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8.4: Angiosperm Families - Biology

8.4: Angiosperm Families - Biology


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Learning Objectives

  • Interpret a floral formula.
  • Describe the characteristics used to identify each of the four largest angiosperm families.

Knowing who is related to whom in the plant world can provide important information. Often, closely related organisms will have similar life history traits, such as defense compounds or other secondary metabolites. Identifying which family an unknown plant belongs to might provide insight into potential interactions with other organisms (see Orchidaceae below), its capability to resist particular environmental conditions or stressors, its edibility or toxicity, or whether it produces compounds that might be used medicinally.

For example, plants like poison oak (Toxicodendron diversilobum) and poison ivy (Toxicodendron radicans) are in the family Anacardiaceae and produce an oil called urushiol. This oil causes a (sometimes severe) allergic reaction in many people, resulting in an itchy, blistering rash. Mango (Mangifera indica) and cashew (Anacardium occidentale). also belong to this family, but these are not known for giving itchy rashes. However, as someone who is quite allergic to poison oak, I unknowingly ate a mango with the skin on and broke out in a rash all around my mouth and lower face, just like how I react to touching poison oak. Indeed, cashews are just the seed of the cashew fruit. The rest of the fruit and seed coat are removed due to the urushiol content.

Additionally, while there are over 350,000 species of flowering plants, there are only around 400 families. Being able to quickly narrow an unknown plant to family is an essential skill in plant identification. So how can we tell who belongs to the same family among plants? For flowering plants, we can use floral formulas.

Floral Formulas

Since there are so many terms about flowers, and at the same time, flower structure and diversity always were of immense importance in botany, two specific ways were developed to make flower description more compact. First is a flower formula. This is an approach where every part of flower is designated with a specific letter, numbers of parts with digits, and some other features (whorls, fusion, position) with other signs.

  • Symmetry: * means radial symmetry, while X means bilateral symmetry.
  • Whorls: K is the calyx, C is the corolla, A is the androecium, and G is the gynoecium. The number that follows each letter represents the number of parts in that whorl. For the gynoecium, a line under the number indicates an inferior ovary, while a line above the number indicates a superior ovary.
  • Fusion: In most representations, connation is indicated by circling the number, while adnation is indicated by drawing a line connecting the numbers of the fused whorls.

Here are a few examples of floral formulas, followed by their interpretation:

(ast K_{4}C_{4}A_{2+4}G_{underline{(2)}}): flower actinomorphic, with four sepals, four petals and six stamens in two whorls, ovary superior, with two fused carpels

(uparrow K_{(5)}[C_{(1,2,2)}A_{2,2}]G_{underline{(2 imes2)}}): flower zygomorphic, with five fused sepals, five unequal fused petals, two-paired stamens attached to petals, superior ovary with two subdivided carpels

(ast K_{(5)}C_{(5)}[A_{5}G_{underline{(3)}}]): actinomorphic flower with five fused sepals and five fused petals, five stamens attached to pistil, ovary inferior, with three fused carpels

The following signs are used to enrich formulas:

PLUS “+” is used to show different whorls; minus “(-)” shows variation; “(vee)” = “or

BRACKETS “[]” and “()” show fusion. In most representations, connation is indicated by circling the fused whorl, while adnation is indicated by drawing a line underneath the formula connecting those whorls.

COMMA “,” shows inequality of flower parts in one whorl

MULTIPLICATION “( imes)” shows splitting

INFINITY “(infty)” shows indefinite number of more than 12 parts

Flower diagram is a graphical way of flower description. This diagram is a kind of cross-section of the flower. Frequently, the structure of pistil is not shown on the diagram. Also, diagrams sometimes contain signs for the description of main stem (axis) and flower-related leaf (bract). The best way to show how to draw diagram is also graphical (Figure (PageIndex{1})); formula of the flower shown there is (ast K_{5}C_{5}A_{5}G_{underline{(5)}}).

Review of Terms and Formula Designations

FLOWER PARTS occur in whorls in the following order—sepals, petals, stamens, pistils.

(The only exceptions are flowers of Eupomatia with stamens then perianth, Lacandonia with pistils then stamens, and some monocots like Triglochin, where stamens in several whorls connect with tepals.)

PEDICEL flower stem

RECEPTACLE base of flower where other parts attach

HYPANTHIUM cup-shaped receptacle (Figure (PageIndex{2}))

PERIANTH = CALYX + COROLLA

SEPALS small and green, collectively called the CALYX, formula: K

PETALS often large and showy, collectively called the COROLLA, formula: C

TEPALS used when sepals and petals are not distinguishable, they form SIMPLE PERIANTH, formula: P

ANDROECIUM collective term for stamens: formula: A

STAMEN = FILAMENT + ANTHER

ANTHER structure containing pollen grains

FILAMENT structure connecting anther to receptacle

GYNOECIUM collective term for pistils/carpels, formula: G. Gynoecium can be composed of:

  1. A single CARPEL = simple PISTIL, this is MONOMERY
  2. Two or more fused CARPELS = compound PISTIL, this is SYNCARPY
  3. Two or more unfused CARPELS = two or more simple PISTILS, this is APOCARPY

(Note that variant #4, several compound pistils, does not exist in nature.)

To determine the number of CARPELS in a compound PISTIL, count LOCULES, points of placentation, number of STYLES, STIGMA and OVARY lobes.PISTIL Collective term for carpel(s). The terms CARPEL and PISTIL are equivalent when there is no fusion, if fusion occurs then you have 2 or more CARPELS united into one PISTIL.

CARPEL structure enclosing ovules, may correspond with locules or placentas

OVARY basal position of pistil where OVULES are located. The ovary develops into the fruit; OVULES develop into seeds after fertilization.

LOCULE chamber containg OVULES

PLACENTA place of attachment of OVULE(S) within ovary

STIGMA receptive surface for pollen

STYLE structure connecting ovary and stigma

FLOWER Floral unit with sterile, male and female zones

ACTINOMORPHIC FLOWER A flower having multiple planes of symmetry, also called radially symmetrical, formula: (ast)

ZYGOMORPHIC FLOWER A flower having only one plane of symmetry, also called bilaterally symmetrical, formula: (uparrow)

PERFECT FLOWER A flower having both sexes

MALE / FEMALE FLOWER A flower having one sex, formula: ♂ / ♀

MONOECIOUS PLANTS A plant with unisexual flowers with both sexes on the same plant

DIOECIOUS PLANTS A plant with unisexual flowers with one sex on each plant, in effect, male and female plants

SUPERIOR OVARY most of the flower is attached below the ovary, formula: (G_{underline{dots}})

INFERIOR OVARY most of the flower is attached on the top of ovary, formula: (G_{overline{dots}})

(Inferior ovary only corresponds with monomeric or syncarpous flowers.)

