Angiosperms

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.

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.