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The flowering plants (also called angiosperms) are the dominant and most familiar group of land plants. The flowering plants and the gymnosperms comprise the two groups in the seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics.
Angiosperm derived characteristics The flowers of flowering plants are the most remarkable feature that distinguishes them from other seed plants. Flowers initiated the differences between gymnosperms and angiosperms by broadening the scope of evolutionary relationships and niches open to flowering plants, allowing them to eventually dominate terrestrial ecosystems. The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized the carpel and some surrounding tissues develop into a fruit, another opportunity for angiosperms to increase their domination of the terrestrial ecosystem with evolutionary adaptations to dispersal mechanisms. Endosperm formation generally begins after fertilization and before the first division of the zygote. Endosperm is a highly nutritive tissue that can provide food for the developing embryo, the cotyledons, and sometimes for the seedling when it first appears. These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. The major exception to the dominance of terrestrial ecosystems by flowering plants is the coniferous forest. Origins The first undisputed evidence of angiosperms appears in the fossil record in the early Cretaceous period with a rapid diversification in the mid-Cretaceous. Some older fossils, such as the upper Triassic Sanmiguelia, have been suggested to represent early angiosperms. Based on current evidence, it seems that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late Triassic (245-202 million years ago). A close relationship between angiosperms and Gnetophytes, suggested on the basis of morphological evidence, has been disputed on the basis of molecular evidence that suggest Gnetophytes are more closely related to other gymnosperms. Fossil plants with some identifiable angiosperm characteristics appear in the Jurassic and early Cretaceous (135-65 million years ago), but in relatively few and primitive forms. The great angiosperm radiation, when a great diversity of angiosperms appear in the fossil record, occurred in the mid-Cretaceous (approximately 100 million years ago). By the late Cretaceous, angiosperms appear to have become the predominant group of land plants, and many fossil plants recognizable as belonging to modern families (including beech, oak, maple, and magnolia) appeared. Classification
Internal classification The internal classification of this group has undergone considerable revision as ideas change about the relationships of the plants that form this group. The Cronquist system, proposed by Arthur Cronquist in 1968 and published in its full form in 1981, is still widely used but is no longer believed to reflect phylogeny. A general consensus about how the flowering plants should be arranged has recently begun to emerge, through the work of the Angiosperm Phylogeny Group, who published an influential reclassification of the angiosperms in 1998. An update incorporating more recent research was published as APG II in 2003. Traditionally, the flowering plants are divided into two groups, which in the Cronquist system are called Magnoliopsida (at the rank of class, formed from the family name Magnoliacae) and Liliopsida (at the rank of class, formed from the family name Liliaceae). Other descriptive names allowed by Art 16 of the ICBN include Dicotyledones or Dicotyledoneae, and Monocotyledones or Monocotyledoneae, which have a long history of use. In English a member of either group may be called a "dicotyledon" (plural "dicotyledons") and "monocotyledon" (plural "monocotyledons"), or abbreviated, as "dicot" (plural "dicots") and "monocot" (plural "monocots"). These names derive from the fact that the dicots usually have two cotyledons (embryonic leaves) within each seed, while the monocots usually have only one. From a diagnostic point of view the number of cotyledons is neither a particularly handy nor reliable character. Recent studies, as by the APG group, show that the monocots are a "good" group (a holophyletic or monophyletic group); this clade is given the name monocots. However, the dicots are not (they are a paraphyletic group). Nevertheless, within the dicots a "good" group does exist, which includes most of the dicots. This clade is called the eudicots or "tricolpates". The name "tricolpates" derives from the type of pollen found throughout this group. The name eudicots is formed by preceding "dicot" by the botanical prefix "eu-" (from the greek 'eu'= "true"), as the eudicots share the characters traditionally attributed to the dicots, such as flowers with four or five parts (four or five petals, four or five sepals). Separating this group of eudicots from the rest of the (former) dicots leaves a remainder, which sometimes are called informally "palaeodicots" (the prefix "palaeo-" means old, and derives from the classic greek). As this remainder group is not a "good" group this is a term of convenience only. Flowering plant diversity The number of species of flowering plants is estimated to be in the range of 250,000 to 400,000. The number of families in APG (1998) was 462. In APG II (2003) it is not settled; at maximum it is 457, but within this number there are 55 optional segregates, so that the minimum number of families in this system is 402. The most diverse families of flowering plants, in order of number of species, are: In the list above (showing only the 9 largest families), the Orchidaceae, Poaceae, Cyperaceae and Araceae are monocot families; the others are dicot families. Plant anatomy The amount and complexity of tissue-formation in flowering plants far exceeds that found in Gymnosperms. The vascular bundles of the stem are arranged such that the xylem and phloem stand side by side on the same radius. In the Dicotyledons, the bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue, known as cambium; by the formation of a layer of cambium between the bundles (interfascicular cambium) a complete ring is formed, and a regular periodical increase in thickness results from it by the development of xylem