WHORL flower parts attached to one node

Major Families

This section will cover some of the larger families of angiosperms, including their floral formula and general characteristics. A selection of angiosperm families within the eudicots, including examples of notable members, can be found in Table (PageIndex{1}) at the end of this section.

Orchidaceae, the Orchid Family

Orchids are one of the most species-rich group of plants, containing over 28,000 species (Figure (PageIndex{1})). These plants tend to be tropical and epiphytic (growing on other plants). However, as can be assumed from their vast diversity, orchids can be found in many ecosystems and growing on a variety of substrates, including rocks! Their flowers are often highly modified, including long nectar spurs, hairy petals, and strange morphologies. Many orchids are fly pollinated. Follow this link to see observations of orchids from across the globe and in your region!

Orchids make a large number of tiny seeds, i.e. the tiny black specks in vanilla bean ice cream are the seeds of the orchid Vanilla plantifolia. An interesting, r-selected strategy with a twist: the seeds parasitize mycorrhizal fungi. Orchid seeds will not germinate if they do not have a fungal partner. As the fungal hyphae penetrate into the orchid cells, they formed coiled structures. The orchid feeds on the fungal hyphae until it has produced its first leaves and can photosynthesize on its own. At this point, the relationship can be shifted toward mutualism, with sugars transfered from plant to fungus. However, some orchids have lost the ability to make chlorophyll and instead, continue to feed from their fungal partner. These plants are referred to as mycoheterotrophs. The mycoheterotrophic orchid Rhizanthella gardneri undergoes its entire life cycle underground!

Asteraceae, the Aster Family or Composite Family

There are more than 32,000 accepted species in Asteraceae—a recent and dramatic increase in described species has sent them soaring past the orchids (for the time being). They have a cosmopolitan distribution, but are better represented in temperate and subtropical regions. This family is primarily herbaceous but does contain some woody species. Members of this family produce head inflorescences with one or two different types of florets that all attach to a common receptacle (Figure (PageIndex{4})). Disc florets have radial symmetry, while ligulate florets have bilateral symmetry (Figure (PageIndex{5})). Some inflorescences contain only disc florets (e.g. thistle), some only ligulate florets (e.g. dandelions), and others contain a combination of the two (e.g. daisy). The inflorescence is subtended by layers of bracts called phyllaries, forming an involucre.

The calyx is reduced to hairs or bristles (pappus, see Figure (PageIndex{6-7})), petals are fused into a tube or ligula (with 5 or 3 teeth). The pollen is lifted up and distributed by the outer sides of the stigmas, called secondary pollen presentation (Figure (PageIndex{6})). Florets have inferior ovaries. The fruit is an achene and the mature seed has almost no endosperm.

Plants iused for oils, vegetables, ornamentals and medicinal plants are distributed in multiple subfamilies, the most commercially important are:

  • Carduoideae: mostly tubular flowers

-Centaurea—knapweed

-Cynara—artichoke

-Carthamus–safflower

  • Cichorioideae: mostly 5-toothed ligulate (pseudo-ligulate) flowers + lacticifers with latex

-Taraxacum—dandelion

-Lactuca—lettuce

  • Asteroideae: tubular + 3-toothed ligulate flowers

-Helianthus—sunflower (BTW, “canola”, or Brassica napus from Cruciferae is the second main source of vegetable oil)

-Artemisia—sagebrush

-Tagetes—marigold and lots of other ornamentals

Fabaceae, the Legume Family

With around 19,000 species, Fabaceae (Figure (PageIndex{8})) is the third largest angiosperm family after orchids and asters. This family is widely distributed throughout the world, but does particularly well in the tropics. These plants form root nodules with nitrogen-fixing bacteria. Leaves are alternate, pinnately compound (once or twice), with stipules. Plants in this family have legumes as fruits (one locule dehiscent along a single suture).

There are three subfamilies (Caesalpinioideae, Mimosoideae, Papilionoideae) with distinct characteristics. In Papilionoideae the petals are mostly free, unequal and have special names: banner, keel and wing (Figure (PageIndex{9})). In Mimosoideae, they fuse and form a tube with radial symmetry. There are usually 10 stamens with 9 fused and one free; in Mimosoideae, stamens are numerous. There is a single carpel in the gynoecium, meaning the ovary will have a single locule.

Representatives of Leguminosae:

  • Mimosoideae: stamens numerous, petals connected

-Acacia—dominant tree of African and Australian savannas, often with phyllodes

-Mimosa—sensitive plant

  • Papilionoideae: stamens 9+1, petals mostly free; this subfamily contains many extremely important food plants with high protein value

-Glycine—soybean

-Arachis—peanut with self-buried fruits

-Phaseolus—bean

-Pisum—pea

Poaceae, the Grass Family

Grasses are monocots. There are approximately 12,000 species of grasses widely distributed throughout the world, but most genera concentrate in the tropics. Grasses tend to be wind dispersed and prefer dry, sunny places. They often form turf (tussocks)—compact structures where old grass stems, rhizomes, roots, and soil parts are intermixed. Grasses form grasslands—specific ecological communities widely represented on Earth (for example, North American prairies are grasslands). Grasses are also important components of other ecosystems (e.g. wetlands). Stems of grasses are usually hollow and round. The leaves have sheathing bases.

Grass florets are reduced, wind-pollinated, usually bisexual (Figure (PageIndex{10})), and form complicated spikelets. Each spikelet bears two glumes; each flower has lemma and palea scales (Figure (PageIndex{11})). The perianth is reduced to lodicules. Stamens from 6 to 1 (most often 3), with large anthers. The fruit of grasses is a caryopsis, what we commonly refer to as a grain. Grasses are incredibly important commercially as food sources, construction, biofuels, and components of many products: rice, corn, wheat, sugarcane, barley, rye, sorghum, and bamboo are all grasses. The study of grasses is called agrostology--it can be both aggravating and glumey.

Table (PageIndex{1}): A selection of eudicot families.

