89-502: The Hylocereeae are a tribe of cacti . Most are found in the tropical forests of Central and northern South America , and are climbers or epiphytes , unlike most cacti. The tribe includes between six and eight genera in different circumscriptions . The plants known as " epiphyllum hybrids " or "epiphyllums", widely grown for their flowers, are hybrids of species within this tribe, particularly Disocactus , Pseudorhipsalis and Selenicereus , less often Epiphyllum , in spite of
178-453: A pericarpel . Tissue derived from the petals and sepals continues the pericarpel, forming a composite tube—the whole may be called a floral tube, although strictly speaking only the part furthest from the base is floral in origin. The outside of the tubular structure often has areoles that produce wool and spines. Typically, the tube also has small scale-like bracts , which gradually change into sepal-like and then petal-like structures, so
267-464: A cactus. Stem shapes vary considerably among cacti. The cylindrical shape of columnar cacti and the spherical shape of globular cacti produce a low surface area-to-volume ratio, thus reducing water loss, as well as minimizing the heating effects of sunlight. The ribbed or fluted stems of many cacti allow the stem to shrink during periods of drought and then swell as it fills with water during periods of availability. A mature saguaro ( Carnegiea gigantea )
356-547: A cluster may share a common root. Other cacti have a quite different appearance. In tropical regions, some grow as forest climbers and epiphytes . Their stems are typically flattened and almost leaf-like in appearance, with few or even no spines. Climbing cacti can be very large; a specimen of Hylocereus was reported as 100 meters (330 ft) long from root to the most distant stem. Epiphytic cacti, such as species of Rhipsalis or Schlumbergera , often hang downwards, forming dense clumps where they grow in trees high above
445-464: A continuous supply of CO 2 during photosynthesis means the stomata must be open, so water vapor is continuously being lost. Plants using the C 3 mechanism lose as much as 97% of the water taken up through their roots in this way. A further problem is that as temperatures rise, the enzyme that captures CO 2 starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%. Crassulacean acid metabolism (CAM)
534-411: A drought. The concentration of salts in the root cells of cacti is relatively high. All these adaptations enable cacti to absorb water rapidly during periods of brief or light rainfall. Thus, Ferocactus cylindraceus reportedly can take up a significant amount of water within 12 hours from as little as 7 mm (0.3 in) of rainfall, becoming fully hydrated in a few days. Although in most cacti,
623-415: A greater volume than the body. Taproots may aid in stabilizing the larger columnar cacti. Climbing, creeping and epiphytic cacti may have only adventitious roots , produced along the stems where these come into contact with a rooting medium. Like their spines, cactus flowers are variable. Typically, the ovary is surrounded by material derived from stem or receptacle tissue, forming a structure called
712-415: A high surface area-to-volume ratio, at maturity they contain little or no water, being composed of fibers made up of dead cells. Spines provide protection from herbivores and camouflage in some species, and assist in water conservation in several ways. They trap air near the surface of the cactus, creating a moister layer that reduces evaporation and transpiration . They can provide some shade, which lowers
801-418: A normal shoot, nodes bearing leaves or flowers would be separated by lengths of stem (internodes). In an areole, the nodes are so close together, they form a single structure. The areole may be circular, elongated into an oval shape, or even separated into two parts; the two parts may be visibly connected in some way (e.g. by a groove in the stem) or appear entirely separate (a dimorphic areole). The part nearer
890-466: A particular plant is an example of, say, Mammillaria mammillaris , they should be able to compare it with the type specimen to which this name is permanently attached. Type specimens are normally prepared by compression and drying, after which they are stored in herbaria to act as definitive references. However, cacti are very difficult to preserve in this way; they have evolved to resist drying and their bodies do not easily compress. A further difficulty
979-545: A protective response. The first such plant receptors were identified in rice and in Arabidopsis thaliana . Plants have some of the largest genomes of all organisms. The largest plant genome (in terms of gene number) is that of wheat ( Triticum aestivum ), predicted to encode ≈94,000 genes and thus almost 5 times as many as the human genome . The first plant genome sequenced was that of Arabidopsis thaliana which encodes about 25,500 genes. In terms of sheer DNA sequence,
