Diversity of Seed Plants and Their Systematics

Diversity of Seed Plants and Their Systematics

1 Diversity of Seed Plants and their Systematics Diversity of Flowering Plants Gurcharan Singh Department of Botany SBD Khalsa College New Delhi Date of Submission: November 20, 2006 2 Diversity of Flowering Plants Flowering plants (Angiosperms) represent the most dominant group of plants on this planet with approximately 255, 265 species (Thorne, 2006) depicting a wide variety of habit and growing in diverse habitats. The group represents tall evergreen trees often reaching more than 100 m in height (Eucalyptus regnans), tall deciduous trees (Populus nigra, Platanus orientalis), evergreen shrubs (species of Rhododendron, Euonymus, Skimmia), herbaceous plants more than a metre tall (Zea mays, Saccharum officinarum, Silybum marianum) or almost microscopic not more than 1-2 mm (Wolfia arrhiza). Several angiosperms with weeker stems may twine around the support (Ipomoea, Convolvulus), climb with the help of tendrils (Cucurbita, Cucumis), hooks (Galium) or thorns (Bougainvillea). The leaves may be as small just 1-2 mm (Pilea microphylla) or over 20 m long (Raphia fainifera). Broad rounded floating leaves of Victoria amazonica often reach 2 m in diameter and can support the weight of a child. Although flowering plants are largely green and photosynthetic, few grow as partial (Viscum) or total parasites on roots (Balanophora) or even leaves (Arceuthobium- growing on pinus leaves and being amongst the smallest angiosperms). A few, mainly the species of orchids (Dendrobium spp.) and Tillandsia grow as epiphytes. Angiosperms form the most successful group of present day plants presenting a unique combination of characters, not found in any other group of plants. Like ferns and some gymnosperms they have macrophyllous leaves with complex parallel (monocots) or reticulate (Dicots) venation patterns. Angiosperms are similar to gymnosperms in producing seeds, the seeds are enclosed in ripened ovary (fruit) and not naked like gymnosperms. Both groups also produce pollen tube (siphonogamous mode of reproduction), but the pollen grains of angiosperms germinate on stigma, whereas they land and germinate directly on ovule. The sieve tubes of angiosperms are associated with companion cells. Double fertilization resulting in triploid endosperm, and highly reduced male and female gametophytes are unique to angiosperms. The angiosperms also have vessels, which though also found in gymnosperms like Gnetum and Ephedra, are of different nature, mainly scalariform pitting in angiosperms, but circular in these gymnosperms. The pollen grains of angiosperms are also unique in having non-laminate endexine and ectexine differentiated into a foot-layer, columellar layer and tectum (tectum absent in Amborellaceae). The angiosperm flower typically is a hermaphrodite structure with carpels surrounded by stamens and the latter by petals and sepals, since insect pollination prevails. Arbuscular mycorrhizae are also unique to angiosperms (except Amborellaceae, Nymphaeales and Austrobaileyales). The vessel elements of angiosperms typically possess scalariform perforations. There may be individual exceptions to most of these characters. Vessels are absent in some angiosperms (Winteraceae) while some gymnosperms have vessels (Gnetales). The flowers are unisexual without perianth in several Amentiferae, which also exhibit anemophily. In spite of these and other exceptions, this combination of characters is unique to angiosperms and not found in any other group of seed plants. Classification of Angiosperms Angiosperms have traditionally been divided into two main groups dicotyledons and monocotyledons, a trend that was consistently following till the end of last century. Linnaeus through his valuable publication Species Plantarum (1753), put in order the great confusion about the naming, establishing binomial nomenclature by providing a name consisting of a generic name combined with specific epithet for every known species of plants. But through his sexual system of classification, a representation of an artificial classification, he relied solely on characters of stamens and carpels, that have not proved to be as reliable as Linnaeus cexpected. In traditional natural classification of Bentham and Hooker (1862-83) widely followed for nearly 100 years in this part of the world, and still followed by some, dicotyledons were divided into three subclasses: Polypetalae, Gamopetalae and Monochlamydeae, further subdivided into series, cohorts (now orders) and natural orders (now regarded as families). Monocots were directly divided into seven series, with no intervening cohorts. The incorporation of phylogeny during last century saw the development of several phylogenetic systems of classification, with attempt to arrange groups according