FamilyExamples
Anacardiaceaepoison ivy, cashews, pistachios
Asteraceaeasters and all the other composite flowers
Brassicaceaecabbage, turnip; Arabidopsis, and other mustards
Cactaceaecacti
Cucurbitaceaesquashes
Euphorbiaceaecassava (manioc)
Fabaceaebeans and all the other legumes
Fagaceaeoaks
Linaceaeflax (source of linen)
Malvaceaecotton
Oleaceaeolives, ashes, lilacs
Rosaceaeroses, apples, peaches, strawberries, almonds
Rubiaceaecoffee
Rutaceaeoranges and other citrus fruits
Solanaceaepotato, tomato, tobacco
Theaceaetea
Vitaceaegrapes

Magnoliidae being the most primitive with flowers of numerous free parts (like water lily, Nymphaea, fossil Archaefructus and Amborella);

Liliidae or monocots are grasses, palms, true lilies and many others with trimerous flowers;

Rosidae with pentamerous or tetramerous flowers and free petals;

Asteridae most advanced, bear flowers with fused petals and reduced number of carpels.

Rosids and asterids each comprise about 1/3 of angiosperm diversity.


8.4: Angiosperm Families - Biology

Common name: Mustard family

Distribution

This family includes 350 genera and 3200 species. The members are cosmopolitan in distribution however they are dominant in temperate and other colder parts of the world. Most of the members are used as vegetables and seeds of some plants are used for oil production.

Vegetative characters

Annual (Brassica), biennial(Raphanus), perennial(Cheiranthes), herbs, sometimes under shrubs(Farsetia), mostly terrestrial, sometimes aquatic(Nasturtium officiate). The plants possess pungent sap having sulphur containing glucosides.

Branched tap root system, sometimes root may become modified into different forms due to the storage of food materials such as fusiform (Raphanus Sativus), napiform (Brassica rapa).

Aerial, erect, with distinct nodes and internodes, branched, sometimes unbranched, herbaceous, rarely woody, cylindrical, solid, glabrous, sometimes pubescent, sometimes stem gets reduced into flattened disc (Raphanus sativus) or sometimes the stem becomes modified into the corm-like structure (Brassica olearacea var Caulorapa).

Cauline and ramal, sometimes radical(Raphanus sativus), exstipulate, alternate, sometimes opposite, the lower leaf is petiolate, the upper leaf is sessile, simple, sometimes pinnately compound (Nasturtium officinate), lower leaves lyrate, upper leaves hastate or lanceolate, pubescent, sometimes glabrous, unicostate reticulate venation.

Floral characters

Inflorescence

Racemose, typical raceme, sometimes corymb (Iberis Amara)

Ebracteate, bracteate in Nasturtium montanum, pedicellate, complete, bisexual, actinomorphic, zygomorphic in Iberis Amara, tetramerous, hypogynous.

Sepals 2+2, 4 sepals arranged in two whorls, polysepalous, imbricate aestivation, petaloid or green, inferior.

Petals 4, polypetalous, cruciform, four petals arranged in cross-like manner and each petal differentiated into limb and claw, valvate aestivation, equal sized but unequal sized in (Iberis Amara), sometimes petals are modified into or reduced into scales (Coronopus) or sometimes petals are completely absent (Lepidium), inferior.

Stamens 2+ 4, polyandrous, tetradynamous (presence of six stamens arranged in two whorls outer two with short filaments and inner four with long filaments. Sometimes either four stamens (Nasturtiumofficinate) or 2 stamens (Coronopus Didymus) up to 16 stamens (Megacarpeapolyandra) may be present. Anther dithecous, basifixed, outer two introrse, inner four extrorse or sometimes introrse only, inferior, nectary glands may be present in some species, dehiscence by longitudinal slits.

Carpels(2), bi carpellary, syncarpous, ovary superior, unilocular but becomes bilocular due to the presence of false septum called replum, one or many ovules in parietal placentation, style simple, short. Stigma simple or capitate or bifid.

Siliqua or silicula or lomentum(Raphanus).

Floral formula

Economic Importance

Vegetables:

  • Brassica campest rais
  • Brassica rapa (Turnip)
  • Raphanus sativus (Radish)
  • Brassica oleracea var.botrytis (Cauliflower)
  • Brassica oleracea var.capitata (Cabbage)

Oil-yielding plants

Ornamental plants

Floral Diagram

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Things to remember
  • The members are cosmopolitan in distribution however they are dominant in temperate and other colder parts of the world.
  • The plants possess pungent sap having sulphur containing glucosides.
  • This family include 350 genera and 3200 species.
  • Most of the members are used as vegetables and seeds of some plants are used for oil production.
  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.

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Malvaceae

The plant may be herbs, shrubs or trees. Flowers are complete, actinomorphic, pentamerous and hypogynous. Epicalyx is present. Calyx has 5 sepals which connect at the base, corolla has 5 petals. Androecium has indefinite Monadelphous stamens and gynoecium has five carpels, syncarpous, ovary superior with axile placentation. Fruit is capsule type and non-endospermic seed. Hibiscus rosa-sinensis (China rose) is one of the best examples of this family. Example: Bhindi, hollyhocks.


Statistics

Species of Angiosperms contained within The Plant List belong to 406 plant families and 14,038 plant genera.

The Plant List includes 904,649 scientific plant names of species rank for the Angiosperms. Of these 273,174 are accepted species names.

The Plant List includes a further 184,795 scientific plant names of infraspecific rank for the Angiosperms. We do not intend The Plant List to be complete for names of infraspecific rank. These are primarily included because names of species rank are synonyms of accepted infraspecific names.

Species names

The status of the 904,649 species names for the Angiosperms recorded in The Plant List , are as follows:

Status Total
◕ ◐ Accepted 273,174 30.2%
◕ ◐ Synonym 421,698 46.6%
◕ ◐ Unplaced 15,282 1.7%
◕ ◐ Unassessed 194,495 21.5%

All names

The status of the 1,089,444 names (including infraspecific names) for the Angiosperms recorded in The Plant List , are as follows:

Status Total
◕ ◐ Accepted 298,536 27.4%
◕ ◐ Synonym 575,340 52.8%
◕ ◐ Unplaced 15,343 1.4%
◕ ◐ Unassessed 200,225 18.4%

A further 1,241 name records indicate where names have been misapplied.

