Pollen

Tulip anther with many grains of pollen
Closeup image of a cactus flower and its stamens
Scanning electron microscope image (500x magnification) of pollen grains from a variety of common plants: sunflower (Helianthus annuus), morning glory (Ipomoea purpurea), prairie hollyhock (Sidalcea malviflora), oriental lily (Lilium auratum), evening primrose (Oenothera fruticosa), and castor bean (Ricinus communis).

Pollen is a fine to coarse powdery substance comprising pollen grains which are male microgametophytes of seed plants, which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophytes during the process of their movement from the stamens to the pistil of flowering plants, or from the male cone to the female cone of coniferous plants. If pollen lands on a compatible pistil or female cone, it germinates, producing a pollen tube that transfers the sperm to the ovule containing the female gametophyte. Individual pollen grains are small enough to require magnification to see detail. The study of pollen is called palynology and is highly useful in paleoecology, paleontology, archaeology, and forensics.

Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to the stigma of another in cross-pollination.[1] In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower.[1]

The structure and formation of pollen

Triporate pollen of Oenothera speciosa
Pollen of Lilium auratum showing single sulcus (monosulcate)
Arabis pollen has three colpi and prominent surface structure.
Pollens/Microspores of Lycopersicon esculentum at coenocytic tetrad stage of development observed through oil immersion microscope; the chromosomes of what will become four pollen grains can be seen.
Apple pollen under microscopy

Pollen itself is not the male gamete.[2] Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell. In flowering plants the vegetative tube cell produces the pollen tube, and the generative cell divides to form the two sperm cells.

Formation

Pollen is produced in the microsporangia in the male cone of a conifer or other gymnosperm or in the anthers of an angiosperm flower. Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (see electron micrograph, right). Pollen grains of pines, firs, and spruces are winged. The smallest pollen grain, that of the forget-me-not (Myosotis spp.),[which?] is around 6 µm (0.006 mm) in diameter.[citation needed] Wind-borne pollen grains can be as large as about 90–100 µm.[3]

In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups of sporogenous cells form within the anther. The fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac. Some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells.

In a process called microsporogenesis, four haploid microspores are produced from each diploid sporogenous cell (microsporocyte, pollen mother cell or meiocyte), after meiotic division. After the formation of the four microspores, which are contained by callose walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record. Two basic types of microsporogenesis are recognised, simultaneous and successive. In simultaneous microsporogenesis meiotic steps I and II are completed prior to cytokinesis, whereas in successive microsporogenesis cytokinesis follows. While there may be a continuum with intermediate forms, the type of microsporogenesis has systematic significance. The predominant form amongst the monocots is successive, but there are important exceptions.[4]

During microgametogenesis, the unicellular microspores undergo mitosis and develop into mature microgametophytes containing the gametes.[5] In some flowering plants,[which?] germination of the pollen grain may begin even before it leaves the microsporangium, with the generative cell forming the two sperm cells.

Structure

Except in the case of some submerged aquatic plants, the mature pollen grain has a double wall. The vegetative and generative cells are surrounded by a thin delicate wall of unaltered cellulose called the endospore or intine, and a tough resistant outer cuticularized wall composed largely of sporopollenin called the exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. The spines may be less than a micron in length (spinulus, plural spinuli) referred to as spinulose (scabrate), or longer than a micron (echina, echinae) referred to as echinate. Various terms also describe the sculpturing such as reticulate, a net like appearance consisting of elements (murus, muri) separated from each other by a lumen (plural lumina).

The pollen wall protects the sperm while the pollen grain is moving from the anther to the stigma; it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall, which prevents the pollen grain from shrinking and crushing the genetic material during desiccation, is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin.

Pollen apertures are regions of the pollen wall that may involve exine thinning or a significant reduction in exine thickness.[6] They allow shrinking and swelling of the grain caused by changes in moisture content. Elongated apertures or furrows in the pollen grain are called colpi (singular: colpus) or sulci (singular: sulcus). Apertures that are more circular are called pores. Colpi, sulci and pores are major features in the identification of classes of pollen.[7] Pollen may be referred to as inaperturate (apertures absent) or aperturate (apertures present). The aperture may have a lid (operculum), hence is described as operculate.[8] However the term inaperturate covers a wide range of morphological types, such as functionally inaperturate (cryptoaperturate) and omniaperturate.[4] Inaperaturate pollen grains often have thin walls, which facilitates pollen tube germination at any position.[6] Terms such as uniaperturate and triaperturate refer to the number of apertures present (one and three respectively).

The orientation of furrows (relative to the original tetrad of microspores) classifies the pollen as sulcate or colpate. Sulcate pollen has a furrow across the middle of what was the outer face when the pollen grain was in its tetrad.[9] If the pollen has only a single sulcus, it is described as monosulcate, has two sulci, as bisulcate, or more, as polysulcate.[10][11] Colpate pollen has furrows other than across the middle of the outer faces.[9] Eudicots have pollen with three colpi (tricolpate) or with shapes that are evolutionarily derived from tricolpate pollen.[12] The evolutionary trend in plants has been from monosulcate to polycolpate or polyporate pollen.[9]

Other Languages
Afrikaans: Stuifmeel
العربية: حبوب اللقاح
asturianu: Polen
azərbaycanca: Çiçək tozu
башҡортса: Һеркә
беларуская: Пылок
български: Цветен прашец
bosanski: Polen
català: Pol·len
Чӑвашла: Шăрка
čeština: Pyl
Cymraeg: Paill
dansk: Pollen
Deutsch: Pollen
eesti: Õietolm
Ελληνικά: Γύρη
español: Polen
Esperanto: Poleno
euskara: Polen
فارسی: گرده
français: Pollen
Gaeilge: Pailin
Gàidhlig: Poilean
galego: Pole
한국어: 꽃가루
Հայերեն: Ծաղկափոշի
हिन्दी: पराग
hrvatski: Cvjetni pelud
Ido: Poleno
Bahasa Indonesia: Serbuk sari
interlingua: Polline
íslenska: Frjóduft
italiano: Polline
қазақша: Тозаң
Kiswahili: Mbelewele
Latina: Pollen
latviešu: Putekšņi
lietuvių: Žiedadulkė
magyar: Virágpor
македонски: Полен
മലയാളം: പരാഗം
Bahasa Melayu: Debunga
Nederlands: Stuifmeel
日本語: 花粉
norsk: Pollen
norsk nynorsk: Pollen
occitan: Pollèn
oʻzbekcha/ўзбекча: Chang donachalari
پنجابی: پراگ (پھل)
Plattdüütsch: Pollen
polski: Pyłek
português: Pólen
română: Polen
Runa Simi: Sisa
русский: Пыльца
Scots: Pollen
sicilianu: Pòllini
Simple English: Pollen
slovenčina: Včelí peľ
slovenščina: Cvetni prah
српски / srpski: Полен
srpskohrvatski / српскохрватски: Pelud
suomi: Siitepöly
svenska: Pollen
தமிழ்: மகரந்தம்
తెలుగు: పుప్పొడి
Türkçe: Polen
Türkmençe: Tozgajyk
українська: Пилок
Tiếng Việt: Phấn hoa
中文: 花粉