The flowering plants or angiosperms are the most speciose group of land plants. The flowering plants and the gymnosperms comprise the two extant groups of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies.
Flowers with closed carpel enclosing the ovules (carpel(s) and accessory parts may become the fruit); stamens with two pairs of pollen sacs; male gametophyte reduced to three cells; female gametophyte reduced to seven cells with eight nuclei; seed contains endosperm formed through double fertilisation; phloem tissue composed of sieve tubes and companion cells.
In the dicotyledons, the vascular 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 the development of xylem on the inside and phloem on the outside. The soft phloem becomes crushed, but the hard xylem persists and forms the 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.
Among the monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They contain no cambium and once formed the stem increases in diameter only in exceptional cases.
The flower, fruit, and seed
The characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, and provide the most trustworthy external characteristics for establishing relationships among angiosperm species. The function of the flower is to ensure 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 violets, a flower arises singly in the axil of an ordinary foliage-leaf. 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). The "female" cells called megaspores which will divide to become the egg-cell (megagametogenesis) are 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 other structures are present and serve to protect the sporophylls and to form an envelope attractive to pollinators. The individual members of these surrounding structures are known as 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 the bud. The inner series (corolla of petals) is generally white or brightly colored, and is more delicate in structure. It functions to attract animal pollinators. Attraction is effected by color, scent and nectar, which may be secreted in some part of the flower.
While the majority of flowers are perfect or hermaphrodite (having both male and female parts in the same flower structure), flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization. Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot easily transfer pollen to the pistil (receptive part of the carpel). Homomorphic flowers may employ a biochemical (physiological) mechanism called self-incompatibility to discriminate between self- and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers.
Fertilization and embryogenesis
Double fertilization refers to a process in 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, 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 seed. When seed develops without fertilization, the process is known as apomixis.
Fruit and seed
As the development of embryo and endosperm proceeds within the embryo-sac, the sac wall enlarges and combines with the nucellus (which is likewise enlarging) and the integument to form the seed-coat. The ovary wall develops to form the fruit or pericarp, whose form 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, e.g. the floral receptacle in the apple, strawberry and others.
The character of the seed-coat bears a definite relation to that of the fruit. They protect the embryo and aid in dissemination; they may also directly promote germination. Among plants with indehiscent fruits, the fruit generally provides protection for of the embryo and secures dissemination. In this case, the seed-coat is only slightly developed. If the fruit is dehiscent and the seed is exposed, the seed-coat is generally well developed, and must discharge the functions otherwise executed by the fruit.
The earliest fossil angiosperm, Archaefructus, comes from the Yixian formation in China and is dated to about 125 million years BP (Sun et al., 2002). Angiosperm pollen has been found in the fossil record perhaps as long ago as 130 million years.
The relationships of angiosperms to other plant taxa remain contentious (see Friedman & Floyd, 2001, for an overview). Morphological data indicated that the gymnosperms were paraphyletic with regard to the angiosperms, and that the Gnetales were the closest living relatives of the angiosperms. However, molecular data have indicated that modern gymnosperms form a monophyletic sister group to angiosperms, with the Gnetales more closely related to (possibly even within) the conifers. However, as the gymnosperm crown group dates back to the Carboniferous, the angiosperm stem must have diverged by that time if the molecular analyses are correct. A number of fossil seed plant groups of uncertain relationships are known from within that time frame, and it seems likely that at least some of these taxa lie on the angiosperm stem. Taylor et al. (2003) demonstrated that oleanane, a diagenetic product of organic compounds found in angiosperms but absent from living gymnosperms, was present in fossils of the Cretaceous Bennettitales and Permian Gigantopteridales, making these two groups likely angiosperm stem candidates. Other fossil taxa that have been suggested as angiosperm relatives include the Glossopteridales and Caytonia.
