The flowering plants known as angiosperms, Angiospermae or Magnoliophyta, are the most diverse group of land plants, with 64 orders, 416 families 13,164 known genera and c. 369,000 known species. Like gymnosperms, angiosperms are seed-producing plants. However, they are distinguished from gymnosperms by characteristics including flowers, endosperm within the seeds, the production of fruits that contain the seeds. Etymologically, angiosperm means a plant; the term comes from the Greek words sperma. The ancestors of flowering plants diverged from gymnosperms in the Triassic Period, 245 to 202 million years ago, the first flowering plants are known from 160 mya, they diversified extensively during the Early Cretaceous, became widespread by 120 mya, replaced conifers as the dominant trees from 100 to 60 mya. 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.
Angiosperm stems are made up of seven layers. 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 phloem form concentric rings. In the dicotyledons, the bundles in the 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, a complete ring is formed, 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 once formed the stem increases in diameter only in exceptional cases; the characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, provide the most trustworthy external characteristics for establishing relationships among angiosperm species; the function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds. The floral apparatus may arise terminally from the axil of a leaf; as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More the flower-bearing portion of the plant is distinguished from the foliage-bearing or vegetative portion, 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.
The "female" cells called megaspores, which will divide to become the egg cell, are contained in the ovule and enclosed in the carpel. The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. 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 petals. The outer series is green and leaf-like, functions to protect the rest of the flower the bud; the inner series is, in general, white or brightly colored, is more delicate in structure. It functions to attract bird pollinators. Attraction is effected by color and nectar, which may be secreted in some part of the flower; the characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans. While the majority of flowers are perfect or hermaphrodite, 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 transfer pollen to the pistil. Homomorphic flowers may employ a biochemical mechanism called self-incompatibility to discriminate between self and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers; the botanical term "Angiosperm", from the Ancient Greek αγγείον, angeíon and σπέρμα, 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; 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
The Catarrhini or catarrhine monkeys or Old World anthropoids are the sister group to the New World monkeys, the Platyrrhini. The Platyrrhini emerged within "monkeys" by migration to South America from Afro-Arabia by ocean. With respect to the ones that stayed behind, Geoffroy in 1812 grouped the apes and the Cercopithecoidea together and established the name Catarrhini, "Old World monkeys", or "singes de l'Ancien continent". Darwin in the late 19th century imagined that apes were the sister to the Cercopithecoidea. There has been some resistance to directly designate apes as monkeys despite the scientific evidence, so "Old World monkey" may be taken to mean the Cercopithecoidea or the Catarrhini; that apes are monkeys was realized by Georges-Louis Leclerc, Comte de Buffon in the 18th century. The apes are further divided into the lesser apes or gibbons and the great apes, consisting of the orangutans, gorillas and humans; the Catarrhini are all native to Asia. Members of this parvorder are called catarrhines.
The technical distinction between the New World platyrrhines and Old World catarrhines is the shape of their noses. The platyrrhines have nostrils; the catarrhines have nostrils. Catarrhines never have prehensile tails, have flat fingernails and toenails, a tubular ectotympanic, eight, not 12, giving them a dental formula of: 22.214.171.124.1.2.3. Most catarrhine species do not form a pair bond. Most, but not all, species live in social groups. Like the platyrrhines, the catarrhines are diurnal, have grasping hands and grasping feet; the apes – in both traditional and phylogenic nomenclature – are catarrhine species. In traditional usage, ape describes any tailless and more ground-dwelling species of catarrhine. "Ape" may be found as part of the common name such as the Barbary ape. In phylogenic usage, the term ape applies only to the superfamily Hominoidea; this grouping comprises the two families Hylobatidae, the lesser apes or gibbons, Hominidae, the great apes, including orangutans, chimpanzees and related extinct genera, such as the prehuman australopithecines and the giant orangutan relative Gigantopithecus.
