From Wikipedia, the free encyclopedia
The
flowering plants (
angiosperms), also known as
Angiospermae or
Magnoliophyta, are the most diverse group of
land plants. Together with
gymnosperms, they are the only extant groups of
seed-producing plants, but they can be distinguished from the gymnosperms by a series of
synapomorphies (derived
characteristics). These characteristics include
flowers,
endosperm within the seeds, and the production of
fruits that contain the seeds.
The ancestors of flowering plants diverged from
gymnosperms around 245–202 million years ago, and the first flowering plants known to exist are from 140 million years ago. They diversified enormously during the
Lower Cretaceous and became widespread around 100 million years ago, but replaced
conifers as the dominant trees only around 60-100 million years ago.
[edit] Angiosperm derived characteristics
The flowers, which are the
reproductive organs of flowering plants, are the most remarkable feature distinguishing them from other seed plants. Flowers aid angiosperms by enabling a wider range of adaptability and broadening the
ecological niches open to them. This has allowed flowering plants to largely 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 become modified through time to prevent
self-fertilization, which has permitted further diversification, allowing angiosperms eventually to fill more niches.
- Reduced male parts, three cells
The male
gametophyte in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants. The smaller pollen decreases the time from pollination — the pollen grain reaching the female plant — to
fertilization of the ovary; in gymnosperms fertilization can occur up to a year after pollination, while in angiosperms the fertilization begins very soon after pollination. The shorter time leads to angiosperm plants setting seeds sooner and faster than gymnosperms, which is a distinct evolutionary advantage.
- 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. This helps to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized, the carpel and some surrounding tissues develop into a fruit. This fruit often serves as an attractant to seed-dispersing animals. The resulting cooperative relationship presents another advantage to angiosperms in the process of
dispersal.
- Reduced female gametophyte, seven cells with eight nuclei
The reduced female gametophyte, like the reduced male gametophyte, may be an adaptation 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 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.
[edit] Evolution
Land plants have existed for about 425 million years.
[3] Early land plants
reproduced sexually with flagellated, swimming sperm, like the green algae from which they evolved. An adaptation to terrestrialization was the development of upright meiosporangia for dispersal by
spores to new habitats. This feature is lacking in the descendants of their nearest algal relatives, the
Charophycean green algae. A later terrestrial adaptation took place with retention of the delicate, avascular sexual stage, the gametophyte, within the tissues of the vascular sporophyte. This occurred by spore germination within sporangia rather than spore release, as in non-seed plants. A current example of how this might have happened can be seen in the precocious spore germination in
Sellaginella, the spike-moss. The result for the ancestors of angiosperms was enclosing them in a case, the
seed. The first seed bearing plants, like the
ginkgo, and
conifers (such as
pines and
firs), did not produce flowers. Interestingly, the pollen grains (males) of
Ginkgo and cycads produce a pair of flagellated, mobile sperm cells that "swim" down the developing pollen tube to the female and her eggs.
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.
[4] However, the fossil record has grown since the time of Darwin, and 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). A close relationship between angiosperms and
gnetophytes, proposed on the basis of
morphological evidence, has more recently been disputed on the basis of
molecular evidence that suggest gnetophytes are instead more closely related to other
gymnosperms.
The earliest known
macrofossil confidently identified as an angiosperm,
Archaefructus liaoningensis, is dated to about 125 million years BP (the
Cretaceous period),
[5] while pollen considered to be of angiosperm origin takes the
fossil record back to about 130 million years BP. However, one study has suggested that the early-middle
Jurassic plant
Schmeissneria, traditionally considered a type of
ginkgo, may be the earliest known angiosperm, or at least a close relative.
[6] Additionally, circumstantial chemical evidence has been found for the existence of angiosperms as early as 250 million years ago.
Oleanane, a
secondary metabolite produced by many flowering plants, has been found in
Permian deposits of that age together with fossils of
gigantopterids.
[7][8] Gigantopterids are a group of extinct seed plants that share many morphological traits with flowering plants, although they are not known to have been flowering plants themselves.
Recent
DNA analysis based on
molecular systematics [9][10] showed that
Amborella trichopoda, found on the Pacific island of
New Caledonia, belongs to a
sister group of the other flowering plants, and morphological studies
[11] suggest that it has features that may have been characteristic of the earliest flowering plants.
The great angiosperm
radiation, when a great diversity of angiosperms appears in the fossil record, occurred in the mid-
Cretaceous (approximately 100 million years ago). However, a study in 2007 estimated that the division of the five most recent (the genus
Ceratophyllum, the family
Chloranthaceae, the
eudicots, the
magnoliids, and the
monocots) of the eight main groups occurred around 140 million years ago.
[12] By the late Cretaceous, angiosperms appear to have dominated environments formerly occupied by
ferns and
cycadophytes, but large canopy-forming trees replaced
conifers as the dominant trees only close to the end of the
Cretaceous 65 millions years ago or even later, at the beginning of the
Tertiary.
[13] The radiation of herbaceous angiosperm occurred much later.
[14] Yet, many fossil plants recognizable as belonging to modern families (including
beech,
oak,
maple, and
magnolia) appeared already at late
Cretaceous.
It is generally assumed that the
function of flowers, from the start, was to involve mobile
animals in their
reproduction processes. That is, pollen can be scattered even if the flower is not brightly
colored or oddly shaped in a way that attracts animals; however, by expending the energy required to create such traits, angiosperms can enlist the aid of animals and thus reproduce more efficiently.
