Angiosperms

Angiosperms 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 of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics. Angiosperm derived characteristics • Flowers The flowers of flowering plants are the most remarkable feature distinguishing them from other seed plants. Flowers aided angiosperms by enabling a wider range of evolutionary relationship and broadening the ecological niches open to them, allowing flowering plants to eventually dominate terrestrial ecosystems. • Stamens with two pairs of pollen sacs Stamens are much lighter than the corresponding organs of gymnosperms and have contributed to the diversification of angiosperms through time with adaptations to specialized pollination syndromes, such as particular pollinators. Stamens have also been modified through time to prevent self-fertilization, again to increase diversity, allowing angiosperms to eventually fill more niches. •Reduced male parts, three cells The reduced male gametophyte in angiosperms may have evolved to decrease the amount of time from pollination, the pollen grain reaching the female plant, to the fertilization of the ovary. In gymnosperms fertilization can occur up to a year after pollination, while in flowering plants the fertilization process begins very soon after pollination, allowing angiosperms, ultimately, to set seeds sooner and faster than gymnosperms. •Closed carpel enclosing the ovules (carpel or carpels and accessory parts may become the fruit) 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. •Reduced female gametophyte, seven cells with eight nuclei The reduced female gametophyte, like the reduce male gametophyte may be evolutionary adaptations allowing for more rapid seed set, eventually leading to such flowering plant adaptations as annual herbaceous life cycles, allowing the flowering plants to fill even more niches. • Endosperm 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. Evolution Pink Hyacinth tree in flower, at sunset Pink Hyacinth tree in flower, at sunset While land plants have existed for about 425 million years, the first ones reproduced by a simple adaptation of their aquatic counterparts: spores. In the sea, plants -- and some animals -- can simply scatter out little living copies of themselves to float away and grow elsewhere. This is how early plants, such as the modern fern, are thought to have reproduced. But plants soon began protecting these copies to deal with drying out and other abuse which is even more likely on land than in the sea. The protection became the seed, but not, yet, flowers. Early seed-bearing plants include the ginkgo, conifers (like pines), and fir trees. The earliest fossil of an angiosperm, a flowering plant, Archaefructus liaoningensis, is dated about 125 million years old[1]. Pollen, considered directly linked to flower development has been found in the fossil record dating back to perhaps 130 million years ago. The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of evolution that it was called an "abominable mystery" by Charles Darwin. Recently discovered angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps. Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. Some older fossils, such as the upper Triassic Sanmiguelia, have been suggested. Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late Triassic (245-202 million years ago). The relationship of the earlier gigantopterids to flowering plants is still enigmatic. 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. Recent DNA analysis (molecular systematics) [2] [3] show that Amborella trichopoda, found on the Pacific island of New Caledonia, is the sister group to the rest of the flowering plants, and morphological studies [4] suggest that it has features which may have been characteristic of the earliest flowering plants. 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. Various flower colors and shapes Various flower colors and shapes A Syrphid fly on a Grape hyacinth A Syrphid fly on a Grape hyacinth The general assumption is that the function of flowers, from the start, was to involve other animals in the reproduction process. Pollen can be scattered without bright colors and obvious shapes, which would therefore be a liability, using the plant's resources, unless they provide some other benefit. One proposed reason for the sudden, fully developed appearance of flowers is that they evolved in an isolated setting like an island, or chain of islands, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a wasp, for example), the way many island species develop today. This symbiotic relationship, with a hypothetical wasp bearing pollen from one plant to another much the way fig wasps do today, could have eventually resulted in both the plant(s) and their partners developing a high degree of specialization. Island genetics is believed to be a common source of speciation, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Note that the wasp example is not incidental; bees, apparently evolved specifically for symbiotic plant relationships, are descended from wasps. Likewise, most fruit used in plant