Pollen is a powdery substance produced by most types of flowers of seed plants for the purpose of sexual reproduction. It consists of pollen grains (highly reduced microgametophytes ), which produce male gametes (sperm cells).
86-516: 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 gymnosperms . 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
172-409: A central cell that contains two nuclei. In select angiosperms, special cases occur in which the female gametophyte is not 7 celled with 8 nuclei. On the small end of the spectrum, some species have mature female gametophytes with only 4 cells, each with one nuclei. Conversely, some species have 10-celled mature female gametophytes consisting of 16 total nuclei. Once double fertilization occurs,
258-766: A 54% increased chance of asthma attacks when exposed to pollen. The number of people in the United States affected by hay fever is between 20 and 40 million, including around 6.1 million children and such allergy has proven to be the most frequent allergic response in the nation. Hay fever affects about 20% of Canadians and the prevalence is increasing. There are certain evidential suggestions pointing out hay fever and similar allergies to be of hereditary origin . Individuals who suffer from eczema or are asthmatic tend to be more susceptible to developing long-term hay fever. Since 1990, pollen seasons have gotten longer and more pollen-filled, and climate change
344-407: A burglar who brushed against a Hypericum bush during a crime, and has even been proposed as an additive for bullets to enable tracking them. In some Native American religions , pollen was used in prayers and rituals to symbolize life and renewal by sanctifying objects, dancing grounds, trails, and sandpaintings . It may also be sprinkled over heads or in mouths. Many Navajo people believed
430-407: A challenge. While seed plant gametophyte tissue is typically composed of mononucleate haploid cells (1 x n), specific circumstances can occur in which the ploidy does vary widely despite still being considered part of the gametophyte. In gymnosperms , the male gametophytes are produced inside microspores within the microsporangia located inside male cones or microstrobili . In each microspore,
516-562: A complete lack of aromatic structures and the presence of a macrocyclic backbone of polyhydroxylated tetraketide-like monomers with pseudo-aromatic 2-pyrone rings. These were crosslinked to a poly(hydroxy acid) chain by ether linkages to form the polymer. Electron microscopy shows that the tapetal cells that surround the developing pollen grain in the anther have a highly active secretory system containing lipophilic globules. These globules are believed to contain sporopollenin precursors. Tracer experiments have shown that phenylalanine
602-423: 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. During microgametogenesis, the unicellular microspores undergo mitosis and develop into mature microgametophytes containing the gametes. In some flowering plants, germination of the pollen grain may begin even before it leaves
688-425: A direct tube cell path from the pollination site to the egg is not needed. In most species the germ cell can be more specifically described as a sperm cell which mates with the egg cell during fertilization, though that is not always the case. In some Gnetophyta species, the germ cell will release two sperm nuclei that undergo a rare gymnosperm double fertilization process occurring solely with sperm nuclei and not with
774-482: A double set of chromosomes. Cell division of the zygote results in a new diploid multicellular organism, the second stage in the life cycle known as the sporophyte . The sporophyte can produce haploid spores by meiosis that on germination produce a new generation of gametophytes. In some multicellular green algae ( Ulva lactuca is one example), red algae and brown algae , sporophytes and gametophytes may be externally indistinguishable (isomorphic). In Ulva ,
860-407: 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 nuclei. 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
946-399: A lumen (plural lumina). These reticulations may also be referred to as brochi. 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
SECTION 10
#17327837875941032-503: A multitude of viruses and eukaryotic parasites are present in the pollen. It is currently unclear if the parasites are introduced by the bee that collected the pollen or if it is from the flower. Though this is not likely to pose a risk to humans, it is a major issue for the bumblebee rearing industry that relies on thousands of tonnes of honey bee collected pollen per year. Several sterilization methods have been employed, though no method has been 100% effective at sterilisation without reducing
1118-504: A pollen allergy is by preventing contact with the material. Individuals carrying the ailment may at first believe that they have a simple summer cold, but hay fever becomes more evident when the apparent cold does not disappear. The confirmation of hay fever can be obtained after examination by a general physician . Antihistamines are effective at treating mild cases of pollinosis; this type of non-prescribed drugs includes loratadine , cetirizine and chlorpheniramine . They do not prevent
