The flowering
plants or angiosperms are
the most widespread 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,
or derived characteristics.
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 relationships and broadening the ecological
niches open to them, allowing flowering plants to eventually
dominate terrestrial ecosystems.
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 to eventually fill more niches.
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.
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.
The reduced female gametophyte, like the reduced male
gametophyte may be 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 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.
Land plants have existed for
about 425 million years. Early land plants reproduced
by spores
like their aquatic counterparts. Marine
organisms can easily scatter copies of themselves to float away and
grow elsewhere. Land plants soon found it advantageous to protect their copies
from drying out and other hazards by enclosing them in a case, the seed. Early seed bearing
plants, like the ginkgo,
and conifers
(such as pines
and firs),
did not produce flowers.
The earliest fossil of an angiosperm, or flowering plant, Archaefructus liaoningensis,
is dated to about 125 million years BP[1]. Pollen, considered directly linked to
flower development, has been found in the fossil record perhaps as long ago as
130 million years.
While there is only hard evidence of such flowers
existing about 130 million years ago, there is some circumstantial evidence
that they may have existed 250 million years ago. A chemical used by plants to
defend their flowers, oleanane, has been detected in fossil plants that old,
including gigantopterids[2],
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 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.[1] However the fossil record has grown since the time
of
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) [3]
[4] show that Amborella trichopoda, found on the Pacific
island of New Caledonia, belongs to a sister group
of the other flowering plants, and morphological studies [5]
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). 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.[6]
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.
However, some authors have proposed an earlier origin for
angiosperms, sometime in the Paleozoic (251 million years ago or more).[2][3][4]
Two Bees
on a Creeping Thistle Cirsium arvense
It is generally assumed that the function of flowers, from the start, was to
involve mobile animals
in their reproduction processes. Pollen can be scattered
without bright colors
and obvious shapes. Expending energy on these structures would appear to be a liability,
unless they provide significant benefit.
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 which 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 flower parts, is frequently a seed disbursal tool which depends
upon animals, who eat or otherwise disturb it, incidentally scattering the
seeds it contains (see frugivory). While many such mutualistic
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.
Flowers are derived from leaf and stem components, arising
from a combination of genes
normally responsible for forming new shoots.[7] 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.[8]
There are eight groups
of living angiosperms:
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, in that order.[6]
From 1736, an illustration of Linnaean classification.
Auxanometer: devise for measuring increase or rate
of growth in plants.
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. 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.
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[7], 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 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[7] in 2003.
A monocot (left), and dicot
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 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 monophyletic group does exist, called
the eudicots
or tricolpates,
and including most of the dicots. The name tricolpates derives from a type of pollen found
widely within this group. The name eudicots
is formed combining dicot with
the prefix eu- (from Greek, for
"well," or "good," botanically indicating
"true"), 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.
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. [8] [9] [10] The number of families in APG (1998) was 462. In APG II[7] (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 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:
In the list above (showing only the 10 largest families),
the Orchidaceae, Poaceae, Cyperaceae and Araceae are monocot families; the
others are dicot families.
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.
Main articles: Flower and Plant
sexuality
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 insect or bird pollinators. Attraction is effected by color, scent, and nectar, which
may be secreted in some part of the flower. The 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.
Main articles: Fertilization
and Plant 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 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.
The fruit of the Aesculus
or Horse Chestnut tree.
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.
A mature wheat field in northern
Agriculture is almost entirely dependent on
angiosperms, either directly or indirectly through livestock
feed. Of all the families 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).
The Fabaceae,
or legume family, comes 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, or 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, for example the coconut
(Cocos
nucifera) on Pacific atolls, and 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 other uses. The main area in which
they are surpassed by other plants is timber
production.