reproduction comes from the enlargement of parts of the flower. This fruit is frequently a tool which depends upon animals wishing to eat it, and thus scattering the seeds it contains. While many such symbiotic relationships remain too fragile to survive competition with mainland animals and spread, flowers proved to be an unusually effective means of production, spreading (whatever their actual origin) to become the dominant form of land plant life. While there is only hard proof of such flowers existing about 130 million years ago, there is some circumstantial evidence that they did exist up to 250 million years ago. A chemical used by plants to defend their flowers, oleanane, has been detected in fossil plants that old, including gigantopterids[5], which evolved at that time and bear many of the traits of modern, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail; one of the earliest examples of petrification. The similarity in leaf and stem structure can be very important, because flowers are genetically just an adaptation of normal leaf and stem components on plants, a combination of genes normally responsible for forming new shoots[6]. The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the ovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least "ovary inferior". Flower evolution continues to the present day; modern flowers have been so profoundly influenced by humans that some of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, perhaps already having symbiotic companion plant relationships with them, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection[7]. Classification A monocot (left), and dicot A monocot (left), and dicot The botanical term "Angiosperm", from the ancient Greek αγγειον (receptacle) and σπερμα (seed), was coined in the form Angiospermae by Paul Hermann in 1690, as the name of that one of his primary divisions of the plant kingdom, which included flowering plants possessing seeds enclosed in capsules, in contradistinction to his Gymnospermae, or flowering plants with achenial or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked. The term and its antonym were maintained by Carolus Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Its use with any approach to its modern scope only became possible after Robert Brown had established in 1827 the existence of truly naked ovules in the Cycadeae and Coniferae, entitling them to be correctly called Gymnosperms. From that time onwards, so long as these Gymnosperms were, as was usual, reckoned as dicotyledonous flowering plants, the term Angiosperm was used antithetically by botanical writers, but with varying limitation, as a group-name for other dicotyledonous plants. The advent in 1851 of Hofmeister's discovery of the changes proceeding in the embryo-sac of flowering plants, and his determination of the correct relationships of these with the Cryptogamia, fixed the position of Gymnosperms as a class distinct from Dicotyledons, and the term Angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than Gymnosperms, and as including therefore the classes of Dicotyledons and Monocotyledons. This is the sense in which the term is nowadays received and in which it is used here. In most taxonomic treatments the flowering plants are treated as a coherent group. Usually this takes the form of a taxonomic grouping, or taxon, which will be assigned a rank. For taxa at a rank above the rank of family Art 16 of the ICBN allows either a descriptive name or a name formed from the name of an included family (that in turn is based on a generic name). The most popular descriptive name has been Angiospermae (Angiosperms), with Anthophyta ("flowering plants") a second choice. These names are not linked to any rank. The Wettstein system and the Engler system use the name Angiospermae, at the assigned rank of subdivision. A name formed from an included family depends on the rank chosen, with different endings for different ranks. The Reveal system treated it as subdivision Magnoliophytina (Frohne & U. Jensen ex Reveal, Phytologia 79: 70 1996), but later split it to Magnoliopsida, Liliopsida and Rosopsida. The Takhtajan system and Cronquist system treat this group at the rank of division, leading to the name Magnoliophyta (from the family name Magnoliaceae). The Dahlgren system and Thorne system (1992) treat this group at the rank of class, leading to the name Magnoliopsida. However, the APG system, of 1998, and the APG II system, of 2003, do not treat it as a formal taxon but rather treat it as a clade without a formal botanical name and use the name angiosperms for this clade. 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 observation that the dicots most often have two cotyledons, or embryonic leaves, within each seed. The monocots usually have only one, but the rule is not absolute either way. 