1204-465: A rigid structure. There is also good evidence that the chemical composition of sporopollenin is not the same in all plants, indicating it is a class of compounds rather than having one constant structure. In 2019, thioacidolysis degradation and solid-state NMR was used to determine the molecular structure of pitch pine sporopollenin, finding it primarily composed of polyvinyl alcohol units alongside other aliphatic monomers, all crosslinked through
1290-402: A series of acetal linkages. Its complex and heterogeneous chemical structure give some protection from the biodegradative enzymes of bacteria, fungi and animals. Some aromatic structures based on p -coumarate and naringenin were also identified within the sporopollenin polymer. These can absorb ultraviolet light and thus prevent it penetrating further into the spore. This has relevance to
1376-418: A single egg cell. The gametophyte becomes a food storage tissue in the seed. In angiosperms, the megagametophyte is reduced to only a few cells, and is sometimes called the embryo sac . A typical embryo sac contains seven cells and eight nuclei, one of which is the egg cell. Two nuclei fuse with a sperm nucleus to form the primary endospermic nucleus which develops to form triploid endosperm , which becomes
1462-429: A single gametophyte is produced, consisting of four haploid cells produced by meiotic division of a diploid microspore mother cell. At maturity, each microspore-derived gametophyte becomes a pollen grain. During its development, the water and nutrients that the male gametophyte requires are provided by the sporophyte tissue until they are released for pollination. The cell number of each mature pollen grain varies between
1548-465: A single kind of gametophyte. However, not all heteromorphic gametophytes come from heterosporous plants. That is, some plants have distinct egg-producing and sperm-producing gametophytes, but these gametophytes develop from the same kind of spore inside the same sporangium; Sphaerocarpos is an example of such a plant. In seed plants, the microgametophyte is called pollen. Seed plant microgametophytes consists of several (typically two to five) cells when
1634-527: A small number feed on pollen as larvae (including some ant larvae). Spiders are normally considered carnivores but pollen is an important source of food for several species, particularly for spiderlings , which catch pollen on their webs . It is not clear how spiderlings manage to eat pollen however, since their mouths are not large enough to consume pollen grains. Some predatory mites also feed on pollen, with some species being able to subsist solely on pollen, such as Euseius tularensis , which feeds on
1720-796: A widespread anemophilous plant. Arizona was once regarded as a haven for people with pollen allergies, although several ragweed species grow in the desert. However, as suburbs grew and people began establishing irrigated lawns and gardens , more irritating species of ragweed gained a foothold and Arizona lost its claim of freedom from hay fever. Anemophilous spring blooming plants such as oak , birch , hickory , pecan , and early summer grasses may also induce pollen allergies. Most cultivated plants with showy flowers are entomophilous and do not cause pollen allergies. Symptoms of pollen allergy include sneezing , itchy, or runny nose, nasal congestion , red, itchy, and watery eyes. Substances, including pollen, that cause allergies can trigger asthma. A study found
1806-452: Is a major precursor, but other carbon sources also contribute. The biosynthetic pathway for phenylpropanoid is very active in tapetal cells, supporting the idea that its products are needed for sporopollenin synthesis. Chemical inhibitors of pollen development and many male sterile mutants have effects on the secretion of these globules by the tapetal cells. Microgametophyte A gametophyte ( / ɡ ə ˈ m iː t ə f aɪ t / )
SECTION 20
#17327837875941892-460: Is also associated with asthma exacerbations in some people, a phenomenon termed thunderstorm asthma . In the US, people often mistakenly blame the conspicuous goldenrod flower for allergies. Since this plant is entomophilous (its pollen is dispersed by animals), its heavy, sticky pollen does not become independently airborne. Most late summer and fall pollen allergies are probably caused by ragweed ,
1978-628: Is also present in a wide variety of plants and plant-derived food products. The U.S. Food and Drug Administration (FDA) has not found any harmful effects of bee pollen consumption, except for the usual allergies. However, FDA does not allow bee pollen marketers in the United States to make health claims about their produce, as no scientific basis for these has ever been proven. Furthermore, there are possible dangers not only from allergic reactions but also from contaminants such as pesticides and from fungi and bacteria growth related to poor storage procedures. A manufacturers's claim that pollen collecting helps
2064-467: Is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of palynology is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Also, pollen analysis has been widely used for reconstructing past changes in vegetation and their associated drivers. Pollen