According to Schrago & Russo, New World monkeys split from their Old World kin about 35 million years ago. They use the major Catarrhine division between Cercopithecoids and Hominoids of about 25 Mya, as a calibration point, from this calculate the gibbons separating from the great apes about 15-19 Mya. According to Begun and Harrison, the Catarrhini split from their New World monkey kin about 44 - 40 Mya, with the first catarrhines appearing in Africa and Arabia, not appearing in Eurasia until 18-17 Mya; the distinction between apes and monkeys is complicated by the traditional paraphyly of monkeys: Apes emerged as a sister group of Old World Monkeys in the catarrhines, which are a sister group of New World Monkeys. Therefore, apes and related contemporary extinct groups such as Parapithecidaea are monkeys as well, for any consistent definition of "monkey". "Old World Monkey" may legitimately be taken to be meant to include all the catarrhines, including apes and extinct species such as Aegyptopithecus, in which case the apes and Aegyptopithecus emerged within the Old World Monkeys.
Order PrimatesSuborder Strepsirrhini: lemurs, etc. Suborder Haplorhini: tarsiers + monkeys, including apes Infraorder Tarsiiformes Family Tarsiidae: tarsiers Infraorder Simiiformes: simians, or higher primates Parvorder Catarrhini Superfamily Cercopithecoidea Family Cercopithecidae Superfamily †Pliopithecoidea Family †Dionysopithecidae Family †Pliopithecidae Superfamily †Propliopithecoidea Family †Propliopithecidae Superfamily †Dendropithecoidea Family †Dendropithecidae Superfamily †Saadanioidea Family †Saadaniidae Superfamily Hominoidea Family Hylobatidae: gibbons Family Hominidae: great apes Parvorder Platyrrhini: New World monkeys Below is a cladogram with extinct species in which the crown Catharrhini emerged within the Dendropithecidae, which emerged in the Propliopithecoidea. Saadanioidea is sister of the Cercopithecoidea rather than of the Crown Catarrhini here, it is indicated. The Platyrrhini may have emerged in e.g. the Oligopithecidae. In 2018 an alternative phylogeny of the group indicated above as the Dendropithecidae was offered by Rossie and Hill with Micropithecus diverging first, as sister to the crown Catharrhini.
Diverging second were the Cercopithecoidea. A monophyletic Dendropithecidae were found as basal Hominoidea. Ekembo was found to be paraphyletic with respect to proconsul and the remaining Hominoidae, despite the original claims that it can be distinguished. Sellers, Bill. "Primate Evolution". Retrieved 2010-08-21. Raaum, Ryan L.. "Catarrhine primate divergence dates estimated from complete mitochondrial genomes: Concordance with fossil and nuclear DNA evidence". Journal of Human Evolution. 48: 237–257. Doi:10.1016/j.jhevol.2004.11.007. PMID 15737392
In botany, a fruit is the seed-bearing structure in flowering plants formed from the ovary after flowering. Fruits are the means. Edible fruits, in particular, have propagated with the movements of humans and animals in a symbiotic relationship as a means for seed dispersal and nutrition. Accordingly, fruits account for a substantial fraction of the world's agricultural output, some have acquired extensive cultural and symbolic meanings. In common language usage, "fruit" means the fleshy seed-associated structures of a plant that are sweet or sour, edible in the raw state, such as apples, grapes, lemons and strawberries. On the other hand, in botanical usage, "fruit" includes many structures that are not called "fruits", such as bean pods, corn kernels and wheat grains; the section of a fungus that produces spores is called a fruiting body. Many common terms for seeds and fruit do not correspond to the botanical classifications. In culinary terminology, a fruit is any sweet-tasting plant part a botanical fruit.