Within the angiosperms, recent phylogenetic analyses have mostly agreed that the so-called ANITA grade (including Amborella, Nymphaeales and Austrobaileyales), includes the basalmost living clades. The Austrobaileyales are most likely closer to the remaining angiosperms than are Amborella and Nymphaeales. Analyses disagree on whether Amborella alone or an Amborella + Nymphaeales clade represents the basalmost branch of angiosperms, but the former option is perhaps the more popular. Saarela et al. (2007) recently demonstrated that the Hydatellaceae also fall in this area as the sister group to Nymphaeales. The other major angiosperm clades (listed below) form a monophyletic group, but relationships between the clades are uncertain.
The great angiosperm radiation occurred in the mid-Cretaceous. 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 had appeared.
Flowers are derived from leaf and stem components, arising from a combination of genes normally responsible for forming new shoots. 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, and to be dominated by the ovary. 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".
The Angiospermae (flowering plants) are traditionally divided into two subgroups, the dicotyledons and monocotyledons (often shortened to dicots and monocots). Dicotyledons have two seed leaves, flower parts in multiples of four or five, vascular tissue surrounding the stem and reticulate leaf veins. Monocotyledons have a single seed leaf, flower parts in multiples of three, vascular tissue in bundles and parallel leaf veins. Recent phylogenetic analyses agree, however, that the dicotyledons are paraphyletic to the monocotyledons. There are eight well-established clades of living angiosperms:
- Amborella - a single species of shrub from New Caledonia
- Nymphaeales + Hydatellaceae
- Austrobaileyales - about 100 species of woody plants from various parts of the world
- Chloranthaceae - several dozen species of aromatic plants with toothed leaves
- Ceratophyllum - about 6 species of aquatic plants, perhaps most familiar as aquarium plants
- magnoliids - about 9,000 species, characterized by trimerous flowers, pollen with one pore, and usually branching-veined leaves, containg the Magnoliales, Laurales, Canellales, Piperales and Aristolochiaceae
- eudicots - about 175,000 species characterized by 4- or 5-merous flowers, pollen with three pores, and usually branching-veined leaves, containing the majority of the taxa previously included in the dicotyledons
- monocots - about 70,000 species characterized by trimerous flowers, a single cotyledon, pollen with one pore, and usually parallel-veined leaves.
History of classification
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 one of his primary divisions of the plant kingdom. This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or flowering plants with achenial or schizocarpic 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 1827, when Robert Brown established the existence of truly naked ovules in the Cycadeae and Coniferae, and applied to them the name "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, with varying scope, as a group-name for other dicotyledonous plants.
In 1851, Hofmeister discovered the changes occurring in the embryo-sac of flowering plants, and determined the correct relationships of these to the Cryptogamia. This 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, including the classes of dicotyledons and monocotyledons. This is the sense in which the term is used today.
In most taxonomies, the flowering plants are treated as a coherent group. 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. The Reveal system treated flowering plants 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. The Dahlgren system and Thorne system 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 angiosperms as a formal taxon but rather an informal clade.
Friedman, W. E., & S. K. Floyd. 2001. Perspective: The origin of flowering plants and their reproductive biology – a tale of two phylogenies. Evolution 55 (2): 217-231.
Saarela, J. M., H. S. Rai, J. A. Doyle, P. K. Endress, S. Mathews, A. D. Marchant, B. G. Briggs & S. W. Graham. 2007. Hydatellaceae identified as a new branch near the base of the angiosperm phylogenetic tree. Nature 446: 312-315.
Sun, G., Q. Ji, D. L. Dilcher, S. Zheng, K. C. Nixon & X. Wang. 2002. Archaefructaceae, a new basal angiosperm family. Science 296: 899-904.
Taylor, D. W., H. Li, J. Dahl, F. J. Fago, D. Zinniker & J. M. Moldowan. 2003. Biogeochemical evidence for the presence of the angiosperm molecular fossil oleanane in Paleozoic and Mesozoic non-angiospermous fossils. Paleobiology 32: 179-190.
Descriptive sections transferred from Wikipedia and edited CKT080513; phylogeny CKT070918.