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 a type of pollen found widely within this group. The name eudicots is formed by preceding dicot by the botanical prefix eu- (from Greek, where "eu-" means either "true" or "good"), 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 (Greek prefix "palaeo-" means "old"). As this remnant group is not monophyletic 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: 1. Orchidaceae (orchid family): 25,000 or more species 2. Asteraceae or Compositae (daisy family): 20,000 species 3. Fabaceae or Leguminosae (pea family): 17,000 4. Rubiaceae (madder family): 13,183 5. Poaceae or Gramineae (grass family): 9,000 6. Euphorbiaceae (spurge family): 5,000 7. Malvaceae (mallow family): 4,300 8. Cyperaceae (sedge family): 4,000 9. Araceae (aroid family): 3700 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. The flower, fruit, and seed Flowers Main articles: Flower and Plant sexuality 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 which will divide to become pollen grains, are the "male" cells and are borne in the stamens (or microsporophylls), and the "female" cells called megaspores, which will divide to become the egg-cell (in a process called megagametogenesis), 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 Main articles: Fertilization and Plant 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 Main articles: Seed and Fruit 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, pearl 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. Angiosperms The angiosperms, or flowering plants, are the largest and most species-rich phylum of plants, with more than 250,000 species estimated. Defining Characteristics The term "angiosperm" derives from two Greek words: angeion, meaning "vessel," and sperma, meaning "seed." The angiosperms are those plants whose seeds develop within a surrounding layer of plant tissue, called the carpel, with seeds attached around the margins. This arrangement is easily seen by slicing into a tomato, for example. Collectively, carpels together with the style and stigma are termed the ovary, and these plus associated structures develop into the mature fruit. The enclosed seeds and the presence of carpels distinguish angiosperms from their closest living relatives, the gymnosperms , in which the seed is not enclosed within a fruit, but rather sits exposed to the environment. Some defining characteristics of angiosperms include flowers, carpels, and the presence of endosperm, a nutritive substance found in seeds, produced via a second fertilization event. However, some current studies suggest that endosperm is not unique to angiosperms. Angiosperm flowers are generically characterized by having four whorls, or sets of organs: sepals , petals, stamens, and carpels. The carpels may be united or fused to form a compound pistil , and the number of stigma lobes may then be indicative of the number of carpels. The pistil also includes the stigma, on which pollen lands, and style, the tube leading to the egg. Stamens are separated into anthers, which produce pollen, and filaments. The mature ovary (part of the pistil containing the seeds) is termed a "fruit." Sepals and petals may be showy and colorful to attract pollinators, or may be quite reduced in wind-pollinated plants, such as grasses. Likewise, fruits may assume a wide variety of forms associated with mode of dispersal, such as fleshy fruits (for example, berries) dispersed by animals, and dry, winged fruits adapted for wind dispersal, such as the samaras of maple trees, which twirl like helicopters as they fall. Evolution and the Angiosperms The angiosperms are a relatively recent group of land plants, and are thought to have originated in the early Cretaceous, only 130 million years ago. The angiosperms increased dramatically in abundance during the Cretaceous. This sudden, dramatic appearance of large numbers of very diverse flowering plant species in the fossil record was referred to by English naturalist Charles Darwin as an "abominable mystery." It is postulated that coevolution with animal pollinators, especially insects, may have contributed to the explosion and abundance of angiosperm species which characterize the modern earth's flora. However, even today, it is not clear what group of nonflowering plants the angiosperms are most closely related to, or what the relationships of the early lineages of flowering plants are to one another. This is in part due to the extremely fast evolution of this group of plants, over a relatively short period of time, and the extinction of many closely related lineages of seed plants, some of which may be more closely related to the modern angiosperms than extant seed plant lineages. Most contemporary studies, which are based on phylogenetic analysis of deoxyribonucleic acid (DNA) sequence data from as many as six different genes, suggest that the closest relatives of the angiosperms are the gymnosperms, which include cycads, Ginkgo, conifers (the group that contains the pines, spruces, firs, and relatives), and Gnetales (a group containing three ancient genera: Ephedra, the Mormon tea; Welwitschia, a bizarre plant of southwest African deserts; and Gnetum, a genus of mostly tropical vines). The origins of angiosperms are not well understood and remain problematic, in part because many seed plant lineages have already gone