2150-438: Is an important aspect of the pollination process. Bee pollen for human consumption is marketed as a food ingredient and as a dietary supplement . The largest constituent is carbohydrates , with protein content ranging from 7 to 35 percent depending on the plant species collected by bees. Honey produced by bees from natural sources contains pollen derived p-coumaric acid , an antioxidant and natural bactericide that
2236-519: 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 before cytokinesis , whereas in successive microsporogenesis cytokinesis follows. While there may be
2322-400: 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. In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower. Pollen
2408-700: Is described as monosulcate , has two sulci, as bisulcate , or more, as polysulcate . Colpate pollen has furrows other than across the middle of the outer faces, and similarly may be described as polycolpate if more than two. Syncolpate pollen grains have two or more colpi that are fused at the ends. Eudicots have pollen with three colpi ( tricolpate ) or with shapes that are evolutionarily derived from tricolpate pollen. The evolutionary trend in plants has been from monosulcate to polycolpate or polyporate pollen. Additionally, gymnosperm pollen grains often have air bladders, or vesicles, called sacci. The sacci are not actually balloons, but are sponge-like, and increase
2494-482: Is first found in the fossil record in the late Devonian period, but at that time it is indistinguishable from spores. It increases in abundance until the present day. Nasal allergy to pollen is called pollinosis , and allergy specifically to grass pollen is called hay fever . Generally, pollens that cause allergies are those of anemophilous plants (pollen is dispersed by air currents.) Such plants produce large quantities of lightweight pollen (because wind dispersal
2580-601: Is frequently portrayed as a sequential process that begins with placement on the vector, moves through travel, and ends with deposition. This transfer can be mediated by the wind, in which case the plant is described as anemophilous (literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, sometimes with air-sacs. Non-flowering seed plants (e.g., pine trees) are characteristically anemophilous. Anemophilous flowering plants generally have inconspicuous flowers. Entomophilous (literally insect-loving) plants produce pollen that
2666-440: Is infrequently used as food and food supplement . Because of agricultural practices, it is often contaminated by agricultural pesticides. Pollen itself is not the male gamete. It is a gametophyte , something that could be considered an entire organism, which then produces the male gamete. Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and
Pollen - Misplaced Pages Continue
2752-437: Is known as a microgametophyte . Species which produce egg and sperm on separate gametophytes plants are termed dioicous , while those that produce both eggs and sperm on the same gametophyte are termed monoicous . In heterosporous plants (water ferns, some lycophytes, as well as all gymnosperms and angiosperms), there are two distinct types of sporangia , each of which produces a single kind of spore that germinates to produce
2838-447: Is longer lived, nutritionally independent, and the sporophytes are attached to the gametophytes and dependent on them. When a moss spore germinates it grows to produce a filament of cells (called the protonema ). The mature gametophyte of mosses develops into leafy shoots that produce sex organs ( gametangia ) that produce gametes. Eggs develop in archegonia and sperm in antheridia . In some bryophyte groups such as many liverworts of
2924-426: Is one of the two alternating multicellular phases in the life cycles of plants and algae . It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the sexual phase in the life cycle of plants and algae. It develops sex organs that produce gametes , haploid sex cells that participate in fertilization to form a diploid zygote which has
3010-563: Is random and the likelihood of one pollen grain landing on another flower is small), which can be carried for great distances and are easily inhaled, bringing it into contact with the sensitive nasal passages. Pollen allergies are common in polar and temperate climate zones, where production of pollen is seasonal. In the tropics pollen production varies less by the season, and allergic reactions less. In northern Europe, common pollens for allergies are those of birch and alder , and in late summer wormwood and different forms of hay . Grass pollen
3096-403: Is relatively heavy, sticky and protein -rich, for dispersal by insect pollinators attracted to their flowers. Many insects and some mites are specialized to feed on pollen, and are called palynivores . In non-flowering seed plants, pollen germinates in the pollen chamber, located beneath the micropyle , underneath the integuments of the ovule. A pollen tube is produced, which grows into