The confidence with which the status of the 904,649 species names recorded in The Plant List for the Angiosperms, are assigned as follows:

Confidence level Accepted Synonym Unplaced Unassessed Total
High confidence 141,434 216,039 0 0 357,473 39.5%
Medium confidence 123,058 175,013 0 0 298,071 32.9%
Low confidence 8,682 30,646 15,282 194,495 249,105 27.5%

The source of the species name record found in The Plant List for the Angiosperms, are as follows:


Contents

Angiosperm derived characteristics Edit

Angiosperms differ from other seed plants in several ways, described in the table below. These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. [a]

Distinctive features of angiosperms
Feature Description
Flowering organs Flowers, the reproductive organs of flowering plants, are the most remarkable feature distinguishing them from the other seed plants. Flowers provided angiosperms with the means to have a more species-specific breeding system, and hence a way to evolve more readily into different species without the risk of crossing back with related species. Faster speciation enabled the Angiosperms to adapt to a wider range of ecological niches. This has allowed flowering plants to largely dominate terrestrial ecosystems. [ citation needed ]
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 specialised 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 gametophyte, three cells The male gametophyte in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants. [10] The smaller size of the pollen reduces the amount of time between pollination — the pollen grain reaching the female plant — and fertilization. In gymnosperms, fertilization can occur up to a year after pollination, whereas in angiosperms, fertilization begins very soon after pollination. [11] The shorter amount of time between pollination and fertilization allows angiosperms to produce seeds earlier after pollination than gymnosperms, providing angiosperms 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 specialised pollination syndromes and controls. This helps to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilised, 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 In general, endosperm formation 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 the seedling when it first appears.

Vascular anatomy Edit

Angiosperm stems are made up of seven layers as shown on the right. 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.

Reproductive anatomy Edit

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. [ citation needed ] The floral apparatus may arise terminally on a shoot or from the axil of a leaf (where the petiole attaches to the stem). [ citation needed ] 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.

There are two kinds of reproductive cells produced by flowers. Microspores, which will divide to become pollen grains, are the "male" cells and are borne in the stamens (or microsporophylls). [ citation needed ] 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, in general, 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. [ citation needed ]

While the majority of flowers are perfect or hermaphrodite (having both pollen and ovule producing 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. Alternatively, in dioecious species, the male and female parts are morphologically separated, developing on different individual flowers. [12]

History of classification Edit

The botanical term "angiosperm", from Greek words angeíon ( ἀγγεῖον 'bottle, vessel') and spérma ( σπέρμα '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 schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked. Both the term and its antonym were maintained by Carl 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 became possible only after 1827, when Robert Brown established the existence of truly naked ovules in the Cycadeae and Coniferae, [13] and applied to them the name Gymnosperms. [ citation needed ] From that time onward, as 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, with Anthophyta (lit. 'flower-plants') a second choice (both unranked). The Wettstein system and Engler system treated them as a subdivision (Angiospermae). The Reveal system also treated them as a subdivision (Magnoliophytina), [14] but later split it to Magnoliopsida, Liliopsida, and Rosopsida. The Takhtajan system and Cronquist system treat them as a division (Magnoliophyta). The Dahlgren system and Thorne system (1992) treat them as a class (Magnoliopsida). The APG system of 1998, and the later 2003 [15] and 2009 [16] revisions, treat the flowering plants as an unranked clade without a formal Latin name (angiosperms). A formal classification was published alongside the 2009 revision in which the flowering plants rank as a subclass (Magnoliidae). [17]

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. Updates incorporating more recent research were published as the APG II system in 2003, [15] the APG III system in 2009, [16] [18] and the APG IV system in 2016.

Traditionally, the flowering plants are divided into two groups,

to which the Cronquist system ascribes the classes Magnoliopsida (from "Magnoliaceae" and Liliopsida (from "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 plain English, their members may be called "dicotyledons" ("dicots") and "monocotyledons" ("monocots"). The Latin behind these names refers 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 broad diagnostic point of view, the number of cotyledons is neither a particularly handy, nor a reliable character. [ citation needed ]

Recent studies, as by the APG, show that the monocots form a monophyletic group (a clade) but that the dicots are paraphyletic. Nevertheless, the majority of dicot species fall into a clade, the eudicots or tricolpates, and most of the remaining fall into another major clade, the magnoliids, containing about 9,000 species. The rest include a paraphyletic grouping of early branching taxa known collectively as the basal angiosperms, plus the families Ceratophyllaceae and Chloranthaceae. [ citation needed ]

Modern classification Edit

There are eight groups of living angiosperms:

    (ANA: Amborella, Nymphaeales, Austrobaileyales)
    • Amborella, a single species of shrub from New Caledonia , about 80 species, [19]water lilies and Hydatellaceae , about 100 species [19] of woody plants from various parts of the world
      , 77 known species [20] of aromatic plants with toothed leaves , about 9,000 species, [19] characterised by trimerous flowers, pollen with one pore, and usually branching-veined leaves—for example magnolias, bay laurel, and black pepper , about 70,000 species, [19] characterised 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 [19] of aquatic plants, perhaps most familiar as aquarium plants , about 175,000 species, [19] characterised 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 relationships among these eight groups is not yet clear, although there is agreement that the first three groups to diverge from the ancestral angiosperm were Amborellales, Nymphaeales, and Austrobaileyales (basal angiosperms) [21] Of the remaining five groups (core angiosperms), the relationships among the three broadest groups remains unclear (magnoliids, monocots, and eudicots). Zeng and colleagues (Fig. 1) describe four competing schemes. [22] The eudicots and monocots are the largest and most diversified, with

    75% and 20% of angiosperm species, respectively. Some analyses make the magnoliids the first to diverge, others the monocots. [23] Ceratophyllum seems to group with the eudicots rather than with the monocots. The APG IV retained the overall higher order relationship described in APG III. [16]

    Nymphaeales Salisbury ex von Berchtold & Presl 1820

    Magnoliales de Jussieu ex von Berchtold & Presl 1820

    Laurales de Jussieu ex von Berchtold & Presl 1820

    Alismatales Brown ex von Berchtold & Presl 1820

    Pandanales Brown ex von Berchtold & Presl 1820

    Commelinales de Mirbel ex von Berchtold & Presl 1820

    Ranunculales de Jussieu ex von Berchtold & Presl 1820

    Proteales de Jussieu ex von Berchtold & Presl 1820

    Dilleniales de Candolle ex von Berchtold & Presl 1820

    Vitales de Jussieu ex von Berchtold & Presl 1820

    Malpighiales de Jussieu ex von Berchtold & Presl 1820

    Cucurbitales de Jussieu ex von Berchtold & Presl 1820

    Geraniales de Jussieu ex von Berchtold & Presl 1820

    Myrtales de Jussieu ex von Berchtold & Presl 1820

    Sapindales de Jussieu ex von Berchtold & Presl 1820

    Malvales de Jussieu ex von Berchtold & Presl 1820

    Santalales Brown ex von Berchtold & Presl 1820

    Gentianales de Jussieu ex von Berchtold & Presl 1820

    Solanales de Jussieu ex von Berchtold & Presl 1820

    Boraginales de Jussieu ex von Berchtold & Presl 1820

    Dipsacales de Jussieu ex von Berchtold & Presl 1820

    Evolutionary history Edit

    Paleozoic Edit

    Fossilised spores suggest that land plants (embryophytes) have existed for at least 475 million years. [24] Early land plants reproduced sexually with flagellated, swimming sperm, like the green algae from which they evolved. [ citation needed ] An adaptation to terrestrialization was the development of upright meiosporangia for dispersal by spores to new habitats. [ citation needed ] 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. [ citation needed ] 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 Selaginella, the spike-moss. The result for the ancestors of angiosperms was enclosing them in a case, the seed.

