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

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.

see also Dicots; Evolution of Plants; Flowers; Gymnosperms; Monocots; Phylogeny; Systematics, Plant; Taxonomy.

Doug Soltis

Pam Soltis

Bibliography

Cronquist, A. An Integrated System of Flowering Plants. New York: Columbia University Press, 1981.

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.

see also Conifers; Fruits; Gymnosperms; Monocots; Nitrogen Fixation; Pollination and Fertilization; Roots; Symbiosis

Molly Nepokroeffand Elizabeth A. Zimmer

Bibliography

Crane, P. R., E. M. Friis, and K. R. Pederson. "The Origin and Early Diversification of Angiosperms." Nature 374 (1995): 27–33.

Judd, Walter S., Christopher S. Campbell, Elizabeth A. Kellogg, and Peter F. Stevens. Plant Systematics: A Phylogenetic Approach. Sunderland, MA: Sinauer Associates, Inc., 1999.

Kenrick, Paul. "The Family Tree Flowers." Nature 402 (1999): 358–359.

Parkinson, C. L., K. L. Adams, and J. D. Palmer. "Multigene Analyses Identify the Three Earliest Lineages of Flowering Plants." Current Biology 9 (1999): 1485–1488.

Angiosperm

Angiosperm is the name given to plants that produce flowers during sexual reproduction. The term literally means “vessel seed” and refers to the fact that seeds are contained in a highly specialized organ called an ovary.

Flowering plants are the most recently evolved of the major groups of plants, arising only about 130 million years ago. Despite their geological youthfulness, angiosperms are the dominant plants of the world today: about 80% of all living plant species are flowering plants. Furthermore, they occupy a greater variety of habitats than any other group of plants. The ancestors of flowering plants are the gymnosperms (e.g., pine and fir), which are the other major group of plants that produce seeds. The gymnosperms, however, produce their seeds on the surface of leaf-like structures, which makes the seeds vulnerable by drying out and to mechanical damage when winds whip the branches back and forth. Most importantly, conifer seeds are vulnerable to insects and other animals, which view seeds as nutritious, energy-packed treats. In angiosperms, the margins of the seed-bearing leaves have become inrolled and fused, so the seeds are no longer exposed but are more safely tucked inside the newly evolved “vessel,” which is the ovary.

The other major advance of the angiosperms over the gymnosperms was the evolution of the flower, which is the structure responsible for sexual reproduction in these plants. Sexual reproduction brings genetic material from two individuals of differing ancestry together, so that the offspring will have a new genetic makeup. The gymnosperms dealt with their immobility by packaging their male component into tiny pollen grains, which can be released into the wind to be blown to the female component of another individual of the same species.

Although this method of pollination succeeds, it is wasteful and inefficient because most of the pollen grains land somewhere other than on a female, such as in people’s noses, where they may cause hay fever. Furthermore, pollen grains are rich in fixed energy and nutrients such as nitrogen, so they are costly to make (native North Americans used to make pancakes out of the pollen of cattail). By evolving bright colors, scents, and nectar, the flowers of angiosperms served to attract animals. By traveling from one flower to another, these animals would accidentally move pollen as well, enabling sexual reproduction to take place. Because flower-seeking animals such as bees, butterflies, and hummingbirds can learn to recognize different types of flowers, they can move pollen from flower to flower quite efficiently. Therefore, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms, and the resources they save can be put into other important functions, such as growth and greater seed production. Therefore, the flower and its ovary have provided angiosperms with tremendous advantages, and have enabled them to become rapidly dominant over their gymnosperm ancestors.

Les C. Cwynar

Angiosperm

Angiosperm is the name given to those plants that produce flowers during sexual reproduction . The term literally means "vessel seed" and refers to the fact that seeds are contained in a highly specialized organ called an ovary.

Flowering plants are the most recently evolved of the major groups of plants, arising only about 130 million years ago. Despite their geological youthfulness, angiosperms are the dominant plants of the world today: about 80% of all living plant species are flowering plants. Furthermore, they occupy a greater variety of habitats than any other group of plants. The ancestors of flowering plants are the gymnosperms (e.g., pine and fir), which are the other major group of plants that produce seeds. The gymnosperms, however, produce their seeds on the surface of leaf-like structures, which makes the seeds vulnerable to mechanical damage when winds whip the branches back and forth, and to drying out. Most importantly, conifer seeds are vulnerable to insects and other animals, which view seeds as nutritious, energy packed treats. In angiosperms, the margins of the seed-bearing leaves have become inrolled and fused, so the seeds are no longer exposed but are more safely tucked inside the newly evolved "vessel," which is the ovary.

The other major advance of the angiosperms over the gymnosperms was the evolution of the flower , which is the structure responsible for sexual reproduction in these plants. The function of sexual reproduction is to bring together genetic material from two individuals of differing ancestry, so that the offspring will have a new genetic makeup. The gymnosperms dealt with their immobility by packaging their male component into tiny pollen grains, which can be released into the wind to be blown to the female component of another individual of the same species. Although this method of pollination succeeds, it is wasteful and inefficient because most of the pollen grains land somewhere other than on a female, such as in one's nose, where they may cause hay fever. Furthermore, pollen grains are rich in fixed energy and nutrients such as nitrogen , so they are costly to make (native North Americans used to make pancakes out of the pollen of cattail). By evolving bright colors, scents, and nectar , the flowers of angiosperms served to attract animals. By traveling from one flower to another, these animals would accidentally move pollen as well, enabling sexual reproduction to take place. Because flower-seeking animals such as bees , butterflies , and hummingbirds can learn to recognize different types of flowers, they can move pollen from flower to flower quite efficiently. Therefore, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms, and the resources they save can be put into other important functions, such as growth and greater seed production. Therefore, the flower and its ovary have provided angiosperms with tremendous advantages, and have enabled them to become rapidly dominant over their gymnosperm ancestors.

Les C. Cwynar

angiosperm Plants with true flowers, as distinct from gymnosperm and other non-flowering plants. They include most trees, bushes, and non-woody herbs. There are two main groups: monocotyledons (which have one seed leaf) such as grasses and daffodils, and dicotyledons (which have two seed leaves) such as peas and oak.