3182-406: Is responsible, according to a new study. The researchers attributed roughly half of the lengthening pollen seasons and 8% of the trend in pollen concentrations to climate changes driven by human activity. In Denmark , decades of rising temperatures cause pollen to appear earlier and in greater amounts, exacerbated by the introduction of new species such as ragweed. The most efficient way to handle
3268-432: Is successful, the male gametophyte continues to develop. If a tube cell was not developed in the microstrobilus, one is created after pollination via mitosis. The tube cell grows into the diploid tissue of the female cone and may branch out into the megastrobilus tissue or grow straight towards the egg cell. The megastrobilus sporophytic tissue provides nutrients for the male gametophyte at this stage. In some gymnosperms,
3354-586: Is thought to be a severe underestimate as the feeding habits are only known for 1,000 species). Similarly, Ladybird beetles mainly eat insects, but many species also eat pollen, as either part or all of their diet. Hemiptera are mostly herbivores or omnivores but pollen feeding is known (and has only been well studied in the Anthocoridae ). Many adult flies, especially Syrphidae , feed on pollen, and three UK syrphid species feed strictly on pollen (syrphids, like all flies , cannot eat pollen directly due to
3440-630: Is usually well preserved in soils and sediments . The exine layer is often intricately sculptured in species-specific patterns, allowing material recovered from (for example) lake sediments to provide useful information to palynologists about plant and fungal populations in the past. Sporopollenin has found uses in the field of paleoclimatology as well. Sporopollenin is also found in the cell walls of several taxa of green alga , including Phycopeltis (an ulvophycean ) and Chlorella . Spores are dispersed by many different environmental factors, such as wind, water or animals. In suitable conditions,
3526-479: 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
Pollen - Misplaced Pages Continue
3612-528: The Rhynie chert . Other fossil gametophytes found in the Rhynie chert shows they were much more developed than present forms, resembling the sporophyte in having a well-developed conducting strand, a cortex, an epidermis and a cuticle with stomata, but were much smaller. In bryophytes ( mosses , liverworts , and hornworts ), the gametophyte is the most visible stage of the life cycle. The bryophyte gametophyte
3698-420: The buoyancy of the pollen grain and help keep it aloft in the wind, as most gymnosperms are anemophilous . Pollen can be monosaccate , (containing one saccus) or bisaccate (containing two sacci). Modern pine , spruce , and yellowwood trees all produce saccate pollen. The transfer of pollen grains to the female reproductive structure ( pistil in angiosperms) is called pollination . Pollen transfer
3784-444: The forget-me-not ( Myosotis spp.), is 2.5–5 μm (0.005 mm) in diameter. Corn pollen grains are large, about 90–100 μm. Most grass pollen is around 20–25 μm. Some pollen grains are based on geodesic polyhedra like a soccer ball . Pollen is produced in the microsporangia in the male cone of a conifer or other gymnosperm or in the anthers of an angiosperm flower . In angiosperms, during flower development
3870-408: The nucellus to provide nutrients for the developing sperm cells. Sperm cells of Pinophyta and Gnetophyta are without flagella , and are carried by the pollen tube, while those of Cycadophyta and Ginkgophyta have many flagella. When placed on the stigma of a flowering plant, under favorable circumstances, a pollen grain puts forth a pollen tube , which grows down the tissue of the style to
3956-514: The ovary , and makes its way along the placenta , guided by projections or hairs, to the micropyle of an ovule . The nucleus of the tube cell has meanwhile passed into the tube, as does also the generative nucleus, which divides (if it has not already) to form two sperm cells. The sperm cells are carried to their destination in the tip of the pollen tube. Double-strand breaks in DNA that arise during pollen tube growth appear to be efficiently repaired in
4042-438: The anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, fertile sporogenous cells, the archespore , form within the anther. The 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
4128-433: The anther. This creates a 2 or 3 celled male gametophyte which becomes known as the pollen grain once dehiscing occurs. One cell is the tube cell, and the remaining cell/cells are the sperm cells. The development of the three celled male gametophyte prior to dehiscing has evolved multiple times and is present in about a third of angiosperm species allowing for faster fertilization after pollination. Once pollination occurs,
4214-465: The bee colonies is also controversial. Pine pollen ( 송화가루 ; Songhwa Garu ) is traditionally consumed in Korea as an ingredient in sweets and beverages. The growing industries in pollen harvesting for human and bee consumption rely on harvesting pollen baskets from honey bees as they return to their hives using a pollen trap . When this pollen has been tested for parasites, it has been found that
