e.g. "female" or "male") . For example, a sporophyte that give rise only to male gametophytes may be described as "male", even though the sporophyte itself is asexual, producing only spores. Similarly, flowers produced by the sporophyte may be described as "unisexual" or "bisexual", meaning that they give rise to either one sex of gametophyte or gametophytes of both sexes. ==Flowering plants==

Basic flower morphology '' flower In angiosperms the flower is the characteristic sexual reproductive structure, which varies enormously across the group. The bisexual flower (termed "perfect" botanically), of Ranunculus glaberrimus in the figure provides an example of the common structures. A calyx of outer sepals and a corolla of inner petals form the perianth, the non-sexual part of the flower. Next inwards grow numerous stamens that produce pollen grains, each grain producing a tiny male gametophyte from a microspore. Stamens collectively form the androecium. Finally in the middle there are carpels, which at maturity contain one or more ovules, and within each ovule is a tiny female gametophyte produced from a megaspore. Carpels also have a stigma which receives pollen and a style which connects the stigma to the ovary and enables the pollen to grow into the ovary for the female gametophyte to achieve fertilization. Carpels collectively form the gynoecium. In other flowering plants, two or more carpels and their styles and stigmas may be fused together to varying degrees in the same flower. This entire structure may be called a pistil. Variations '', the common or European alder, has unisexual flowers and is monoecious. The male-flower catkins are hanging down on the left, the much smaller female flowers are above and last season's fruit on the right. '' has unisexual flowers and is dioecious: (above and top right) a 'shoot' with flowers from a male plant, showing robust stamens with pollen, and a female-flower stigma, reduced and sterile; and (below and bottom right) a shoot with flowers from a female plant, showing a robust stigma and male-flower stamens (staminodes), reduced, sterile, with no pollen. A flower with functioning stamens and carpels is described as "bisexual" or "hermaphroditic". A unisexual flower is one in which either the stamens or the carpels are missing, vestigial or otherwise sterile. Staminate unisexual flowers have only functional stamens and are thus male, and carpellate or pistillate unisexual flowers have only functional carpels and are thus female. If only bisexual flowers are found on plants of a species, it is described as homoecious, If both staminate and carpellate unisexual flowers are always found on the same plant, the species is described as monoecious. If each plant has either only staminate or carpellate flowers, the species is described as dioecious. A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species. • Andromonoecious: having both bisexual and male flowers on the same plant. • Gynoecious: having only female flowers (the female of a dioecious population); producing seed but not pollen. • Gynomonoecious: having both bisexual and female flowers on the same plant. (From the Greek monos "single" + oikia "house". See also the Wiktionary entry for .) • Perfect: (of flowers) see bisexual. • Subdioecious: having some individuals in otherwise dioecious populations with flowers that are not clearly male or female. The population produces normally male or female plants with unisexual flowers, but some plants may have bisexual flowers, some both male and female flowers, and others some combination thereof, such as female and bisexual flowers. The condition is thought to represent a transition between bisexuality and dioecy. Resource-allocation constraints may be important in the evolution of dioecy, for example, with wind-pollination, separate male flowers arranged in a catkin that vibrates in the wind may provide better pollen dispersal. In climbing plants, rapid upward growth may be essential, and resource allocation to fruit production may be incompatible with rapid growth, thus giving an advantage to delayed production of female flowers. Dioecy has evolved separately in many different lineages, and monoecy in the plant lineage correlates with the evolution of dioecy, suggesting that dioecy can evolve more readily from plants that already produce separate male and female flowers. == See also ==