    The apparently sudden appearance in the fossil record of nearly modern flowers, and in great diversity, initially posed such a problem for the theory of gradual evolution that Charles Darwin called it an "abominable mystery". [25] However, the fossil record has considerably 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. [ citation needed ] Several groups of extinct gymnosperms, in particular seed ferns, have been proposed as the ancestors of flowering plants, but there is no continuous fossil evidence showing how flowers evolved, and botanists still regard it as a mystery. [26] Some older fossils, such as the upper Triassic Sanmiguelia lewisi, have been suggested. [ citation needed ]

    The first seed bearing plants, like the ginkgo, and conifers (such as pines and firs), did not produce flowers. The pollen grains (male gametophytes) 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.

    Oleanane, a secondary metabolite produced by many flowering plants, has been found in Permian deposits of that age together with fossils of gigantopterids. [27] [28] 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. [ citation needed ]

    Triassic and Jurassic Edit

    Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms in the Triassic period (245–202 million years ago). Fossil angiosperm-like pollen from the Middle Triassic (247.2–242.0 Ma) suggests an older date for their origin. [29] 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. [30] [31]

    The fossil plant species Nanjinganthus dendrostyla from Early Jurassic China seems to share many exclusively angiosperm features, such as a thickened receptacle with ovules, and thus might represent a crown-group or a stem-group angiosperm. [32] However, these have been disputed by other researchers, who contend that the structures are misinterpreted decomposed conifer cones. [33] [34]

    The evolution of seed plants and later angiosperms appears to be the result of two distinct rounds of whole genome duplication events. [35] These occurred at 319 million years ago and 192 million years ago . Another possible whole genome duplication event at 160 million years ago perhaps created the ancestral line that led to all modern flowering plants. [36] That event was studied by sequencing the genome of an ancient flowering plant, Amborella trichopoda, [37] and directly addresses Darwin's "abominable mystery".

    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. [38]

    Cretaceous Edit

    Whereas the earth had previously been dominated by ferns and conifers, angiosperms appeared and quickly spread during the Cretaceous. They now comprise about 90% of all plant species including most food crops. [39] It has been proposed that the swift rise of angiosperms to dominance was facilitated by a reduction in their genome size. During the early Cretaceous period, only angiosperms underwent rapid genome downsizing, while genome sizes of ferns and gymnosperms remained unchanged. Smaller genomes—and smaller nuclei—allow for faster rates of cell division and smaller cells. Thus, species with smaller genomes can pack more, smaller cells—in particular veins and stomata [ citation needed ] —into a given leaf volume. Genome downsizing therefore facilitated higher rates of leaf gas exchange (transpiration and photosynthesis) and faster rates of growth. This would have countered some of the negative physiological effects of genome duplications, facilitated increased uptake of carbon dioxide despite concurrent declines in atmospheric CO2 concentrations, and allowed the flowering plants to outcompete other land plants. [40]

    The oldest known fossils definitively attributable to angiosperms are reticulated monosulcate pollen from the late Valanginian (Early or Lower Cretaceous - 140 to 133 million years ago) of Italy and Israel, likely representative of the basal angiosperm grade. [33]

    The earliest known macrofossil confidently identified as an angiosperm, Archaefructus liaoningensis, is dated to about 125 million years BP (the Cretaceous period), [41] whereas pollen considered to be of angiosperm origin takes the fossil record back to about 130 million years BP, [42] with Montsechia representing the earliest flower at that time. [43]

    In 2013 flowers encased in amber were found and dated 100 million years before present. The amber had frozen the act of sexual reproduction in the process of taking place. Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. The pollen was sticky, suggesting it was carried by insects. [44] In August 2017, scientists presented a detailed description and 3D model image of what the first flower possibly looked like, and presented the hypothesis that it may have lived about 140 million years ago. [45] [46] A Bayesian analysis of 52 angiosperm taxa suggested that the crown group of angiosperms evolved between 178 million years ago and 198 million years ago . [47]

    Recent DNA analysis based on molecular systematics [48] [49] 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 [50] suggest that it has features that may have been characteristic of the earliest flowering plants. The orders Amborellales, Nymphaeales, and Austrobaileyales diverged as separate lineages from the remaining angiosperm clade at a very early stage in flowering plant evolution. [51]

    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 [52] estimated that the division of the five most recent of the eight main groups occurred around 140 million years ago. (the genus Ceratophyllum, the family Chloranthaceae, the eudicots, the magnoliids, and the monocots) .

    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 specialised 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 the development of a high degree of specialisation in both the plant(s) and their partners. Note that the wasp example is not incidental bees, which, it is postulated, evolved specifically due to mutualistic plant relationships, are descended from wasps. [53]

    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). Although many such mutualistic relationships remain too fragile to survive competition and to 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. [ citation needed ]

    Flower ontogeny uses a combination of genes normally responsible for forming new shoots. [54] The most primitive flowers probably had a variable number of flower parts, often separate from (but in contact with) each other. The flowers 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 evolved, 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 flower species were formerly simple weeds, which sprouted only 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. [55]

    A few paleontologists have also proposed that flowering plants, or angiosperms, might have evolved due to interactions with dinosaurs. One of the idea's strongest proponents is Robert T. Bakker. He proposes that herbivorous dinosaurs, with their eating habits, provided a selective pressure on plants, for which adaptations either succeeded in deterring or coping with predation by herbivores. [56]

    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 66 million years ago or even later, at the beginning of the Paleogene. [57] The radiation of herbaceous angiosperms occurred much later. [58] Yet, many fossil plants recognisable as belonging to modern families (including beech, oak, maple, and magnolia) had already appeared by the late Cretaceous. Flowering plants appeared in Australia about 126 million years ago. This also pushed the age of ancient Australian vertebrates, in what was then a south polar continent, to 126-110 million years old. [43]