extinct. However, studies indicate that the earliest lineage of flowering plants, or basal angiosperms, may include the family Amborellaceae (with the single living species Amborella trichopoda, a shrub from the South Pacific island of New Caledonia). Other early diverging lineages of angiosperms include Nympheales, the water lilies; Illiciales, or star anise; a group called the magnoliids, which includes magnolias, laurels, and black pepper; and the very large group called the monocots . A final lineage, the eudicots , contains all other flowering plants and comprises the bulk (approximately three-quarters) of the flowering plant species. Monocots, Dicots, and Eudicots The angiosperms have historically been divided into two groups: the monocotyledons (monocots) and the dicotyledons (dicots). These terms derive from the number of seed leaves, or cotyledons , the plants have upon germination. Dicots have recently been shown not to be an evolutionarily natural group. The monocots do form an evolutionarily natural, or monophyletic , group, and include familiar plants such as lilies, grasses, and palm trees. The monocots are characterized by having a single cotyledon, an adventitious root system, stems with scattered vascular bundles, absence of woody growth, leaves with parallel venation, flower parts usually in sets of threes, and monoaperturate pollen (that is, pollen with one large, groovelike aperture). The dicots have historically included all those plants with two cotyledons, tap root systems, stems with vascular bundles in a ring, leaf venation forming a netlike pattern, and flower parts in fours or fives. Current studies indicate that the dicots do not form an evolutionarily monophyletic group, but instead include several different lineages, some of which are more closely related to the monocots. Two groups that are well supported in contemporary studies are the eudicots ("true dicots"), characterized by having triaperturate pollen (that is, pollen with three long, groovelike apertures), and the noneudicots, which are characterized by having inaperturate pollen; that is, pollen lacking apertures. Noneudicot, basal angiosperms include the monocots, the laurels and avocados, the magnolias, black pepper, Amborella, water lilies and Illiciaceae (the star anise family). Evolutionary relationships among these noneudicot groups are not well understood. The eudicots include many familiar plants, including most trees, and include two major groups of flowering plants, the asterids (including the composite family, and the economically important Solanaceae, the potato family) and the rosids (including the rose family and the economically important legume family). Diversity and Symbioses Some of the most species-rich families of flowering plants include the monocot species of Orchidaceae, the orchids (19,500 species), the Poaceae or grass family (8,700), the Cyperaceae or sedge family (4,500), and the eudicot families of Euphorbiaceae or spurge family (6,900), the Fabaceae or legume family (18,000), the Rosaceae or rose family (3,000), Brassicaceae or mustard family (4,130), Rubiaceae or coffee family (9,000), the Lamiaceae or mint family (6,970), the Apiaceae or carrot family (4,250), and the Asteraceae or composite family (23,000). The angiosperms are of great ecological importance and are principal components of nearly all of the major land habitats. Correspondingly, flowering plants are quite diverse in morphology , growth form, and habitat, and range from the minute aquatic plants in the duckweed family (genus Lemna ) to the massive forest trees, such as oak and maple. Angiosperm flowers can be quite reduced, as in the grasses, where the most visible floral parts are the stamens and stigmas, to quite elaborate floral structures exhibiting fusion of parts and development of complex shapes, such as those evolved to attract insect pollinators in the orchids, mints, and snapdragons. An important aspect of angiosperm evolution is their well-documented relationships with other organisms such as animal pollinators, mycorrhizal (fungal) root associations, and even bacteria. Indeed, one of the most successful families of flowering plants, in terms of number of species, are the orchids, which have very specialized relationships with both pollinators and mycorrhizal interactions. Another highly successful family, the legume family, has evolved symbiotic relationships with nitrogen-fixing bacterial symbionts . Some flowering plants, such as the acacias of the legume family, obtain protection from herbivores via symbiotic relationships with ants. Through agriculture, humans have developed their own complex relationship with angiosperms. It is these relationships with other organisms that is the hallmark of angiosperms, and as such have contributed to the success of the flowering plants in the modern earth's flora. Angiosperms The angiosperms, or flowering plants (division Anthophyta or Magnoliophyta), comprise more than 230,000 species and are thus by far the largest division of plants; they represent the dominant group of land plants today. In both vegetative