Island genetics provides one proposed explanation for the sudden, fully developed appearance of flowering plants. Island genetics is believed to be a common source of
speciation in general, especially when it comes to radical adaptations that seem to have required inferior transitional forms. Flowering plants may have evolved in an isolated setting like an
island or island chain, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a
wasp, for example). Such a relationship, with a hypothetical wasp carrying pollen from one plant to another much the way
fig wasps do today, could result in both the plant(s) and their partners developing a high degree of
specialization. Note that the wasp example is not incidental;
bees, which apparently evolved specifically due to mutualistic plant relationships, are descended from wasps.
Animals are also involved in the distribution of seeds.
Fruit, which is formed by the enlargement of flower parts, is frequently a seed-dispersal tool that attracts animals to eat or otherwise disturb it, incidentally scattering the seeds it contains (see
frugivory). While many such
mutualistic relationships remain too fragile to survive
competition and spread widely, flowering proved to be an unusually effective means of reproduction, spreading (whatever its origin) to become the dominant form of land plant life.
Flower
ontogeny uses a combination of
genes normally responsible for forming new shoots.
[15] 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.
[16]
[edit] Classification
| |
| The phylogeny of the flowering plants, as of APG III (2009). |
There are eight groups of living angiosperms:
- Amborella — a single species of shrub from New Caledonia
- Nymphaeales — about 80 species[17] — water lilies and Hydatellaceae
- Austrobaileyales — about 100 species[17] of woody plants from various parts of the world
- Chloranthales — several dozen species of aromatic plants with toothed leaves
- Magnoliidae — about 9,000 species,[17] characterized by trimerous flowers, pollen with one pore, and usually branching-veined leaves — for example magnolias, bay laurel, and black pepper
- Monocotyledonae — about 70,000 species,[17] characterized by trimerous flowers, a single cotyledon, pollen with one pore, and usually parallel-veined leaves — for example grasses, orchids, and palms
- Ceratophyllum — about 6 species[17] of aquatic plants, perhaps most familiar as aquarium plants
- Eudicotyledonae — about 175,000 species,[17] characterized by 4- or 5- merous flowers, pollen with three pores, and usually branching-veined leaves — for example sunflowers, petunia, buttercup, apples and oaks
The exact relationship between these eight groups is not yet clear, although it has been determined that the first three groups to diverge from the ancestral angiosperm were
Amborellales,
Nymphaeales, and
Austrobaileyales.
[18] The term
basal angiosperms refers to these three groups. The five other groups form the clade Mesangiospermae, with the Chloranthales and Magnoliidae forming the basal mesangiosperms.
Ceratophyllum seems to group with the
eudicots rather than with the
monocots.
[edit] History of classification
From 1736, an illustration of Linnaean classification
The botanical term "Angiosperm", from the
Ancient Greek αγγείον,
angeíon (receptacle, vessel) and σπέρμα, (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. This included flowering plants possessing seeds enclosed in capsules, distinguished from 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 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.
Auxanometer: Device for measuring increase or rate of growth in 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 (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,
[19] 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.
The internal classification of this group has undergone considerable revision. 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 accurately reflect
phylogeny. A consensus about how the flowering plants should be arranged has recently begun to emerge through the work of the
Angiosperm Phylogeny Group (APG), which published an influential reclassification of the angiosperms in 1998. An update incorporating more recent research was published as APG II
[19] in 2003.
Monocot (left) and dicot seedlings
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 Article 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, show that the
monocots form a
monophyletic group (
clade) but that the dicots do not (they are
paraphyletic). Nevertheless, the majority of dicot species do form a monophyletic group, called the
eudicots or
tricolpates. Of the remaining dicot species, most belong to a third major clade known as the
Magnoliidae, containing about 9,000 species. The rest include a paraphyletic grouping of primitive species known collectively as the
basal angiosperms, plus the families
Ceratophyllaceae and
Chloranthaceae.
[edit] Flowering plant diversity
Various flower colors and shapes
The number of
species of flowering plants is estimated to be in the range of 250,000 to 400,000.
[20] [21] [22] The number of
families in
APG (1998) was 462. In
APG II[19] (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. In
APG III (2009) there are 415 families.
The diversity of flowering plants is not evenly distributed. Nearly all species belong to the eudicot (75%), monocot (23%) and magnoliid (2%) clades. The remaining 5 clades contain a little over 250 species in total, i.e., less than 0.1% of flowering plant diversity, divided among 9 families.
The most diverse families of flowering plants, in their APG circumscriptions, in order of number of species, are:
- Asteraceae or Compositae (daisy family): 23,600 species[23]
- Orchidaceae (orchid family): 22,075 species[23]
- Fabaceae or Leguminosae (pea family): 19,400[23]
- Rubiaceae (madder family): 13,150[24]
- Poaceae or Gramineae (grass family): 10,035[23]
- Lamiaceae or Labiatae (mint family): 7,173[23]
- Euphorbiaceae (spurge family): 5,735[23]
- Melastomataceae (melastome family): 5,005[23]
- Myrtaceae (myrtle family): 4,620[23]
- Apocynaceae (dogbane family): 4,555[23]
In the list above (showing only the 10 largest families), the Orchidaceae and Poaceae are monocot families; the others are eudicot families.
[edit] Vascular anatomy
The amount and
complexity of tissue-formation in flowering plants exceeds that of gymnosperms. The
vascular bundles of the stem are arranged such that the
xylem and
phloem form concentric rings.
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 the development of xylem on the inside and phloem on the outside. The soft phloem becomes crushed, but the hard wood 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, called
annual rings.
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.