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#17330862262021068-420: A range of physical and biotic stresses which cause DNA damage , but they can tolerate and repair much of this damage. Plants reproduce to generate offspring, whether sexually , involving gametes , or asexually , involving ordinary growth. Many plants use both mechanisms. When reproducing sexually, plants have complex lifecycles involving alternation of generations . One generation, the sporophyte , which
1157-620: A specimen, which, according to David Hunt , ensured he "left a trail of nomenclatural chaos that will probably vex cactus taxonomists for centuries." In 1984, it was decided that the Cactaceae Section of the International Organization for Succulent Plant Study should set up a working party, now called the International Cactaceae Systematics Group (ICSG), to produce consensus classifications down to
1246-554: A spiny plant whose identity is now not certain. Cacti occur in a wide range of shapes and sizes. They are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north, with the exception of Rhipsalis baccifera , which is also found in Africa and Sri Lanka . Cacti are adapted to live in very dry environments, including the Atacama Desert , one of
1335-404: Is diploid (with 2 sets of chromosomes ), gives rise to the next generation, the gametophyte , which is haploid (with one set of chromosomes). Some plants also reproduce asexually via spores . In some non-flowering plants such as mosses, the sexual gametophyte forms most of the visible plant. In seed plants (gymnosperms and flowering plants), the sporophyte forms most of the visible plant, and
1424-433: Is a mechanism adopted by cacti and other succulents to avoid the problems of the C 3 mechanism. In full CAM, the stomata open only at night, when temperatures and water loss are lowest. CO 2 enters the plant and is captured in the form of organic acids stored inside cells (in vacuoles ). The stomata remain closed throughout the day, and photosynthesis uses only this stored CO 2 . CAM uses water much more efficiently at
1513-547: Is a similar process. Structures such as runners enable plants to grow to cover an area, forming a clone . Many plants grow food storage structures such as tubers or bulbs which may each develop into a new plant. Some non-flowering plants, such as many liverworts, mosses and some clubmosses, along with a few flowering plants, grow small clumps of cells called gemmae which can detach and grow. Plants use pattern-recognition receptors to recognize pathogens such as bacteria that cause plant diseases. This recognition triggers
1602-401: Is close to the ancestral species from which all cacti evolved) does have long-lasting leaves, which are, however, thickened and succulent in many species. Other species of cactus with long-lasting leaves, such as the opuntioid Pereskiopsis , also have succulent leaves. A key issue in retaining water is the ratio of surface area to volume. Water loss is proportional to surface area, whereas
1691-546: Is considered close to the ancestral species from which all cacti evolved. In tropical regions, other cacti grow as forest climbers and epiphytes (plants that grow on trees). Their stems are typically flattened, almost leaf-like in appearance, with fewer or even no spines, such as the well-known Christmas cactus or Thanksgiving cactus (in the genus Schlumbergera ). Cacti have a variety of uses: many species are used as ornamental plants, others are grown for fodder or forage, and others for food (particularly their fruit). Cochineal
1780-537: Is derived through Latin from the Ancient Greek κάκτος ( kaktos ), a name used by Theophrastus for a spiny plant, which may have been the cardoon ( Cynara cardunculus ). Later botanists, such as Philip Miller in 1754, divided cacti into several genera, which, in 1789, Antoine Laurent de Jussieu placed in his newly created family Cactaceae. By the early 20th century, botanists came to feel Linnaeus's name Cactus had become so confused as to its meaning (was it
1869-496: Is known as botany , a branch of biology . All living things were traditionally placed into one of two groups, plants and animals . This classification dates from Aristotle (384–322 BC), who distinguished different levels of beings in his biology , based on whether living things had a "sensitive soul" or like plants only a "vegetative soul". Theophrastus , Aristotle's student, continued his work in plant taxonomy and classification. Much later, Linnaeus (1707–1778) created