However, in botany, a fruit is the ripened ovary or carpel that contains seeds, a nut is a type of fruit and not a seed, a seed is a ripened ovule. Examples of culinary "vegetables" and nuts that are botanically fruit include corn, eggplant, sweet pepper, tomato. In addition, some spices, such as allspice and chili pepper, are fruits. In contrast, rhubarb is referred to as a fruit, because it is used to make sweet desserts such as pies, though only the petiole of the rhubarb plant is edible, edible gymnosperm seeds are given fruit names, e.g. ginkgo nuts and pine nuts. Botanically, a cereal grain, such as corn, rice, or wheat, is a kind of fruit, termed a caryopsis. However, the fruit wall is thin and is fused to the seed coat, so all of the edible grain is a seed; the outer edible layer, is the pericarp, formed from the ovary and surrounding the seeds, although in some species other tissues contribute to or form the edible portion. The pericarp may be described in three layers from outer to inner, the epicarp and endocarp.
Fruit that bears a prominent pointed terminal projection is said to be beaked. A fruit results from maturation of one or more flowers, the gynoecium of the flower forms all or part of the fruit. Inside the ovary/ovaries are one or more ovules where the megagametophyte contains the egg cell. After double fertilization, these ovules will become seeds; the ovules are fertilized in a process that starts with pollination, which involves the movement of pollen from the stamens to the stigma of flowers. After pollination, a tube grows from the pollen through the stigma into the ovary to the ovule and two sperm are transferred from the pollen to the megagametophyte. Within the megagametophyte one of the two sperm unites with the egg, forming a zygote, the second sperm enters the central cell forming the endosperm mother cell, which completes the double fertilization process; the zygote will give rise to the embryo of the seed, the endosperm mother cell will give rise to endosperm, a nutritive tissue used by the embryo.
As the ovules develop into seeds, the ovary begins to ripen and the ovary wall, the pericarp, may become fleshy, or form a hard outer covering. In some multiseeded fruits, the extent to which the flesh develops is proportional to the number of fertilized ovules; the pericarp is differentiated into two or three distinct layers called the exocarp and endocarp. In some fruits simple fruits derived from an inferior ovary, other parts of the flower, fuse with the ovary and ripen with it. In other cases, the sepals, petals and/or stamens and style of the flower fall off; when such other floral parts are a significant part of the fruit, it is called an accessory fruit. Since other parts of the flower may contribute to the structure of the fruit, it is important to study flower structure to understand how a particular fruit forms. There are three general modes of fruit development: Apocarpous fruits develop from a single flower having one or more separate carpels, they are the simplest fruits. Syncarpous fruits develop from a single gynoecium having two or more carpels fused together.
Multiple fruits form from many different flowers. Plant scientists have grouped fruits into three main groups, simple fruits, aggregate fruits, composite or multiple fruits; the groupings are not evolutionarily relevant, since many diverse plant taxa may be in the same group, but reflect how the flower organs are arranged and how the fruits develop. Simple fruits can be either dry or fleshy, result from the ripening of a simple or compound ovary in a flower with only one pistil. Dry fruits may be either dehiscent, or indehiscent. Types of dry, simple fruits, examples of each, include: achene – most seen in aggregate fruits capsule – caryopsis – cypsela – an achene-like fruit derived from the individual florets in a capitulum. Fibrous drupe – follicle – is formed from a single carpel, opens by one suture
A mouse, plural mice, is a small rodent characteristically having a pointed snout, small rounded ears, a body-length scaly tail and a high breeding rate. The best known mouse species is the common house mouse, it is a popular pet. In some places, certain kinds of field mice are locally common, they are known to invade homes for shelter. Species of mice are found in Rodentia, are present throughout the order. Typical mice are found in the genus Mus. Mice are distinguished from rats by their size; when someone discovers a smaller muroid rodent, its common name includes the term mouse, while if it is larger, the name includes the term rat. Common terms rat and mouse are not taxonomically specific. Scientifically, the term mouse is not confined to members of Mus for the deer mouse. Domestic mice sold as pets differ in size from the common house mouse; this is attributable both to different conditions in the wild. The best-known strain, the white lab mouse, has more uniform traits that are appropriate to its use in research.