and floral morphology the angiosperms are highly diverse. In size, for example, they range from the duckweeds (the genus Lemna ), which are roughly one millimeter in length, to Eucalyptus trees, which are well over one hundred meters. Although all are characterized by the possession of flowers, these structures are also highly diverse in form and size. The smallest flowers are less than a millimeter in size (the flowers of duckweeds) while the largest flowers are approximately one meter in diameter (the flowers of Rafflesia ). Features unique, or nearly so, to angiosperms include the flower; the presence of seeds within a closed structure (actually a modified leaf) referred to as the carpel ; the reduction of the female gametophyte to eight nuclei and seven cells; double fertilization (the MAJOR ANGIOSPERM GROUPS Major Clades and Representative Families Common Name Number of Species in Family (approximate) Eurosid Rosaceae Rose family 3,500 Fabaceae Pea or legume family 17,000 Brassicaceae Mustard family 3,000 Fagaceae Beech or oak family 1,000 Cucurbitaceae Pumpkin or gourd family 700 Euphorbiaceae Spurge family 5,000 Juglandaceae Walnut or hickory family 50 Begoniacae Begonia family 1,000 Geraniaceae Geranium family 750 Malvaceae Cotton family 1,000 Euasterid Cornaceae Dogwood family 100 Ericaceae Heath family 3,000 Lamiaceae Mint family 3,000 Solanaceae Tomato or potato family 2,500 Asteraceae Sunflower family 25,000 Apiaceae Parsley family 3,000 Hydrangeaceae Hydrangea family 170 Caryophyllales Cactaceae Cactus family 2,000 Caryophyllaceae Carnation or pink family 2,000 Aizoaceae Mesembryanthemum family 2,300 Portulacaceae Portulaca family 500 Polygonaceae Buckwheat or rhubarb family 750 Magnoliids* Magnoliaceae Magnolia family 200 Lauraceae Avocado or cinnamon family 2,500 Piperaceae Pepper family 3,000 Myristicaceae Nutmeg family 380 Annonaceae Sweetsop family 2,000 Monocots* Orchidaceae Orchid family 18,000 Poaceae Grass family 9,000 Arecaceae Palm family 2,800 Araceae Arum family 2,000 * Indicates major clades that are noneudicots; other major clades are eudicots. fusion of egg and sperm resulting in a zygote and the simultaneous fusion of the second sperm with the two polar nuclei, resulting in a triploid nucleus) and subsequent endosperm formation; a male or microgametophyte composed of three nuclei; stamens with two pairs of pollen sacs; and sieve tube elements and companion cells in the phloem. Nearly all angiosperms also possess vessel elements in the xylem, but vessel elements also occur in Gnetales and some ferns. Origins of Angiosperms Because of the sudden appearance of a diverse array of early angiosperms in the fossil record, Charles Darwin referred to the origin of the flowering plants as "an abominable mystery." Although there are reports of earlier angiosperm remains, the oldest fossils that are indisputably angiosperms are from the early Cretaceous period, about 130 million years ago. Based on fossil evidence, it is clear that angiosperms radiated rapidly after their origin, with great diversity already apparent 115 million years ago. By 90 to 100 million years ago the angiosperms had already become the dominant floristic element on Earth. By 75 million years ago, many modern orders and families were present. The closest relatives and ancestor of the flowering plants have long been topics of great interest and debate. There was widespread belief during the last decades of the twentieth century that the Gnetales, a group of gymnosperms having three existing, highly divergent members (Gnetum, Ephedra, Welwitschia ), were the closest living relatives of the flowering plants among existing gymnosperms. These three genera resemble angiosperms in having special water-conducting vessels in the wood and reproductive structures organized into compound strobili similar in organization to compound flower clusters. In addition, some Gnetales (the genus Gnetum ) have angiosperm-like leaves. Gnetales also have a process that, in part, resembles double fertilization, a feature unique to angiosperms. In Gnetales, both sperm produced by the male gametophyte (in the pollen) fuse with nuclei in the female gametophyte. However, in Gnetales the second fusion produces an additional embryo and does not result in the triploid endosperm characteristic of flowering plants. Beginning in the mid-1980s, however, phylogenetic trees derived from gene sequence data have indicated instead a close relationship of Gnetales to conifers, with all of the living gymnosperms forming a clade that is the sister group to the angiosperms. The molecular evidence is compelling and indicates that Gnetales are probably not the closest living relatives of the flowering plants. Several fossil lineages have been suggested as close relatives of the angiosperms. These include Pentoxylon and Bennettitales, and these plants must be considered as possible candidates for the closest relatives of the flowering plants. Taxonomy Traditionally, angiosperms have been divided into two major groups or classes: dicotyledons (Magnoliopsida) and monocotyledons (Liliopsida). In recent