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#17330862262021958-835: Is particularly true of tree-living cacti, such as Rhipsalis and Schlumbergera , but also of some ground-living cacti, such as Ariocarpus . The spines of cacti are often useful in identification, since they vary greatly between species in number, color, size, shape and hardness, as well as in whether all the spines produced by an areole are similar or whether they are of distinct kinds. Most spines are straight or at most slightly curved, and are described as hair-like, bristle-like, needle-like or awl-like, depending on their length and thickness. Some cacti have flattened spines (e.g. Sclerocactus papyracanthus ). Other cacti have hooked spines. Sometimes, one or more central spines are hooked, while outer spines are straight (e.g., Mammillaria rekoi ). In addition to normal-length spines, members of
2047-474: Is present in Pereskia species. By studying the ratio of C to C incorporated into a plant—its isotopic signature —it is possible to deduce how much CO 2 is taken up at night and how much in the daytime. Using this approach, most of the Pereskia species investigated exhibit some degree of CAM-cycling, suggesting this ability was present in the ancestor of all cacti. Pereskia leaves are claimed to only have
2136-557: Is said to be able to absorb as much as 200 U.S. gallons (760 L; 170 imp gal) of water during a rainstorm. A few species differ significantly in appearance from most of the family. At least superficially, plants of the genera Leuenbergeria , Rhodocactus and Pereskia resemble other trees and shrubs growing around them. They have persistent leaves, and when older, bark-covered stems. Their areoles identify them as cacti, and in spite of their appearance, they, too, have many adaptations for water conservation . Leuenbergeria
2225-416: Is said to be able to absorb as much as 200 U.S. gallons (760 L; 170 imp gal) of water during a rainstorm. The outer layer of the stem usually has a tough cuticle , reinforced with waxy layers, which reduce water loss. These layers are responsible for the grayish or bluish tinge to the stem color of many cacti. The stems of most cacti have adaptations to allow them to conduct photosynthesis in
2314-461: Is that many cacti were given names by growers and horticulturalists rather than botanists; as a result, the provisions of the International Code of Nomenclature for algae, fungi, and plants (which governs the names of cacti, as well as other plants) were often ignored. Curt Backeberg , in particular, is said to have named or renamed 1,200 species without one of his names ever being attached to
2403-721: Is the product of an insect that lives on some cacti. Many succulent plants in both the Old and New World – such as some Euphorbiaceae (euphorbias) – are also spiny stem succulents and because of this are sometimes incorrectly referred to as "cactus". The 1,500 to 1,800 species of cacti mostly fall into one of two groups of "core cacti": opuntias (subfamily Opuntioideae ) and "cactoids" (subfamily Cactoideae ). Most members of these two groups are easily recognizable as cacti. They have fleshy succulent stems that are major organs of photosynthesis . They have absent, small, or transient leaves . They have flowers with ovaries that lie below
2492-402: The C 3 mechanism with CAM restricted to stems. More recent studies show that "it is highly unlikely that significant carbon assimilation occurs in the stem"; Pereskia species are described as having "C 3 with inducible CAM." Leafless cacti carry out all their photosynthesis in the stem, using full CAM. As of February 2012 , it is not clear whether stem-based CAM evolved once only in
2581-679: The Cretaceous so rapid that Darwin called it an " abominable mystery ". Conifers diversified from the Late Triassic onwards, and became a dominant part of floras in the Jurassic . In 2019, a phylogeny based on genomes and transcriptomes from 1,153 plant species was proposed. The placing of algal groups is supported by phylogenies based on genomes from the Mesostigmatophyceae and Chlorokybophyceae that have since been sequenced. Both
2670-457: The carpels or ovaries , which develop into fruits that contain seeds . Fruits may be dispersed whole, or they may split open and the seeds dispersed individually. Plants reproduce asexually by growing any of a wide variety of structures capable of growing into new plants. At the simplest, plants such as mosses or liverworts may be broken into pieces, each of which may regrow into whole plants. The propagation of flowering plants by cuttings
2759-618: The eukaryotes that form the kingdom Plantae ; they are predominantly photosynthetic . This means that they obtain their energy from sunlight , using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars from carbon dioxide and water, using the green pigment chlorophyll . Exceptions are parasitic plants that have lost the genes for chlorophyll and photosynthesis, and obtain their energy from other plants or fungi. Most plants are multicellular , except for some green algae. Historically, as in Aristotle's biology ,