Cats, wild dogs, birds of prey and certain kinds of arthropods have been known to prey upon mice. Because of its remarkable adaptability to any environment, the mouse is one of the most successful mammalian genera living on Earth today. Mice, in certain contexts, can be considered vermin which are a major source of crop damage, causing structural damage and spreading diseases through their parasites and feces. In North America, breathing dust that has come in contact with mouse excrement has been linked to hantavirus, which may lead to hantavirus pulmonary syndrome. Nocturnal animals, mice compensate for their poor eyesight with a keen sense of hearing, rely on their sense of smell to locate food and avoid predators. Mice build long intricate burrows in the wild; these have long entrances and are equipped with escape tunnels or routes. In at least one species, the architectural design of a burrow is a genetic trait. Order Dasyuromorphia marsupial mice, smaller species of Dasyuridae order Rodentia suborder Castorimorpha family Heteromyidae Kangaroo mouse, genus Microdipodops Pocket mouse, tribe Perognathinae Spiny pocket mouse, genus Heteromys suborder Anomaluromorpha family Anomaluridae flying mouse suborder Myomorpha family Cricetidae Brush mouse, Peromyscus boylii Florida mouse Golden mouse American Harvest mouse, genus Reithrodontomys family Muridae typical mice, the genus Mus Field mice, genus Apodemus Wood mouse, Apodemus sylvaticus Yellow-necked mouse, Apodemus flavicollis Large Mindoro forest mouse Big-eared hopping mouse Luzon montane forest mouse Forrest's mouse Pebble-mound mouse Bolam's mouse Eurasian Harvest mouse, genus Micromys Mice are common experimental animals in laboratory research of biology and psychology fields because they are mammals, because they share a high degree of homology with humans.
They are the most used mammalian model organism, more common than rats. The mouse genome has been sequenced, all mouse genes have human homologs; the mouse has 2.7 billion base pairs and 20 pairs of chromosomes. They can be manipulated in ways that are illegal with humans, although animal rights activists object. A knockout mouse is a genetically modified mouse that has had one or more of its genes made inoperable through a gene knockout. Reasons for common selection of mice are small size, inexpensive varied diet maintained, can reproduce quickly. Several generations of mice can be observed in a short time. Mice are very docile if raised from birth and given sufficient human contact. However, certain strains have been known to be quite temperamental. Mice and rats have the same organs in the same places, with the difference of size. Many people buy mice as companion pets, they can be playful and can grow used to being handled. Like pet rats, pet mice should not be left unsupervised outside as they have many natural predators, including birds, lizards and dogs.
Male mice tend to have a stronger odor than the females. However, mice are as pets they never need bathing. Well looked-after mice can make ideal pets; some common mouse care products are: Cage – Usually a hamster or gerbil cage, but a variety of special mouse cages are now available. Most should have a secure door. Food – Special pelleted and seed-based food is available. Mice can eat most rodent food Bedding – Usually made of hardwood pulp, such as aspen, sometimes from shredded, uninked paper or recycled virgin wood pulp. Using corn husk bedding is avoided because it promotes Aspergillus fungus, can grow mold once it gets wet, rough on their feet. In nature, mice are herbivores, consuming any kind of fruit or grain from plants. However, mice adapt well to urban areas and are known for eating all types of food scraps. In captivity, mice are fed commercial pelleted mouse diet; these diets are nutritionally complete. Mice are a staple in the diet of many small carnivores. Humans have eaten mice since prehistoric times and still eat them as a delicacy throughout eastern Zambia and northern Malawi, where they are a seasonal source of protein.
Mice are no longer consumed by humans elsewhere. However, in Victorian Britain, fried mice were still given to children as a folk remedy for bed-wetting. Prescribed cures in Ancient Egypt included mice as medicine. In Ancient Egypt, when infant
Rabbits are small mammals in the family Leporidae of the order Lagomorpha. Oryctolagus cuniculus includes the European rabbit species and its descendants, the world's 305 breeds of domestic rabbit. Sylvilagus includes 13 wild rabbit species, among them the 7 types of cottontail; the European rabbit, introduced on every continent except Antarctica, is familiar throughout the world as a wild prey animal and as a domesticated form of livestock and pet. With its widespread effect on ecologies and cultures, the rabbit is, in many areas of the world, a part of daily life—as food, clothing, a companion, as a source of artistic inspiration. Male rabbits are called bucks. An older term for an adult rabbit is coney. Another term for a young rabbit is bunny, though this term is applied informally to rabbits especially domestic ones. More the term kit or kitten has been used to refer to a young rabbit. A group of rabbits is known as a nest. A group of baby rabbits produced from a single mating is referred to as a litter, a group of domestic rabbits living together is sometimes called a herd.