4300-472: The body became holy when it traveled over a trail sprinkled with pollen. For agricultural research purposes, assessing the viability of pollen grains can be necessary and illuminating. A very common, efficient method to do so is known as Alexander's stain. This differential stain consists of ethanol , malachite green , distilled water , glycerol , phenol , chloral hydrate , acid fuchsin , orange G , and glacial acetic acid . (A less-toxic variation omits
4386-752: The clade that includes Ophioglossaceae and Psilotaceae , the gametophytes are subterranean and subsist by forming mycotrophic relationships with fungi. Homosporous ferns secrete a chemical called antheridiogen . Extant lycophytes produce two different types of gametophytes. In the homosporous families Lycopodiaceae and Huperziaceae , spores germinate into bisexual free-living, subterranean and mycotrophic gametophytes that derive nutrients from symbiosis with fungi. In Isoetes and Selaginella , which are heterosporous, microspores and megaspores are dispersed from sporangia either passively or by active ejection. Microspores produce microgametophytes which produce sperm. Megaspores produce reduced megagametophytes inside
SECTION 50
#17327837875944472-556: The discharge of histamine , but it has been proven that they do prevent a part of the chain reaction activated by this biogenic amine , which considerably lowers hay fever symptoms. Decongestants can be administered in different ways such as tablets and nasal sprays . Allergy immunotherapy (AIT) treatment involves administering doses of allergens to accustom the body to pollen, thereby inducing specific long-term tolerance. Allergy immunotherapy can be administered orally (as sublingual tablets or sublingual drops), or by injections under
4558-415: The dominant sporophyte tissue for nutrients and water. With the exception of mature pollen , if the gametophyte tissue is separated from the sporophyte tissue it will not survive. Due to this complex relationship and the small size of the gametophyte tissue—in some situations single celled—differentiating with the human eye or even a microscope between seed plant gametophyte tissue and sporophyte tissue can be
4644-507: The egg cell becomes the zygote which is then considered sporophyte tissue. Scholars still disagree on whether the fertilized central cell is considered gametophyte tissue. Some botanists consider this endospore as gametophyte tissue with typically 2/3 being female and 1/3 being male, but as the central cell before double fertilization can range from 1n to 8n in special cases, the fertilized central cells range from 2n (50% male/female) to 9n (1/9 male, 8/9th female). However, other botanists consider
4730-407: The female gametophyte stays singled celled. Mitosis does occur, but no cell divisions are ever made. This results in the mature female gametophyte in some Gnetophyta having many free nuclei in one cell. Once mature, this single celled gametophyte is 90% smaller than the female gametophytes in other gymnosperm orders. After fertilization, the remaining female gametophyte tissue in gymnosperms serves as
4816-416: The fertilized endospore as sporophyte tissue. Some believe it is neither. In heterosporic plants, there are two distinct kinds of gametophytes. Because the two gametophytes differ in form and function, they are termed heteromorphic , from hetero - "different" and morph "form". The egg-producing gametophyte is known as a megagametophyte , because it is typically larger, and the sperm producing gametophyte
4902-407: The fusion of developed cells. After fertilization is complete in all orders, the remaining male gametophyte tissue will deteriorate. The female gametophyte in gymnosperms differs from the male gametophyte as it spends its whole life cycle in one organ, the ovule located inside the megastrobilus or female cone. Similar to the male gametophyte, the female gametophyte normally is fully dependent on
4988-431: The gametes are isogamous , all of one size, shape and general morphology. In land plants , anisogamy is universal. As in animals, female and male gametes are called, respectively, eggs and sperm. In extant land plants, either the sporophyte or the gametophyte may be reduced (heteromorphic). No extant gametophytes have stomata , but they have been found on fossil species like the early Devonian Aglaophyton from
5074-402: The gametophyte is free-living and develops outside of the spore wall. Exosporic gametophytes can either be bisexual, capable of producing both sperm and eggs in the same thallus ( monoicous ), or specialized into separate male and female organisms (dioicous). In heterosporous vascular plants (plants that produce both microspores and megaspores), the gametophytes develop endosporically (within
5160-425: The generative cell that carries the male genomic information to be passed on to the next plant generation. However, the vegetative cell that is responsible for tube elongation appears to lack this DNA repair capability. The sporopollenin outer sheath of pollen grains affords them some resistance to the rigours of the fossilisation process that destroy weaker objects; it is also produced in huge quantities. There
5246-474: 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
SECTION 60