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2848-443: The monophyly of three of these subfamilies (not Pereskioideae), but have not supported all of the tribes or even genera below this level; indeed, a 2011 study found only 39% of the genera in the subfamily Cactoideae sampled in the research were monophyletic . Classification of the cacti currently remains uncertain and is likely to change. A 2005 study suggested the genus Pereskia as then circumscribed ( Pereskia sensu lato)
2937-441: The sepals and petals , often deeply sunken into a fleshy receptacle (the part of the stem from which the flower parts grow). All cacti have areoles —highly specialized short shoots with extremely short internodes that produce spines , normal shoots, and flowers. The remaining cacti fall into only two groups: three tree-like genera, Leuenbergeria , Pereskia and Rhodocactus (all formerly placed in Pereskia ), and
3026-1003: The "chlorophyte algae" and the "streptophyte algae" are treated as paraphyletic (vertical bars beside phylogenetic tree diagram) in this analysis, as the land plants arose from within those groups. The classification of Bryophyta is supported both by Puttick et al. 2018, and by phylogenies involving the hornwort genomes that have also since been sequenced. Rhodophyta [REDACTED] Glaucophyta [REDACTED] Chlorophyta [REDACTED] Prasinococcales Mesostigmatophyceae Chlorokybophyceae Spirotaenia [REDACTED] Klebsormidiales [REDACTED] Chara [REDACTED] Coleochaetales [REDACTED] Hornworts [REDACTED] Liverworts [REDACTED] Mosses [REDACTED] Lycophytes [REDACTED] [REDACTED] Gymnosperms [REDACTED] Angiosperms [REDACTED] Plant cells have distinctive features that other eukaryotic cells (such as those of animals) lack. These include
3115-484: The "columns" may be horizontal rather than vertical. Thus, Stenocereus eruca can be described as columnar even though it has stems growing along the ground, rooting at intervals. Cacti whose stems are even smaller may be described as globular (or globose). They consist of shorter, more ball-shaped stems than columnar cacti. Globular cacti may be solitary, such as Ferocactus latispinus , or their stems may form clusters that can create large mounds. All or some stems in
3204-406: The 2017 study proposed revised circumscriptions of some of the genera and of the tribe to make them monophyletic according to their molecular phylogeny: The table below compares the 1993 and 2017 classifications. The tribe Hylocereeae includes eight genera in the 2017 classification by Korotkova et al. As of March 2021, the acceptance of some of these genera varied. For example, Plants of
3293-480: The Cactaceae A cactus ( pl. : cacti , cactuses , or less commonly, cactus ) is a member of the plant family Cactaceae ( / k æ k ˈ t eɪ s i . iː , - ˌ aɪ / ), a family of the order Caryophyllales comprising about 127 genera with some 1,750 known species. The word cactus derives, through Latin, from the Ancient Greek word κάκτος ( káktos ), a name originally used by Theophrastus for
3382-857: The Viridiplantae, along with the red algae and the glaucophytes , in the clade Archaeplastida . There are about 380,000 known species of plants, of which the majority, some 260,000, produce seeds . They range in size from single cells to the tallest trees . Green plants provide a substantial proportion of the world's molecular oxygen; the sugars they create supply the energy for most of Earth's ecosystems and other organisms , including animals, either eat plants directly or rely on organisms which do so. Grain , fruit , and vegetables are basic human foods and have been domesticated for millennia. People use plants for many purposes , such as building materials , ornaments, writing materials , and, in great variety, for medicines . The scientific study of plants
3471-593: The World Online accepted Aporocactus and the sinking of Hylocereus into Selenicereus , but did not accept Kimnachia . Selenicereus is the largest genus in the tribe, and is native from Texas though Central America and the Caribbean into South America as far as Northeast Argentina . Other genera have a more restricted distribution within this area; for example, the two species of Aporocactus are native to Mexico . Cactus See also Classification of
3560-403: The absence of leaves. This is discussed further below under Metabolism . Many cacti have roots that spread out widely, but only penetrate a short distance into the soil. In one case, a young saguaro only 12 cm (4.7 in) tall had a root system with a diameter of 2 m (7 ft), but no more than 10 cm (4 in) deep. Cacti can also form new roots quickly when rain falls after
3649-801: The amount of cytoplasm stays the same. Most plants are multicellular . Plant cells differentiate into multiple cell types, forming tissues such as the vascular tissue with specialized xylem and phloem of leaf veins and stems , and organs with different physiological functions such as roots to absorb water and minerals, stems for support and to transport water and synthesized molecules, leaves for photosynthesis, and flowers for reproduction. Plants photosynthesize , manufacturing food molecules ( sugars ) using energy obtained from light . Plant cells contain chlorophylls inside their chloroplasts, which are green pigments that are used to capture light energy. The end-to-end chemical equation for photosynthesis is: This causes plants to release oxygen into
