Plant reproductive morphology

Plant reproductive morphology is the study of the physical form and structure (the morphology) of those parts of plants directly or indirectly concerned with sexual reproduction. Plant reproductive morphology is the study of the physical form and structure (the morphology) of those parts of plants directly or indirectly concerned with sexual reproduction. Among all living organisms, flowers, which are the reproductive structures of angiosperms, are the most varied physically and show a correspondingly great diversity in methods of reproduction. Plants that are not flowering plants (green algae, mosses, liverworts, hornworts, ferns and gymnosperms such as conifers) also have complex interplays between morphological adaptation and environmental factors in their sexual reproduction. The breeding system, or how the sperm from one plant fertilizes the ovum of another, depends on the reproductive morphology, and is the single most important determinant of the genetic structure of nonclonal plant populations. Christian Konrad Sprengel (1793) studied the reproduction of flowering plants and for the first time it was understood that the pollination process involved both biotic and abiotic interactions. Charles Darwin's theories of natural selection utilized this work to build his theory of evolution, which includes analysis of the coevolution of flowers and their insect pollinators. Plants have complex lifecycles involving alternation of generations. One generation, the sporophyte, gives rise to the next generation asexually via spores. 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, the gametophyte, produces gametes, eggs and/or sperm. A gametophyte can be monoicous (bisexual), producing both eggs and sperm or dioicous (unisexual), either female (producing eggs) or male (producing 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, is the sporophyte, and the gametophyte is very small. In seed plants, each female gametophyte, and the spore that gives rise to it, is hidden within the sporophyte and is entirely dependent on it for nutrition. Each male gametophyte typically consists of from two to four cells enclosed within the protective wall of a pollen grain. The sporophyte of a flowering plant is often described using sexual terms (e.g. 'female' or 'male') based on the sexuality of the gametophyte it gives rise to. For example, a sporophyte that produces spores 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 both sexes of gametophyte. The flower is the characteristic structure concerned with sexual reproduction in flowering plants (angiosperms). Flowers vary enormously in their construction (morphology). A 'complete' flower, like that of Ranunculus glaberrimus shown in the figure, has a calyx of outer sepals and a corolla of inner petals. The sepals and petals together form the perianth. Next inwards there are numerous stamens, which produce pollen grains, each containing a microscopic male gametophyte. Stamens may be called the 'male' parts of a flower and 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. Carpels may be called the 'female' parts of a flower and collectively form the gynoecium. Each carpel in Ranunculus species is an achene that produces one ovule, which when fertilized becomes a seed. If the carpel contains more than one seed, as in Eranthis hyemalis, it is called a follicle. Two or more carpels may be fused together to varying degrees and the entire structure, including the fused styles and stigmas may be called a pistil. The lower part of the pistil, where the ovules are produced, is called the ovary. It may be divided into chambers (locules) corresponding to the separate carpels. A 'perfect' flower has both stamens and carpels, and may be described as 'bisexual' or 'hermaphroditic'. A 'unisexual' flower is one in which either the stamens or the carpels are missing, vestigial or otherwise non-functional. Each flower is either 'staminate' (having only functional stamens) and thus 'male', or 'carpellate' (or 'pistillate') (having only functional carpels) and thus 'female'. If separate staminate and carpellate flowers are always found on the same plant, the species is called monoecious. If separate staminate and carpellate flowers are always found on different plants, the species is called dioecious. A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species. Members of the birch family (Betulaceae) are examples of monoecious plants with unisexual flowers. A mature alder tree (Alnus species) produces long catkins containing only male flowers, each with four stamens and a minute perianth, and separate stalked groups of female flowers, each without a perianth. (See the illustration of Alnus serrulata.)