    Contents

    Seagrasses are a paraphyletic group of marine angiosperms which evolved three to four times from land plants back to the sea. The following characteristics can be used to define a seagrass species. It lives in an estuarine or in the marine environment, and nowhere else. The pollination takes place underwater with specialized pollen. The seeds which are dispersed by both biotic and abiotic agents are produced underwater. [2] The seagrass species have specialized leaves with a reduced cuticle, an epidermis which lacks stomata and is the main photosynthetic tissue. The rhizome or underground stem is important in anchoring. The roots can live in an anoxic environment and depend on oxygen transport from the leaves and rhizomes but are also important in the nutrient transfer processes. [3] [2]

    Seagrasses profoundly influence the physical, chemical, and biological environments of coastal waters. [2] Though seagrasses provide invaluable ecosystem services by acting as breeding and nursery ground for a variety of organisms and promote commercial fisheries, many aspects of their physiology are not well investigated. Several studies have indicated that seagrass habitat is declining worldwide. [4] [5] Ten seagrass species are at elevated risk of extinction (14% of all seagrass species) with three species qualifying as endangered. Seagrass loss and degradation of seagrass biodiversity will have serious repercussions for marine biodiversity and the human population that depends upon the resources and ecosystem services that seagrasses provide. [6] [2]

    Terrestrial plants evolved perhaps as early as 450 million years ago from a group of green algae. [7] Seagrasses then evolved from terrestrial plants which migrated back into the ocean. [8] [9] Between about 70 million and 100 million years ago, the three independent seagrass lineages (Hydrocharitaceae, Cymodoceaceae complex, and Zosteraceae) evolved from a single lineage of monocotyledonous flowering plants. [10]

    Other plants that colonised the sea, such as salt marsh plants, mangroves, and marine algae, have more diverse evolutionary lineages. In spite of their low species diversity, seagrasses have succeeded in colonising the continental shelves of all continents except Antarctica. [11]

    Around 140 million years ago, seagrasses evolved from early monocotyledonous land plants, which succeeded in conquering the marine environment. Today, they are a polyphyletic group of marine angiosperms with around 60 species in five families (Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae), which belong to the order Alismatales according to the Angiosperm Phylogeny Group IV System. [12] The genus Ruppia, which occurs in brackish water, is not regarded as a "real" seagrass by all authors and has been shifted to the Cymodoceaceae by some authors. [13] The APG IV system and The Plant List Webpage [14] do not share this family assignment. [15]

    Seagrasses form important coastal ecosystems. [16] The worldwide endangering of these sea meadows, which provide food and habitat for many marine species, prompts the need for protection and understanding of these valuable resources. Recent sequencing of the genomes of Zostera marina and Zostera muelleri has given better understanding angiosperm adaption to the sea. [17] [18] During the evolutionary step back to the ocean, different genes have been lost (e.g., stomatal genes) or have been reduced (e.g., genes involved in the synthesis of terpenoids) and others have been regained, such as in genes involved in sulfation. [18] [15]

    Genome information has shown further that adaption to the marine habitat was accomplished by radical changes in cell wall composition. [17] [18] However the cell walls of seagrasses are not well understood. In addition to the ancestral traits of land plants one would expect habitat-driven adaption processs to the new environment characterized by multiple abiotic (high amounts of salt) and biotic (different seagrass grazers and bacterial colonization) stressors. [15] The cell walls of seagrasses seem intricate combinations of features known from both angiosperm land plants and marine macroalgae with new structural elements. [15]

    Seagrass populations are currently threatened by a variety of anthropogenic stressors. [22] [5] The ability of seagrasses to cope with environmental perturbations depends, to some extent, on genetic variability, which is obtained through sexual recruitment. [23] [24] [25] By forming new individuals, seagrasses increase their genetic diversity and thus their ability to colonise new areas and to adapt to environmental changes. [26] [27] [28] [29] [30] [21] [ excessive citations ]

    Seagrasses have contrasting colonisation strategies. [31] Some seagrasses form seed banks of small seeds with hard pericarps that can remain in the dormancy stage for several months. These seagrasses are generally short-lived and can recover quickly from disturbances by not germinating far away from parent meadows (e.g., Halophila sp., Halodule sp., Cymodocea sp., Zostera sp. and Heterozostera sp. [31] [32] In contrast, other seagrasses form dispersal propagules. This strategy is typical of long-lived seagrasses that can form buoyant fruits with inner large non-dormant seeds, such as the genera Posidonia sp., Enhalus sp. and Thalassia sp. [31] [33] Accordingly, the seeds of long-lived seagrasses have a large dispersal capacity compared to the seeds of the short-lived type, [34] which permits the evolution of species beyond unfavourable light conditions by the seedling development of parent meadows. [21]

    The seagrass Posidonia oceanica (L.) Delile is one of the oldest and largest species on Earth. An individual can form meadows measuring nearly 15 km wide and can be as much as 100,000 years old. [35] P. oceanica meadows play important roles in the maintenance of the geomorphology of Mediterranean coasts, which, among others, makes this seagrass a priority habitat of conservation. [36] Currently, the flowering and recruitment of P. oceanica seems to be more frequent than that expected in the past. [37] [38] [39] [40] [41] Further, this seagrass has singular adaptations to increase its survival during recruitment. The large amounts of nutrient reserves contained in the seeds of this seagrass support shoot and root growth, even up to the first year of seedling development. [35] In the first months of germination, when leaf development is scarce, P. oceanica seeds perform photosynthetic activity, which increases their photosynthetic rates and thus maximises seedling establishment success. [42] [43] Seedlings also show high morphology plasticity during their root system development [44] [45] by forming adhesive root hairs to help anchor themselves to rocky sediments. [37] [46] [47] However, many factors about P. oceanica sexual recruitment remain unknown, such as when photosynthesis in seeds is active or how seeds can remain anchored to and persist on substrate until their root systems have completely developed. [21]