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3738-413: The amount of water present is proportional to volume. Structures with a high surface area-to-volume ratio, such as thin leaves, necessarily lose water at a higher rate than structures with a low area-to-volume ratio, such as thickened stems. Spines , which are modified leaves, are present on even those cacti with true leaves, showing the evolution of spines preceded the loss of leaves. Although spines have
3827-431: The atmosphere. Green plants provide a substantial proportion of the world's molecular oxygen, alongside the contributions from photosynthetic algae and cyanobacteria. Plants that have secondarily adopted a parasitic lifestyle may lose the genes involved in photosynthesis and the production of chlorophyll. Growth is determined by the interaction of a plant's genome with its physical and biotic environment. Factors of
3916-461: The basis of the modern system of scientific classification , but retained the animal and plant kingdoms , naming the plant kingdom the Vegetabilia. When the name Plantae or plant is applied to a specific group of organisms or taxa , it usually refers to one of four concepts. From least to most inclusive, these four groupings are: There are about 382,000 accepted species of plants, of which
4005-573: The cactus and providing some shade. In the absence of true leaves, cacti's enlarged stems carry out photosynthesis . Cactus spines are produced from specialized structures called areoles , a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers , which are usually tubular and multipetaled. Many cacti have short growing seasons and long dormancies and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system that quickly absorbs any water reaching
4094-611: The common name. The members of the tribe are very variable in their morphology, especially when the terrestrial Acanthocereus is included. Many species form aerial roots. The hylocereoid clade ( Selenicereus , Weberocereus and probably Aporocactus ) are mostly climbing or epiphytic , and have spiny ribbed stems. The phyllocactoid clade ( Epiphyllum , Disocactus , Kimnachia and Pseudorhipsalis ) are mainly epiphytic, and have spineless flattened leaf-like stems. Flowers and pollination syndromes are equally diverse, ranging from large white nocturnal flowers to bright red flowers opening in
4183-600: The core cacti, or separately in the opuntias and cactoids; CAM is known to have evolved convergently many times. To carry out photosynthesis, cactus stems have undergone many adaptations. Early in their evolutionary history, the ancestors of modern cacti (other than Leuenbergeria species) developed stomata on their stems and began to delay developing bark. However, this alone was not sufficient; cacti with only these adaptations appear to do very little photosynthesis in their stems. Stems needed to develop structures similar to those normally found only in leaves. Immediately below
4272-542: The daytime. The group was first identified by Britton and Rose in 1920 as the subtribe Hylocereinae of the tribe Cereeae (subtribe Hylocereanae of tribe Cereanae in their terminology). In 1958, Buxbaum revised the subtribe Hylocereinae, placing it in a newly created tribe Hylocereeae, which included four other subtribes, some containing genera removed from Hylocereinae, others containing different genera. The International Cactaceae Systematics Group classification, based on Barthlott & Hunt (1993), recognized six genera within
4361-483: The development of forests in swampy environments dominated by clubmosses and horsetails, including some as large as trees, and the appearance of early gymnosperms , the first seed plants . The Permo-Triassic extinction event radically changed the structures of communities. This may have set the scene for the evolution of flowering plants in the Triassic (~ 200 million years ago ), with an adaptive radiation in
4450-521: The driest places on Earth. Because of this, cacti show many adaptations to conserve water. For example, almost all cacti are succulents , meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Most species of cacti have lost true leaves, retaining only spines , which are highly modified leaves. As well as defending against herbivores , spines help prevent water loss by reducing air flow close to
4539-669: The end into more than one stigma . The stamens usually arise from all over the inner surface of the upper part of the floral tube, although in some cacti, the stamens are produced in one or more distinct "series" in more specific areas of the inside of the floral tube. The flower as a whole is usually radially symmetrical ( actinomorphic ), but may be bilaterally symmetrical ( zygomorphic ) in some species. Flower colors range from white through yellow and red to magenta. All cacti have some adaptations to promote efficient water use. Most cacti— opuntias and cactoids —specialize in surviving in hot and dry environments (i.e. are xerophytes ), but
