The flower, fruit, and seed

Flowers
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.

[edit] Fertilization and embryogenesis
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.

[edit] Fruit and seed
Main articles: Seed and Fruit

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.

[edit] Economic importance

A mature wheat field in northern Israel.
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.

Flowering plant diversity

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:
Asteraceae or Compositae (daisy family): 23,600 species[11]
Orchidaceae (orchid family): 21,950 species[11]
Fabaceae or Leguminosae (pea family): 19,400[11]
Rubiaceae (madder family): 13,183[12]
Poaceae or Gramineae (grass family): 10,035[11]
Lamiaceae or Labiatae (mint family): 7,173[11]
Euphorbiaceae (spurge family): 5,735[11]
Cyperaceae (sedge family): 4,350[11]
Malvaceae (mallow family): 4,225[11]
Araceae (aroid family): 4,025[11]
In the list above (showing only the 10 largest families), the Orchidaceae, Poaceae, Cyperaceae and Araceae are monocot families; the others are dicot families.

Classification

There are eight groups of living angiosperms:
Amborella - a single species of shrub from New Caledonia
Nymphaeales - about 80 species - water lilies and Hydatellaceae
Austrobaileyales - about 100 species of woody plants from various parts of the world
Chloranthales - 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 - for example magnolias, bay laurel, and black pepper
eudicots - about 175,000 species, characterized by 4- or 5- merous flowers, pollen with three pores, and usually branching-veined leaves - for example sunflowers, petunia, buttercup, apples and oaks
monocots - about 70,000 species, characterized by trimerous flowers, a single cotyledon, pollen with one pore, and usually parallel-veined leaves - for example grasses, orchids, and palms
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

Evolution

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 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). The relationship of the earlier gigantopterids to flowering plants is still enigmatic.
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

Angiosperm derived characteristics

Flowers
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 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 to eventually 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 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.
Reduced female gametophyte, seven cells with eight nuclei
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
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.

Flowering plant

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.

Rosa virginiana

Rosa virginiana, commonly known as the Virginia Rose [2], Common Wild Rose or Prairie Rose, is a woody perennial in the rose family native to eastern North America. In the Canadian maritimes, it grows with at an almost invasive rhythm. It is seen in meadows, vacant lots, ditches, even in dumps.