#17327837875945332-549: The gymnosperm orders. Cycadophyta have 3 celled pollen grains while Ginkgophyta have 4 celled pollen grains. Gnetophyta may have 2 or 3 celled pollen grains depending on the species, and Coniferophyta pollen grains vary greatly ranging from single celled to 40 celled. One of these cells is typically a germ cell and other cells may consist of a single tube cell which grows to form the pollen tube, sterile cells, and/or prothallial cells which are both vegetative cells without an essential reproductive function. After pollination
5418-540: The identification of classes of pollen. 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 . However the term inaperturate covers a wide range of morphological types, such as functionally inaperturate (cryptoaperturate) and omniaperturate. Inaperaturate pollen grains often have thin walls, which facilitates pollen tube germination at any position. Terms such as uniaperturate and triaperturate refer to
5504-557: The male gametophyte and soon degrade. The female gametophyte of angiosperms develops in the ovule (located inside the female or hermaphrodite flower ). Its precursor is a diploid megaspore that undergoes meiosis which produces four haploid daughter cells. Three of these independent gametophyte cells degenerate and the one that remains is the gametophyte mother cell which normally contains one nucleus. In general, it will then divide by mitosis until it consists of 8 nuclei separated into 1 egg cell, 3 antipodal cells , 2 synergid cells , and
5590-467: The mature female gametophyte varies drastically between gymnosperm orders. In Cycadophyta, Ginkgophyta, Coniferophyta, and some Gnetophyta, the single celled female gametophyte undergoes many cycles of mitosis ending up consisting of thousands of cells once mature. At a minimum, two of these cells are egg cells and the rest are haploid somatic cells , but more egg cells may be present and their ploidy, though typically haploid, may vary. In select Gnetophyta,
5676-467: The microsporangium, with the generative cell forming the two sperm cells. 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
5762-415: The number of apertures present (one and three respectively). Spiraperturate refers to one or more apertures being spirally shaped. 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. If the pollen has only a single sulcus, it
5848-558: The nutrient source for the developing zygote (even in Gnetophyta where the diploid zygote cell is much smaller at that stage, and for a while lives within the single celled gametophyte). The precursor to the male angiosperm gametophyte is a diploid microspore mother cell located inside the anther . Once the microspore undergoes meiosis, 4 haploid cells are formed, each of which is a singled celled male gametophyte. The male gametophyte will develop via one or two rounds of mitosis inside
5934-491: The nutritional value of the pollen In forensic biology , pollen can tell a lot about where a person or object has been, because regions of the world, or even more particular locations such a certain set of bushes, will have a distinctive collection of pollen species. Pollen evidence can also reveal the season in which a particular object picked up the pollen. Pollen has been used to trace activity at mass graves in Bosnia , catch
6020-472: The order Marchantiales , the gametes are produced on specialized structures called gametophores (or gametangiophores). All vascular plants are sporophyte dominant, and a trend toward smaller and more sporophyte-dependent female gametophytes is evident as land plants evolved reproduction by seeds. Those vascular plants, such as clubmosses and many ferns, that produce only one type of spore are said to be homosporous. They have exosporic gametophytes — that is,
6106-481: The phenol and chloral hydrate.) In angiosperms and gymnosperms non-aborted pollen grain will appear red or pink, and aborted pollen grains will appear blue or slightly green. Sporopollenin Sporopollenin is a biological polymer found as a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable (one of the most inert among biopolymers) and
6192-476: The pollen grains exit the sporangium. The megagametophyte develops within the megaspore of extant seedless vascular plants and within the megasporangium in a cone or flower in seed plants. In seed plants, the microgametophyte (pollen) travels to the vicinity of the egg cell (carried by a physical or animal vector) and produces two sperm by mitosis. In gymnosperms, the megagametophyte consists of several thousand cells and produces one to several archegonia , each with
6278-474: The pollen of dozens of plant species. Members of some beetle families such as Mordellidae and Melyridae feed almost exclusively on pollen as adults, while various lineages within larger families such as Curculionidae , Chrysomelidae , Cerambycidae , and Scarabaeidae are pollen specialists even though most members of their families are not (e.g., only 36 of 40,000 species of ground beetles , which are typically predatory, have been shown to eat pollen—but this
6364-453: The pollen wall that may involve exine thinning or a significant reduction in exine thickness. They allow shrinking and swelling of the grain caused by changes in moisture content. The process of shrinking the grain is called harmomegathy. 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