classification schemes, each class was then divided into a number of subclasses. In this scheme, dicots were divided into six subclasses: Magnoliidae, Hamamelidae, Caryophyllidae, Rosidae, Dilleniidae, and Asteridae. The monocots were similarly divided into subclasses: Alismatidae, Arecidae, Commelinidae, Zingiberidae, and Liliidae. Although the division of angiosperms into monocots and dicots, with subsequent division into subclasses, has long been followed in classifications and textbooks, phylogenetic studies have dramatically revised views of angiosperm relationships. In fact, trees derived from deoxyribonucleic acid (DNA) sequence data have stimulated the most dramatic changes in views of angiosperm relationships during the past 150 years. As reviewed next, DNA data indicate that many of these groups do not hold together (that is, they do not form distinct clades—they are not monophyletic); hence they should not be recognized. Until recently, the radiation of the angiosperms was thought to have occurred so rapidly that many scientists believed that it might not be possible to identify the earliest angiosperms (this is also known as Darwin's "abominable mystery"). However, a series of recent molecular systematic (DNA) studies using different genes and molecular approaches all identify the very same first branches of the angiosperm tree of life. The evidence from these studies indicates that the angiosperms, formerly grouped as dicots and monocots, are best classified as either eudicots (true dicots) or noneudicots. The noneudicots are further divided into the monocots and the basal angiosperms. This scheme reflects what is now known about angiosperm evolution: The basal angiosperms are those plants thought to have evolved first and are ancestral to both monocots and eudicots. This group is represented by the Magnoliaceae (Magnolia family), Lauraceae (Laurel family), Nymphaeaceae (water lily family), Amborella (a shrub endemic to New Caledonia), and a group of shrubs that include Illicium, Schisandra, Trimenia, and Austrobaileya. Many of these early diverging angiosperms possess pollen with a single groove, or aperture (line of weakness). The monocots, which also have pollen with a single aperture, are believed to have arisen as one line of this earliest group of plants, probably more than 120 million years ago. Eudicots have pollen with three apertures. The details of their origins from basal angiosperms is less clear, but they are believed to have split off perhaps 127 million years ago. The term dicot, therefore, refers to plants that include both the eudicots and the basal angiosperms. Since the basal angiosperms are ancestral to the monocots as well, dicot cannot be meaningfully contrasted to monocot, and is thus not considered to be a taxonomically useful label. Whereas monocot remains a useful term, dicot does not represent a natural group of flowering plants and should be abandoned. That there is no monocot-versus-dicot split in the angiosperms is not a total surprise—botanists have long theorized that the monocots were derived from an ancient group of dicots during the early diversification of the angiosperms, and recent phylogenetic analyses simply confirm this hypothesis. The early branching angiosperms (or noneudicots) comprise not only the monocots, but many of those families (fewer than twenty-five) traditionally placed in the subclass Magnoliidae. Many of these families of early branching flowering plants possess oil cells that produce highly volatile oils referred to as ethereal oils. These ethereal oils are the basis of the characteristic fragrance of these plants; these compounds are responsible for the characteristic aroma of many spices, including sassafras, cinnamon, laurel or bay leaves, nutmeg, star anise, and black pepper. The noneudicots are also highly diverse in floral morphology. Familiar families of noneudicot or early diverging angiosperms include woody families such as the magnolia family (Magnoliaceae), the laurel or cinnamon family (Lauraceae), the nutmeg family (Myristicaceae), and the sweetsop or custard-apple family (Annonaceae). Members of these families often have relatively large flowers with numerous parts that may be spirally arranged. Other early branching angiosperms include plants often referred to as paleoherbs. As the name implies, paleo-herbs are predominantly herbaceous and have small flowers with very few flower parts. The paleoherbs include the black pepper family (Piperaceae) and wild-ginger family (Aristolochiaceae). Once the noneudicots are excluded, the remaining dicots form a well-supported clade referred to as the eudicots. This group contains, by far, the vast majority of angiosperm species; approximately 75 percent of all angiosperms are eudicots. Eudicots include most familiar angiosperm families. Recent phylogenetic trees demonstrate that the eudicots comprise a number of well-supported lineages that differ from traditional circumscriptions . The earliest branches of eudicots are members of the order Ranunculales, which include the Ranunculaceae (buttercup family), Papaveraceae (poppy