Rabbits and hares were classified in the order Rodentia until 1912, when they were moved into a new order, Lagomorpha. Below are some of the species of the rabbit. Order Lagomorpha Family Leporidae Hares are precocial, born mature and mobile with hair and good vision, while rabbits are altricial, born hairless and blind, requiring closer care. Hares live a solitary life in a simple nest above the ground, while most rabbits live in social groups underground in burrows or warrens. Hares are larger than rabbits, with ears that are more elongated, with hind legs that are larger and longer. Hares have not been domesticated, while descendants of the European rabbit are bred as livestock and kept as pets. Rabbits have long been domesticated. Beginning in the Middle Ages, the European rabbit has been kept as livestock, starting in ancient Rome. Selective breeding has generated a wide variety of rabbit breeds, many of which are kept as pets; some strains of rabbit have been bred as research subjects. As livestock, rabbits are bred for their fur.
The earliest breeds were important sources of meat, so became larger than wild rabbits, but domestic rabbits in modern times range in size from dwarf to giant. Rabbit fur, prized for its softness, can be found in a broad range of coat colors and patterns, as well as lengths; the Angora rabbit breed, for example, was developed for its long, silky fur, hand-spun into yarn. Other domestic rabbit breeds have been developed for the commercial fur trade, including the Rex, which has a short plush coat; because the rabbit's epiglottis is engaged over the soft palate except when swallowing, the rabbit is an obligate nasal breather. Rabbits have two sets of one behind the other; this way they can be distinguished from rodents, with which they are confused. Carl Linnaeus grouped rabbits and rodents under the class Glires. However, recent DNA analysis and the discovery of a common ancestor has supported the view that they do share a common lineage, thus rabbits and rodents are now referred to together as members of the superorder Glires.
Since speed and agility are a rabbit's main defenses against predators, rabbits have large hind leg bones and well developed musculature. Though plantigrade at rest, rabbits are on their toes while running, assuming a more digitigrade form. Rabbits use their strong claws for defense; each front foot has four toes plus a dewclaw. Each hind foot has four toes. Most wild rabbits have full, egg-shaped bodies; the soft coat of the wild rabbit is agouti in coloration. The tail of the rabbit is dark on white below. Cottontails have white on the top of their tails; as a result of the position of the eyes in its skull, the rabbit has a field of vision that encompasses nearly 360 degrees, with just a small blind spot at the bridge of the nose. The anatomy of rabbits' hind limbs are structurally similar to that of other land mammals and contribute to their specialized form of locomotion; the Bones of the hind limbs consist of long bones as well as short bones. These bones are created through endochondral ossification during development.
Like most land mammals, the round head of the femur articulates with the acetabulum of the ox coxae. The femur articulates with the tibia, but not the fibula, fused to the tibia; the tibia and fibula articulate with the tarsals of the pes called the foot. The hind limbs of the rabbit are longer than the front limbs; this allows them to produce their hopping form of locomotion. Longer hind limbs are more capable of producing faster speeds. Hares, which have longer legs than cottontail rabbits, are able to move faster. Rabbits stay just on their toes; the hind feet have four long toes that allow for this and are webbed to prevent them from spreading when hopping. Rabbits do not have paw
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument, forming its outer layer, the nucellus, the female gametophyte in its center; the female gametophyte — termed a megagametophyte— is called the embryo sac in angiosperms. The megagametophyte produces an egg cell for the purpose of fertilization. In flowering plants, the ovule is located inside the portion of the flower called the gynoecium; the ovary of the gynoecium produces one or more ovules and becomes the fruit wall. Ovules are attached to the placenta in the ovary through a stalk-like structure known as a funiculus. Different patterns of ovule attachment, or placentation, can be found among plant species, these include: Apical placentation: The placenta is at the apex of the ovary. Simple or compound ovary. Axile placentation: The ovary is divided into radial segments, with placentas in separate locules. Ventral sutures of carpels meet at the centre of the ovary.