6450-555: The role of pollen and spores in transporting and dispersing the gametes of plants. The DNA of the gametes is readily damaged by the ultraviolet component of daylight. Sporopollenin thus provides some protection from this damage as well as a physically robust container. Analysis of sporopollenin from the clubmoss Lycopodium in the late 1980s have shown distinct structural differences from that of flowering plants. In 2020, more detailed analysis of sporopollenin from Lycopodium clavatum provided more structural information. It showed
6536-429: The skin (subcutaneous). Discovered by Leonard Noon and John Freeman in 1911, allergy immunotherapy represents the only causative treatment for respiratory allergies. Most major classes of predatory and parasitic arthropods contain species that eat pollen, despite the common perception that bees are the primary pollen-consuming arthropod group. Many Hymenoptera other than bees consume pollen as adults, though only
6622-399: The spore wall). These gametophytes are dioicous , producing either sperm or eggs but not both. In most ferns , for example, in the leptosporangiate fern Dryopteris , the gametophyte is a photosynthetic free living autotrophic organism called a prothallus that produces gametes and maintains the sporophyte during its early multicellular development. However, in some groups, notably
6708-496: The spore wall. At maturity, the megaspore cracks open at the trilete suture to allow the male gametes to access the egg cells in the archegonia inside. The gametophytes of Isoetes appear to be similar in this respect to those of the extinct Carboniferous arborescent lycophytes Lepidodendron and Lepidostrobus . The seed plant gametophyte life cycle is even more reduced than in basal taxa (ferns and lycophytes). Seed plant gametophytes are not independent organisms and depend upon
6794-438: The sporogenous cells. The archespore cells divide by mitosis and differentiate to form pollen mother cells (microsporocyte, meiocyte ). In a process called microsporogenesis , four haploid microspores are produced from each diploid pollen mother cell, 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
6880-412: The sporopollenin-rich walls of pollen grains and spores can persist in the fossil record for hundreds of millions of years, since sporopollenin is resistant to chemical degradation by organic and inorganic chemicals. The chemical composition of sporopollenin has long been elusive due to its unusual chemical stability, insolubility and resistance to degradation by enzymes and strong chemical reagents. It
6966-405: The structure of their mouthparts, but can consume pollen contents that are dissolved in a fluid). Some species of fungus, including Fomes fomentarius , are able to break down grains of pollen as a secondary nutrition source that is particularly high in nitrogen. Pollen may be valuable diet supplement for detritivores , providing them with nutrients needed for growth, development and maturation. It
7052-417: The surrounding sporophytic tissue for nutrients and the two organisms cannot be separated. However, the female gametophytes of Ginkgo biloba do contain chlorophyll and can produce some of their own energy, though, not enough to support itself without being supplemented by the sporophyte. The female gametophyte forms from a diploid megaspore that undergoes meiosis and starts being singled celled. The size of
7138-458: The tube cell grows in size and if the male gametophyte is only 2 cells at this stage, the single sperm cell undergoes mitosis to create a second sperm cell. Just like in gymnosperms, the tube cell in angiosperms obtains nutrients from the sporophytic tissue, and may branch out into the pistil tissue or grow directly towards the ovule. Once double fertilization is completed, the tube cell and other vegetative cells, if present, are all that remains of
7224-459: The tube cell will create a direct channel from the site of pollination to the egg cell, in other gymnosperms, the tube cell will rupture in the middle of the megastrobilus sporophyte tissue. This occurs because in some gymnosperm orders, the germ cell is nonmobile and a direct pathway is needed, however, in Cycadophyta and Ginkgophyta, the germ cell is mobile due to flagella being present and
7310-443: Was once thought to consist of polymerised carotenoids but the application of more detailed analytical methods since the 1980s has shown that this is not correct. Analyses have revealed a complex biopolymer , containing mainly long-chain fatty acids , phenylpropanoids , phenolics and traces of carotenoids in a random co-polymer. It is likely that sporopollenin derives from several precursors that are chemically cross-linked to form
7396-505: Was suggested that obtaining nutrients from pollen, deposited on the forest floor during periods of pollen rains, allows fungi to decompose nutritionally scarce litter. Some species of Heliconius butterflies consume pollen as adults, which appears to be a valuable nutrient source, and these species are more distasteful to predators than the non-pollen consuming species. Although bats , butterflies and hummingbirds are not pollen eaters per se , their consumption of nectar in flowers
#593406