family), Proteaceae (protea family), and Platanaceae (sycamore family). Following these early branching taxa , most remaining eudicots form a large clade (referred to as the core eudicots), comprised of three main branches and several smaller ones. The main branches of core eudicots are: eurosids, or true rosids (made up of members of the traditional subclasses Rosidae, Dilleniidae, and Hamamelidae) the euasterids, or true asterids (containing members of the traditional subclasses Asteridae, Dilleniidae, and Rosidae) the Caryophyllales (the traditional subclass Caryophyllidae, plus some Dilleniidae). Importantly, there is no clade that corresponds to the traditionally recognized subclasses Dilleniidae and Hamamelidae. As noted, these subclasses have members scattered throughout the eudciots—hence, they are not natural, or monophyletic groups. Because of the enormous insights that DNA-based studies have provided into relationships within the angiosperms, the use of long-recognized subclass names and group delineations, such as Magnoliidae, Rosidae, Asteridae, Dilleniidae, has been abandoned in recent classification schemes. Evolution and Adapations Based on the earliest branches of the angiosperm tree reconstructed from DNA sequence data, as well as fossil evidence, early angiosperms were likely woody shrubs with a moderate-sized flower possessing a moderate number of spirally arranged flower parts. There was no differentiation between sepals and petals (that is, tepals were present). The stamens did not exhibit well-differentiated anther and filament regions (these are often referred to as laminar stamens). The carpels , the structures that enclose the seeds, were folded and sealed by a sticky secretion rather than being fused shut, as is the typical condition in later-flowering plants. In contrast, later angiosperms (the eudicots, for example) have well-differentiated sepals and petals and flower parts in distinct whorls . Their stamens are well-differentiated into anther and filament regions and the carpels are fused during development. By eighty to ninety million years ago the angiosperms were dominant floristic elements. Obvious reasons for their success include the evolution of more efficient means of pollination (the flower for the attraction of pollinators) and seed dispersal (via the mature carpel, or fruit). One important innovation was the evolution of the bisexual flower; that is, the presence of both carpels and stamens in one flower. In contrast, gymnosperms have separate male and female reproductive structures or cones. Bisexual structures may have an advantage over unisexual structures in that the pollinator can both deliver and pick up pollen at each visit to a flower. Other possible reasons for the success of flowering plants involve morphological, chemical, and anatomical attributes. These include the presence in angiosperms of more efficient means of water and carbohydrate (sugar) conduction via vessel elements and sieve tube elements/companion cells, respectively. These anatomical features may be viewed as adaptations for drought resistance. Vessel elements are found in only a few groups other than flowering plants, and the presence of the sieve tube/companion cell pair is unique to the flowering plants. The evolution of the deciduous habit was also important in the success of the flowering plants. This feature permitted woody plants to lose their leaves and to become inactive physiologically during periods of drought and cold. Other important evolutionary innovations in angiosperms that also may have contributed to their success include a more efficient source of nutrition for the developing embryo through the production of triploid endosperm (in other seed plants the haploid female gametophyte tissue nourishes the young embryo) and the protection of ovules and developing seeds inside a novel, closed structure, the carpel. Compared to other plants, the angiosperms also possess enormous biochemical diversity, which includes a diverse array of chemicals that presumably act in defense against herbivores and pathogens . The first seed-producing plants (various lineages of gymnosperms) were wind-pollinated. The angiosperms, in contrast, as well as some gymnosperms (cycads and Gnetales), typically employ a more efficient system—insects feeding on pollen or nectar transfer pollen from one plant to the next. The more attractive the flower of the plant is to the insect, the more frequently the flowers will be visited and the more seed produced. The first angiosperms likely were pollinated by beetles that foraged on pollen and in so doing moved pollen from one flower to the next. Plants with flowers that provided special sources of food for pollinators, such as nectar, had a selective advantage. In this general way angiosperms and insects coevolved, or diversified. The rise and diversification of the diverse array of flower visitors we see today, such as bees, moths, and butterflies, occurred in concert with the increasing diversification of flowers. Both pollinators and angiosperms were influenced by the diversification of the other.