Placentae are along fused margins of carpels. Two or more carpels. Basal placentation: The placenta is at the base of the ovary on a protrusion of the thalamus. Simple or compound carpel, unilocular ovary. Free-central placentation: Derived from axile as partitions are absorbed, leaving ovules at the central axis. Compound unilocular ovary. Marginal placentation: Simplest type. There is only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel's margins; this is conspicuous in legumes. Simple carpel, unilocular ovary. Parietal placentation: Placentae on inner ovary wall within a non-sectioned ovary, corresponding to fused carpel margins. Two or more carpels, unilocular ovary. Superficial: Similar to axile, but placentae are on inner surfaces of multilocular ovary Ovule orientation may be anatropous, such that when inverted the micropyle faces the placenta, campylotropous, or orthotropous. In gymnosperms such as conifers, ovules are borne on the surface of an ovuliferous scale within an ovulate cone.
In some extinct plants and ovules were borne on the surface of leaves. In other extinct taxa, a cupule surrounds the ovule; the ovule appears to be a megasporangium with integuments surrounding it. Ovules are composed of diploid maternal tissue, which includes a megasporocyte. Megaspores remain inside the ovule and divide by mitosis to produce the haploid female gametophyte or megagametophyte, which remains inside the ovule; the remnants of the megasporangium tissue surround the megagametophyte. Megagametophytes produce archegonia. After fertilization, the ovule contains a diploid zygote and after cell division begins, an embryo of the next sporophyte generation. In flowering plants, a second sperm nucleus fuses with other nuclei in the megagametophyte forming a polyploid endosperm tissue, which serves as nourishment for the young sporophyte. An integument is a protective cell layer surrounding the ovule. Gymnosperms have one integument while angiosperms have two integuments; the evolutionary origin of the inner integument has been proposed to be by enclosure of a megasporangium by sterile branches.
Elkinsia, a preovulate taxon, has a lobed structure fused to the lower third of the megasporangium, with the lobes extending upwards in a ring around the megasporangium. This might, through fusion between lobes and between the structure and the megasporangium, have produced an integument; the origin of the second or outer integument has been an area of active contention for some time. The cupules of some extinct taxa have been suggested as the origin of the outer integument. A few angiosperms produce vascular tissue in the outer integument, the orientation of which suggests that the outer surface is morphologically abaxial; this suggests that cupules of the kind produced by the Caytoniales or Glossopteridales may have evolved into the outer integument of angiosperms. The integuments develop into the seed coat; the integuments do not enclose the nucellus but retain an opening at the apex referred to as the micropyle. The micropyle opening allows the pollen to enter the ovule for fertilization. In gymnosperms, the pollen is drawn into the ovule on a drop of fluid that exudes out of the micropyle, the so-called pollination drop mechanism.
Subsequently, the micropyle closes. In angiosperms, only a pollen tube enters the micropyle. During germination, the seedling's radicle emerges through the micropyle. Located opposite from the micropyle is the chalaza. Nutrients from the plant travel through the phloem of the vascular system to the funiculus and outer integument and from there apoplastically and symplastically through the chalaza to the nucellus inside the ovule. In chalazogamous plants, the pollen tubes enter the ovule through the chalaza instead of the micropyle opening; the nucellus is part of the inner structure of the ovule, forming a layer of dipl
Squamata is the largest order of reptiles, comprising lizards and amphisbaenians, which are collectively known as squamates or scaled reptiles. With over 10,000 species, it is the second-largest order of extant vertebrates, after the perciform fish, equal in number to the Saurischia. Members of the order are distinguished by their skins, which bear horny shields, they possess movable quadrate bones, making it possible to move the upper jaw relative to the neurocranium. This is visible in snakes, which are able to open their mouths wide to accommodate comparatively large prey. Squamata is the most variably sized order of reptiles, ranging from the 16 mm dwarf gecko to the 5.21 m green anaconda and the now-extinct mosasaurs, which reached lengths of over 14 m. Among other reptiles, squamates are most related to the tuatara, which superficially resembles lizards. Squamates are a monophyletic sister group to the rhynchocephalians, members of the order Rhynchocephalia; the only surviving member of Rhynchocephalia is the tuatara.