    Seagrasses occurring in the intertidal and subtidal zones are exposed to highly variable environmental conditions due to tidal changes. [49] [50] Subtidal seagrasses are more frequently exposed to lower light conditions, driven by plethora of natural and human-caused influences that reduce light penetration by increasing the density of suspended opaque materials. Subtidal light conditions can be estimated, with high accuracy, using artificial intelligence, enabling more rapid mitigation than was available using in situ techniques. [51] Seagrasses in the intertidal zone are regularly exposed to air and consequently experience extreme high and low temperatures, high photoinhibitory irradiance, and desiccation stress relative to subtidal seagrass. [50] [52] [53] Such extreme temperatures can lead to significant seagrass dieback when seagrasses are exposed to air during low tide. [54] [55] [56] Desiccation stress during low tide has been considered the primary factor limiting seagrass distribution at the upper intertidal zone. [57] Seagrasses residing the intertidal zone are usually smaller than those in the subtidal zone to minimize the effects of emergence stress. [58] [55] Intertidal seagrasses also show light-dependent responses, such as decreased photosynthetic efficiency and increased photoprotection during periods of high irradiance and air exposure. [59] [60]

    In contrast, seagrasses in the subtidal zone adapt to reduced light conditions caused by light attenuation and scattering due to the overlaying water column and suspended particles. [62] [63] Seagrasses in the deep subtidal zone generally have longer leaves and wider leaf blades than those in the shallow subtidal or intertidal zone, which allows more photosynthesis, in turn resulting in greater growth. [53] Seagrasses also respond to reduced light conditions by increasing chlorophyll content and decreasing the chlorophyll a/b ratio to enhance light absorption efficiency by using the abundant wavelengths efficiently. [64] [65] [66] As seagrasses in the intertidal and subtidal zones are under highly different light conditions, they exhibit distinctly different photoacclimatory responses to maximize photosynthetic activity and photoprotection from excess irradiance.

    Seagrasses assimilate large amounts of inorganic carbon to achieve high level production. [67] [68] Marine macrophytes, including seagrass, use both CO2 and HCO −
    3 (bicarbonate) for photosynthetic carbon reduction. [69] [70] [71] Despite air exposure during low tide, seagrasses in the intertidal zone can continue to photosynthesize utilizing CO2 in the air. [72] Thus, the composition of inorganic carbon sources for seagrass photosynthesis probably varies between intertidal and subtidal plants. Because stable carbon isotope ratios of plant tissues change based on the inorganic carbon sources for photosynthesis, [73] [74] seagrasses in the intertidal and subtidal zones may have different stable carbon isotope ratio ranges.

    Seagrass holobiont Edit

    The concept of the holobiont, which emphasizes the importance and interactions of a microbial host with associated microorganisms and viruses and describes their functioning as a single biological unit, [77] has been investigated and discussed for many model systems, although there is substantial criticism of a concept that defines diverse host-microbe symbioses as a single biological unit. [78] The holobiont and hologenome concepts have evolved since the original definition, [79] and there is no doubt that symbiotic microorganisms are pivotal for the biology and ecology of the host by providing vitamins, energy and inorganic or organic nutrients, participating in defense mechanisms, or by driving the evolution of the host. [80] Although most work on host-microbe interactions has been focused on animal systems such as corals, sponges, or humans, there is a substantial body of literature on plant holobionts. [81] Plant-associated microbial communities impact both key components of the fitness of plants, growth and survival, [82] and are shaped by nutrient availability and plant defense mechanisms. [83] Several habitats have been described to harbor plant-associated microbes, including the rhizoplane (surface of root tissue), the rhizosphere (periphery of the roots), the endosphere (inside plant tissue), and the phyllosphere (total above-ground surface area). [75]

    Seagrass beds/meadows can be either monospecific (made up of a single species) or in mixed beds. In temperate areas, usually one or a few species dominate (like the eelgrass Zostera marina in the North Atlantic), whereas tropical beds usually are more diverse, with up to thirteen species recorded in the Philippines.

    Seagrass beds are diverse and productive ecosystems, and can harbor hundreds of associated species from all phyla, for example juvenile and adult fish, epiphytic and free-living macroalgae and microalgae, mollusks, bristle worms, and nematodes. Few species were originally considered to feed directly on seagrass leaves (partly because of their low nutritional content), but scientific reviews and improved working methods have shown that seagrass herbivory is an important link in the food chain, feeding hundreds of species, including green turtles, dugongs, manatees, fish, geese, swans, sea urchins and crabs. Some fish species that visit/feed on seagrasses raise their young in adjacent mangroves or coral reefs.

    Seagrasses trap sediment and slow down water movement, causing suspended sediment to settle out. Trapping sediment benefits coral by reducing sediment loads, improving photosynthesis for both coral and seagrass. [84]

    White-spotted puffers, often found in seagrass areas

    Underwater footage of seagrass meadow, bull huss and conger eel

    Although often overlooked, seagrasses provide a number of ecosystem services. [85] [86] Seagrasses are considered ecosystem engineers. [87] [9] [8] This means that the plants alter the ecosystem around them. This adjusting occurs in both physical and chemical forms. Many seagrass species produce an extensive underground network of roots and rhizome which stabilizes sediment and reduces coastal erosion. [88] This system also assists in oxygenating the sediment, providing a hospitable environment for sediment-dwelling organisms. [87] Seagrasses also enhance water quality by stabilizing heavy metals, pollutants, and excess nutrients. [89] [9] [8] The long blades of seagrasses slow the movement of water which reduces wave energy and offers further protection against coastal erosion and storm surge. Furthermore, because seagrasses are underwater plants, they produce significant amounts of oxygen which oxygenate the water column. These meadows account for more than 10% of the ocean's total carbon storage. Per hectare, it holds twice as much carbon dioxide as rain forests and can sequester about 27.4 million tons of CO2 annually. [90]

    Seagrass meadows provide food for many marine herbivores. Sea turtles, manatees, parrotfish, surgeonfish, sea urchins and pinfish feed on seagrasses. Many other smaller animals feed on the epiphytes and invertebrates that live on and among seagrass blades. [91] Seagrass meadows also provide physical habitat in areas that would otherwise be bare of any vegetation. Due to this three dimensional structure in the water column, many species occupy seagrass habitats for shelter and foraging. It is estimated that 17 species of coral reef fish spend their entire juvenile life stage solely on seagrass flats. [92] These habitats also act as a nursery grounds for commercially and recreationally valued fishery species, including the gag grouper (Mycteroperca microlepis), red drum, common snook, and many others. [93] [94] Some fish species utilize seagrass meadows and various stages of the life cycle. In a recent publication, Dr. Ross Boucek and colleagues discovered that two highly sought after flats fish, the common snook and spotted sea trout provide essential foraging habitat during reproduction. [95] Sexual reproduction is extremely energetically expensive to be completed with stored energy therefore, they require seagrass meadows in close proximity to complete reproduction. [95] Furthermore, many commercially important invertebrates also reside in seagrass habitats including bay scallops (Argopecten irradians), horseshoe crabs, and shrimp. Charismatic fauna can also be seen visiting the seagrass habitats. These species include West Indian manatee, green sea turtles, and various species of sharks. The high diversity of marine organisms that can be found on seagrass habitats promotes them as a tourist attraction and a significant source of income for many coastal economies along the Gulf of Mexico and in the Caribbean.