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#17330862262024628-490: The ends of stems, which are still growing and forming new areoles. In Pereskia , a genus close to the ancestor of cacti, areoles remain active for much longer; this is also the case in Opuntia and Neoraimondia . The great majority of cacti have no visible leaves ; photosynthesis takes place in the stems (which may be flattened and leaflike in some species). Exceptions occur in three (taxonomically, four) groups of cacti. All
4717-404: The first ancestors of modern cacti were already adapted to periods of intermittent drought. A small number of cactus species in the tribes Hylocereeae and Rhipsalideae have become adapted to life as climbers or epiphytes , often in tropical forests, where water conservation is less important. The absence of visible leaves is one of the most striking features of most cacti. Pereskia (which
4806-409: The gametophyte is very small. Flowering plants reproduce sexually using flowers, which contain male and female parts: these may be within the same ( hermaphrodite ) flower, on different flowers on the same plant , or on different plants . The stamens create pollen , which produces male gametes that enter the ovule to fertilize the egg cell of the female gametophyte. Fertilization takes place within
4895-570: The genera Leuenbergeria , Pereskia and Rhodocactus , the branches are covered with leaves, so the species of these genera may not be recognized as cacti. In most other cacti, the branches are more typically cactus-like, bare of leaves and bark and covered with spines, as in Pachycereus pringlei or the larger opuntias . Some cacti may become tree-sized but without branches, such as larger specimens of Echinocactus platyacanthus . Cacti may also be described as shrubby , with several stems coming from
4984-535: The genera and tribe). A 2020 phylogenetic study, which also included morphological features of the stem, produced the righthand cladogram below. This resolved the position of Aporocactus , placing it within the hyloceroid clade, but left relationships within the phyllocactoid clade unresolved. Acanthocereus Aporocactus Selenicereus Weberocereus Pseudorhipsalis Kimnachia Epiphyllum Disocactus The authors of
5073-874: The genus Mammillaria and outgrowths almost like leaves in Ariocarpus species. The stem may also be ribbed or fluted in shape. The prominence of these ribs depends on how much water the stem is storing: when full (up to 90% of the mass of a cactus may be water), the ribs may be almost invisible on the swollen stem, whereas when the cactus is short of water and the stems shrink, the ribs may be very visible. The stems of most cacti are some shade of green, often bluish or brownish green. Such stems contain chlorophyll and are able to carry out photosynthesis; they also have stomata (small structures that can open and close to allow passage of gases). Cactus stems are often visibly waxy. Areoles are structures unique to cacti. Although variable, they typically appear as woolly or hairy areas on
5162-529: The genus or the family?) that it should not be used as a genus name. The 1905 Vienna botanical congress rejected the name Cactus and instead declared Mammillaria was the type genus of the family Cactaceae. It did, however, conserve the name Cactaceae, leading to the unusual situation in which the family Cactaceae no longer contains the genus after which it was named. The difficulties continued, partly because giving plants scientific names relies on " type specimens ". Ultimately, if botanists want to know whether
5251-575: The great majority, some 283,000, produce seeds . The table below shows some species count estimates of different green plant (Viridiplantae) divisions . About 85–90% of all plants are flowering plants. Several projects are currently attempting to collect records on all plant species in online databases, e.g. the World Flora Online . Plants range in scale from single-celled organisms such as desmids (from 10 micrometres (μm) across) and picozoa (less than 3 μm across), to
5340-558: The ground or from branches very low down, such as in Stenocereus thurberi . Smaller cacti may be described as columnar. They consist of erect, cylinder-shaped stems, which may or may not branch, without a very clear division into trunk and branches. The boundary between columnar forms and tree-like or shrubby forms is difficult to define. Smaller and younger specimens of Cephalocereus senilis , for example, are columnar, whereas older and larger specimens may become tree-like. In some cases,