to their inferred evolutionary sequence. Such classifications were proposed by Bessey (1915), Hutchinson (1973), Dahlgren (1989), Cronquist (1988) and Takhtajan (1997). These classifications were similar in many respects considering Magnoliaceae or Winteraceae as most primitive, and Asteraceae, Verbenaceae and Lamiaceae amongst the advanced families. They also depicted the relationships between the groups with the help of line diagrams (Hutchinson) or bubble diagrams, where the branch length represented apomorphy (degree of advancement), and the bubble size the relative number of species. The last two decades have seen a great departure in the understanding and consequently the classification of angiosperms. Angiosperm Phylogeny Group (APG), led by Bremer (who first in 1981 questioned the 3 recognition of monocots and dicots as distinct groups, as such a division renders dicots as paraphyletic-derived from single immediate ancestor, but not including all the descendents), is attempting to divide angiosperms into a number of monophyletic groups (including all the descendents of the common ancestor). Views are further consolidating to interpolate monocot taxa between primitive dicots and the more advanced ones, because it had long been suggested that monocots arose from primitive dicotyledons. The success above the family level, however, is much short of any level of consistency for any meaningful application. As many as 30 families still have to find their place. The artificial clades (given informal group names and not formal taxonomic names) above the order level are too arbitrary, and it may take a long time before the system may have a practical approach for the purpose for which the classifications are meant. From the lot of contemporary authors Thorne, with his latest version (2006), his classification having undergone periodic electronic revisions, has attempted to upgrade his classification in light of recent developments in the field of molecular systematics, but retaining the hierarchical grouping, so essential for any working system of classification. The discussion in the following pages also includes description, illustration of important structures, floral diagram, floral formula of representative species of families with identification of family based on Bentham and Hooker’s System of classification. Identification of families based on Classification of Bentham and Hooker Class Dicotyledons (Embryo with two cotyledons, stem with open bundles, venation reticulate, flowers tetramerous or pentamerous). Subclass 1. Polypetalae (flowers with two whorls of perianth, inner (petals) free). Series 1. Thalamiflorae (sepals usually free, free from ovary, flowers hypogynous, stamens borne on thalamus, disc usually absent). Order Ranales (stamens usually many, carpels free, endosperm conspicuous, embryo small) Family Ranunculaceae: Herbs, leaf base sheathing, flowers spirocyclic, stamens and carpels many, spirally arranged, carpel one or many and free, fruit achene or follicle. Order Parietales (stamens few or many, carpels united, placentation parietal) Family Brassicaceae (Cruciferae) : Herbs, inflorescence racemose, flowers tetramerous, petals cruciform, stamens 6, tetradynamous, ovary with false septum and replum, fruit siliqua or silicula. Order Malvales (stamens many, carpels united, placentation axile) Family Malvaceae: Plants mucilaginous, stipules present, epicalyx present, sepals united, stamens epipetalous, stamens monadelphous, anthers monothecous, fruit a capsule or schizocarpic. Series 2. Disciflorae (sepals free or united, free or adnate to ovary, flowers hypogynous, disc conspicuous, stamens borne on outer or inner base of disc). Order Geraniales (carpels united, placentation axile, ovules 1-2, disc annular) Family Rutaceae: Leaves gland-dotted, sepals united, stamens free or polyadelphous, fruit a berry. Series 3. Calyciflorae (sepals united, adnate to ovary, flowers perigynous or epigynous). Order Rosales (flowers usually perigynous, carpel 1 or free in bud; styles distinct) Family Fabaceae (Leguminosae): Leaves usually compound, leaf base pulvinate, stipules present, bracts usually caducous, odd sepal anterior, carpel 1, placentation marginal, fruit a legume. Three subfamilies: Faboideae (Papilionoideae): Flowers zygomorphic, sepals united, corolla papilionaceous, aestivation vexillary, posterior petal outermost in bud, stamens diadelphous). Caesalpinioideae: Flowers zygomorphic, sepals free, corolla not papilionaceous, posterior petal innermost in bud, stamens free, usually in two whorls. Mimosoideae: Flowers actinomorphic, sepals united, petals valvate, stamens usually many, free or united. Order Umbellales (flowers epigynous, stamens 5, carpels united, placentation axile, ovule 1 in each locule) Family Apiaceae

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