Squamata and Rhynchocephalia form the subclass Lepidosauria, the sister group to Archosauria, the clade that contains crocodiles and birds, their extinct relatives. Fossils of rhynchocephalians first appear in the Early Triassic, meaning that the lineage leading to squamates must have existed at the time. Scientists believe crown group squamates originated in the Early Jurassic based on the fossil record; the first fossils of geckos and snakes appear in the Middle Jurassic. Other groups like iguanians and varanoids appeared in the Cretaceous. Polyglyphanodontians, a distinct clade of lizards, mosasaurs, a group of predatory marine lizards that grew to enormous sizes appeared in the Cretaceous. Squamates suffered a mass extinction at the Cretaceous–Paleogene boundary, which wiped out polyglyphanodontians and many other distinct lineages; the relationships of squamates is debatable. Although many of the groups recognized on the basis of morphology are still accepted, our understanding of their relationships to each other has changed radically as a result of studying their genomes.
Iguanians were long thought to be the earliest crown group squamates based on morphological data, genetic data suggests that geckoes are the earliest crown group squamates. Iguanians are now united with anguimorphs in a clade called Toxicofera. Genetic data suggests that the various limbless groups. A study in 2018 found that Megachirella, an extinct genus of lepidosaur that lived about 240 million years ago during the Middle Triassic, was a stem-squamate, making it the oldest known squamate; the phylogenetic analysis was conducted by performing high-resolution microfocus X-ray computed tomography scans on the fossil specimen of Megachirella to gather detailed data about its anatomy. This data was compared with a phylogenetic dataset combining the morphological and molecular data of 129 extant and extinct reptilian taxa; the comparison revealed. The study found that geckos are the earliest crown group squamates not iguanians; the male members of the group Squamata have hemipenes, which are held inverted within their bodies, are everted for reproduction via erectile tissue like that in the human penis.
Only one is used at a time, some evidence indicates that males alternate use between copulations. The hemipenis has a variety of shapes, depending on the species, it bears spines or hooks, to anchor the male within the female. Some species have forked hemipenes. Due to being everted and inverted, hemipenes do not have a enclosed channel for the conduction of sperm, but rather a seminal groove that seals as the erectile tissue expands; this is the only reptile group in which both viviparous and ovoviviparous species are found, as well as the usual oviparous reptiles. Some species, such as the Komodo dragon, can reproduce asexually through parthenogenesis. There have been studies on how sexual selection manifests itself in lizards. Snakes use a variety of tactics in acquiring mates. Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids, in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward.
It is common for neck biting to occur. Parthenogenesis is a natural form of reproduction in which the growth and development of embryos occur without fertilization. Agkistrodon contortrix and Agkistrodon piscivorus can reproduce by facultative parthenogenesis; that is, they are capable of switching from a sexual mode of reproduction to an asexual mode. The type of parthenogenesis that occurs is automixis with terminal fusion, a process in which two terminal products from the same meiosis fuse to form a diploid zygote; this process leads to genome wide homozygosity, expression of deleterious recessive alleles and to developmental abnormalities. Both captive-born and wild-born A. contortrix and A. piscivorus appear to be capable of this form of parthenogenesis. Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, females a ZW pair. However, the Colombian Rainbow boa, Epicrates maurus, can reproduce by facultative parthenogenesis resulting in production of WW female pr