5429-552: The ground surface. Cactus stems are often ribbed or fluted with a number of ribs which corresponds to a number in the Fibonacci numbers (2, 3, 5, 8, 13, 21, 34 etc.). This allows them to expand and contract easily for quick water absorption after rain, followed by retention over long drought periods. Like other succulent plants, most cacti employ a special mechanism called " crassulacean acid metabolism " (CAM) as part of photosynthesis. Transpiration , during which carbon dioxide enters
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#17330862262025518-501: The ground. The leafless, spiny stem is the characteristic feature of the majority of cacti (all belonging to the largest subfamily, the Cactoideae ). The stem is typically succulent, meaning it is adapted to store water. The surface of the stem may be smooth (as in some species of Opuntia ) or covered with protuberances of various kinds, which are usually called tubercles. These vary from small "bumps" to prominent, nipple-like shapes in
5607-441: The growing season and then lost (as in many species of Opuntia ). The small genus Maihuenia also relies on leaves for photosynthesis. The structure of the leaves varies somewhat between these groups. Opuntioids and Maihuenia have leaves that appear to consist only of a midrib. Even those cacti without visible photosynthetic leaves do usually have very small leaves, less than 0.5 mm (0.02 in) long in about half of
5696-401: The highest possible volume for water storage with the lowest possible surface area for water loss from transpiration . The tallest free-standing cactus is Pachycereus pringlei , with a maximum recorded height of 19.2 m (63 ft), and the smallest is Blossfeldia liliputiana , only about 1 cm (0.4 in) in diameter at maturity. A fully grown saguaro ( Carnegiea gigantea )
5785-517: The land 1,200 million years ago , but it was not until the Ordovician , around 450 million years ago , that the first land plants appeared, with a level of organisation like that of bryophytes. However, fossils of organisms with a flattened thallus in Precambrian rocks suggest that multicellular freshwater eukaryotes existed over 1000 mya. Primitive land plants began to diversify in
5874-412: The large water-filled central vacuole , chloroplasts , and the strong flexible cell wall , which is outside the cell membrane . Chloroplasts are derived from what was once a symbiosis of a non-photosynthetic cell and photosynthetic cyanobacteria . The cell wall, made mostly of cellulose , allows plant cells to swell up with water without bursting. The vacuole allows the cell to change in size while
5963-507: The largest trees ( megaflora ) such as the conifer Sequoia sempervirens (up to 120 metres (380 ft) tall) and the angiosperm Eucalyptus regnans (up to 100 m (325 ft) tall). The naming of plants is governed by the International Code of Nomenclature for algae, fungi, and plants and the International Code of Nomenclature for Cultivated Plants . The ancestors of land plants evolved in water. An algal scum formed on
6052-656: The late Silurian , around 420 million years ago . Bryophytes, club mosses, and ferns then appear in the fossil record. Early plant anatomy is preserved in cellular detail in an early Devonian fossil assemblage from the Rhynie chert . These early plants were preserved by being petrified in chert formed in silica-rich volcanic hot springs. By the end of the Devonian, most of the basic features of plants today were present, including roots, leaves and secondary wood in trees such as Archaeopteris . The Carboniferous period saw
6141-483: The level of genera. Their system has been used as the basis of subsequent classifications. Detailed treatments published in the 21st century have divided the family into around 125–130 genera and 1,400–1,500 species, which are then arranged into a number of tribes and subfamilies. The ICSG classification of the cactus family recognized four subfamilies, the largest of which was divided into nine tribes. The subfamilies were: Molecular phylogenetic studies have supported
6230-455: The much smaller Maihuenia . These two groups are rather different from other cacti, which means any description of cacti as a whole must frequently make exceptions for them. Species of the first three genera superficially resemble other tropical forest trees. When mature, they have woody stems that may be covered with bark and long-lasting leaves that provide the main means of photosynthesis. Their flowers may have superior ovaries (i.e., above
6319-421: The outer epidermis, a hypodermal layer developed made up of cells with thickened walls, offering mechanical support. Air spaces were needed between the cells to allow carbon dioxide to diffuse inwards. The center of the stem, the cortex, developed " chlorenchyma " – a plant tissue made up of relatively unspecialized cells containing chloroplasts , arranged into a "spongy layer" and a " palisade layer " where most of