on the inside and phloem on the outside. The soft phloem soon becomes crushed, but the hard wood persists, and forms the great bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings. In the smaller group, the Monocotyledons, the bundles are more numerous in the young stem and scattered through the ground tissue. Moreover they contain no cambium and the stem once formed increases in diameter only in exceptional cases. Flowers The characteristic feature of angiosperms is the flower, which shows remarkable variation in form and elaboration, and provides the most trustworthy external characteristics for establishing relationships among angiosperm species. The function of the flower is that of ensuring fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally on a shoot or from the axil of a leaf. Occasionally, as in violet, a flower arises singly in the axil of an ordinary foliage-leaf. However, more typically, the flower-bearing portion of the plant is sharply distinguished from the foliage-bearing or vegetative portion, and forms a more or less elaborate branch-system called an inflorescence. The reproductive cells produced by flowers are of two kinds, microspores or pollen grains, are the "male" cells and are borne in the stamens (or microsporophylls), and the "female" cells called megaspores, in which the egg-cell is developed, contained in the ovule and enclosed in the carpel (or megasporophyll). The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. Usually, however, other structures are present and serve both to protect the sporophylls and to form an envelope attractive to pollinating insects. The individual members of these surrounding structures are called sepals and petals (or tepals in flowers such as Magnolia where sepals and petals are not distinguishable from each other). The outer series (calyx of sepals) is usually green and leaf-like, and functions to protect the rest of the flower, especially in the bud. The inner series (corolla of petals) is generally white or brightly coloured, and more delicate in structure, and functions in attracting a particular insect or bird by agency of which pollination is effected. This attraction involves colour and scent, and frequently also nectar which is secreted in some part of the flower. These characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans. While the majority of flowers are perfect or hermaphrodite (having both male and female parts in the same flower structure), flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization. Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot easily transfer pollen to the pistil (receptive part of the carpel). Homomorphic flowers may employ a biochemical (physiological) mechanism called self-incompatibility to discriminate between self- and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers. Fertilization and embryogenesis Double fertilization refers to a process in flowering plants during reproduction, in which two sperm cells fertilize two cells in the ovary. The pollen grain adheres to the stigma of the carpel (female reproductive structure) and grows a pollen tube that penetrates the ovum through a tiny pore called a micropyle. Two sperm cells are released into the ovary through this tube. One of the two sperm cells fertilizes the egg cell, forming a diploid zygote or embryo, also called the ovule. The other sperm cell fuses with two haploid polar nuclei in the center of the embryo sac. The resulting cell is triploid (3n). This triploid cell divides through mitosis and forms the endosperm, a nutrient-rich tissue inside the fruit. When seed develops without fertilization, the process is known as apomixis. Fruit and seed As the development of embryo and endosperm proceeds within the embryo-sac, its wall enlarges and commonly absorbs the substance of the nucellus (which is likewise enlarging) to near its outer limit, and combines with it and the integument to form the seed-coat; or the whole nucellus and even the integument may be absorbed. The ovary wall has developed to form the fruit or pericarp, the structure of which is closely associated with the manner of distribution of the seed. Frequently the influence of fertilization is felt beyond the ovary, and other parts of the flower take part in the formation of the fruit, as the floral receptacle in the apple, strawberry and others. The character of the seed-coat bears a definite relation to that of the fruit. Their function is the twofold one of protecting the embryo and of aiding in dissemination; they may also directly promote germination. If the fruit is a dehiscent one and the seed is therefore soon exposed, the seed-coat has to provide for the protection of the embryo and may also have to secure dissemination. On the other hand, indehiscent fruits discharge these functions for the embryo, and the seed-coat is only slightly developed. Economic importance Agriculture is almost entirely dependent on angiosperms, either directly or indirectly through livestock feed. Of all the families of flowering plants, the Poaceae, or grass family, is by far the most important, providing the bulk of all feedstocks (rice, corn (maize), wheat, barley, rye, oats, millet, sugar cane, sorghum), with the Fabaceae, or legume family, in second place. Also of high importance are the Solanaceae, or nightshade family (potatoes, tomatoes, and peppers, among others), the Cucurbitaceae, or gourd family (also including pumpkins and melons), the Brassicaceae, or mustard plant family (including rapeseed and cabbage), and the Apiaceae, or parsley family. Many of our fruits come from the Rutaceae, or rue family, and the Rosaceae (rose family, including apples, pears, cherries, apricots, plums, etc). In some parts of the world, certain single species assume paramount importance because of their variety of uses. An example is the coconut (Cocos nucifera) on Pacific atolls. Another example is the olive (Olea europaea) in the Mediterranean. Flowering plants also provide economic resources in the form of wood, paper, fiber (cotton, flax, and hemp, among others), medicines (digitalis, camphor), decorative and landscaping plants, and many, many other uses. See also | |||||||||||||||||||||||||
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