    Diminishing meadows Edit

    The storage of carbon is an essential ecosystem service as we move into a period of elevated atmospheric carbon levels. However, some climate change models suggest that some seagrasses will go extinct – Posidonia oceanica is expected to go extinct, or nearly so, by 2050. [ citation needed ]

    The UNESCO world heritage site around the Balearic islands of Mallorca and Formentera includes about 55,000 hectares (140,000 acres) of Posidonia oceanica, which has global significance because of the amount of carbon dioxide it absorbs. However, the meadows are being threatened by rising temperatures, which slows down its growth, as well as damage from anchors. [96]


    The 2011 paper How Many Species Are There on Earth and in the Ocean? indirectly answers this question as well as any other source you'll find I imagine. It estimates how many species there are total based on the rate of discovery of higher taxa it includes plots of number of taxa over time for the major groups of life in Figure S1. Which gives:

    Animalia - 5300 families in 2011 (the plots are given with only one significant figure, the second one's my estimate) (estimated total: 5800)

    Chromista - 270 families (estimated total: 360)

    Fungi - 550 families (estimated total: 620)

    Plantae - 750 families (estimated total: 800)

    Protozoa - 280 families (estimated total: 310)

    Archaea - 27 families (no estimated total the number has been increasing exponentially so far)

    Bacteria - 300 families (same as for Archaea)

    Which gives us a total of 7477 families in 2011, with an estimated total of (ignoring Archaea and Bacteria, who don't really fall in the same kind of classification anyway) 7890 families. (make that 7500 families discovered by 2011 and 8000 estimated in total given the imprecision involved in my reading the plots).


    ANGIOSPERMIC FAMILIES

    Monosexual : When one of stamen or carpel is present.

    Bisexual: When both stamen and carpel are present.

    Complete: Calyx, corolla both present.

    Incomplete: One of the calyx or corolla is absent.

    Hermaphrodite: Both stamen and carpels are present.

    Actinomorphic: When a flower can be cut into two equal halves by more than one plane, it is called actinomorphic flower.

    Zygomorphic: When a flower can be cut into two equal halves by only one plane, it is called zygomorphic flower.

    Regular: When actinomorphic symmetry.

    Irregular: When zygomorphic symmetry.

    Hypogynous: In this case, the thalamus (upper part of pedicel) is convex. The carpel attach at the tip. The stamen, sepals are inserted below the gyneoecium on the side. So ovary is superior.

    Perigynous: In this case, the thalamus is flattened. Ovary is present in the centre. The stamens, sepals. petals are inserted on the rim of the disc around the gyncoecium. So ovary is superior.

    I. Gamosepalous: Sepal fused

    Polysepalous: Sepal free

    Persistant sepals: If sepals do not fall after the opening of • flower, they are called persistant sepals.

    Gamopetalous: Petal fused

    2. Polypetalous: Petal flee

    When sepals and petals cannot differentiated from each other, then term perianth is used both of these.

    Polyandrous: Stamens free •

    Epipetalous stamen: Stamens are attached with the petals

    Adelphous: Stamens are fused by their filaments

    Monoadelphous: The filament of stamens fused to form single group.

    Basifixed: Stamen attached at the base of anther

    Versatile: Filament at in the back of anther Fherefore,
    anther can swing on filament

    Apocarpous: Carpel free

    Synearpous: Carpel fused

    Monocarpillary: Single carpel

    Polycarpillary: Many camels

    Simple pistil: In this case. the carpels are not fused.

    Compound pistil: In this case, one or more camels are fused.

    Unilocular ovary: In this case. the ovary has single chamber.

    The attachment of ovule in the ovary is called placentation. There are different types of placentation:

    Basal: In this case, the ovule is attached at the base of ovary.

    Axile: In the case there is a central axil (rod) inside the ovary. The ovule attach on this axil

    Marginal: In this case the ovules are attached on the inner wall of the ovary.

    Diagnostic characters

    Roots: Tap root or adventitious roots fibrous or tuberous root

    Stem: Herbaceous or %soody, spiny or without spines Cylindrical Aerial. climbing or mderground stem (rhizome. corm, bulb or tuber).

    Leaves: Sessile or petiolate Leavy phyllotaxis (alternate or opposite) simple or compound: Stipu .tte or exstipulate: parallel or reticulate venation.

    Inflorescence: Racemose or cymose, ‘type of racemose of c ‘nose)

    Honer: Sessile or pedicillate bracteate or ebracteatc: actinomcrphic bi zygomorphic: Regular or irregular: complete or incomplete unisexual or hermaphrodite: bypogynous. perigynous or epigynous:

    Calyx: Number of sepals: free or fused imbricate: green or petaloid

    Stamens: Number of stamens: adelphous or free: attachment of anther on filament.

    Carpel: Monocarpillary or Polycarpillary Simple or compound ovary: apocarpous or syncarpous: type or placentation.


    Seed phylogeny - Morphological and physiological trends in seed evolution

    Seed biodiversity has attracted the attention of many researchers and is a hallmark of seed biology. This great diversity of morphological and physiological features have evolved to control germination and dormancy in response to different environments. The evolution of seed structure, germination and dormancy is summarized by the Tansley review by Bill Finch-Savage and Gerd Leubner (2006) and the references cited therein. The cited references include key literature to seed evolution like the book "Seeds" from Baskin and Baskin (1998), and the publications by Baskin and Baskin (2004), by Forbis et al. (2002), and by Nikolaeva (2004). The work on seed morphology is based on a publication by Martin (The comparative internal morphology of seeds. The American Midland Naturalist 36: 513-660, 1946).

    The most obvious morphological difference in mature angiosperm seeds is their "embryo to seed" size ratios resulting from the extent to which the endosperm is obliterated during seed development by incorporating the nutrients into the storage cotyledons. Based on the internal morphology of 1287 mature seeds Martin (1946) defined the following seed types with distinct embryo to endosperm ratios:


    8.4: Angiosperm Families - Biology

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    Watch the video: Angiosperm Families: Monocots (May 2022).