6408-449: The photosynthesis occurs. Naming and classifying cacti has been both difficult and controversial since the first cacti were discovered for science. The difficulties began with Carl Linnaeus . In 1737, he placed the cacti he knew into two genera, Cactus and Pereskia . However, when he published Species Plantarum in 1753—the starting point for modern botanical nomenclature—he relegated them all to one genus, Cactus . The word "cactus"
6497-501: The physical or abiotic environment include temperature , water , light, carbon dioxide , and nutrients in the soil. Biotic factors that affect plant growth include crowding, grazing, beneficial symbiotic bacteria and fungi, and attacks by insects or plant diseases . Frost and dehydration can damage or kill plants. Some plants have antifreeze proteins , heat-shock proteins and sugars in their cytoplasm that enable them to tolerate these stresses . Plants are continuously exposed to
6586-442: The plant and water escapes, does not take place during the day at the same time as photosynthesis, but instead occurs at night. The plant stores the carbon dioxide it takes in as malic acid , retaining it until daylight returns, and only then using it in photosynthesis. Because transpiration takes place during the cooler, more humid night hours, water loss is significantly reduced. Many smaller cacti have globe-shaped stems, combining
6675-538: The plant kingdom encompassed all living things that were not animals , and included algae and fungi . Definitions have narrowed since then; current definitions exclude the fungi and some of the algae. By the definition used in this article, plants form the clade Viridiplantae (green plants), which consists of the green algae and the embryophytes or land plants ( hornworts , liverworts , mosses , lycophytes , ferns , conifers and other gymnosperms , and flowering plants ). A definition based on genomes includes
6764-453: The points of attachment of the sepals and petals) and areoles that produce further leaves. The two species of Maihuenia have succulent but non-photosynthetic stems and prominent succulent leaves. Cacti show a wide variety of growth habits , which are difficult to divide into clear, simple categories. Cacti can be tree-like (arborescent), meaning they typically have a single more-or-less woody trunk topped by several to many branches . In
6853-453: The price of limiting the amount of carbon fixed from the atmosphere and thus available for growth. CAM-cycling is a less water-efficient system whereby stomata open in the day, just as in plants using the C 3 mechanism. At night, or when the plant is short of water, the stomata close and the CAM mechanism is used to store CO 2 produced by respiration for use later in photosynthesis. CAM-cycling
6942-431: The sepals and petals cannot be clearly differentiated (and hence are often called " tepals "). Some cacti produce floral tubes without wool or spines (e.g. Gymnocalycium ) or completely devoid of any external structures (e.g. Mammillaria ). Unlike the flowers of most other cacti, Pereskia flowers may be borne in clusters. Cactus flowers usually have many stamens , but only a single style , which may branch at
7031-465: The smallest published genome is that of the carnivorous bladderwort ( Utricularia gibba) at 82 Mb (although it still encodes 28,500 genes) while the largest, from the Norway spruce ( Picea abies ), extends over 19.6 Gb (encoding about 28,300 genes). Plants are distributed almost worldwide. While they inhabit several biomes which can be divided into a multitude of ecoregions , only the hardy plants of
7120-481: The species of Leuenbergeria , Pereskia and Rhodocactus are superficially like normal trees or shrubs and have numerous leaves with a midrib and a flattened blade (lamina) on either side. This group is paraphyletic , forming two taxonomic clades . Many cacti in the opuntia group (subfamily Opuntioideae ) also have visible leaves, which may be long-lasting (as in Pereskiopsis species) or produced only during
7209-666: The species studied and almost always less than 1.5 mm (0.06 in) long. The function of such leaves cannot be photosynthesis; a role in the production of plant hormones, such as auxin , and in defining axillary buds has been suggested. Botanically, " spines " are distinguished from "thorns": spines are modified leaves, and thorns are modified branches. Cacti produce spines, always from areoles as noted above. Spines are present even in those cacti with leaves, such as Pereskia , Pereskiopsis and Maihuenia , so they clearly evolved before complete leaflessness. Some cacti only have spines when young, possibly only when seedlings. This
7298-513: The stem acts as the main organ for storing water, some cacti have in addition large taproots . These may be several times the length of the above-ground body in the case of species such as Copiapoa atacamensis , which grows in one of the driest places in the world, the Atacama Desert in northern Chile. Photosynthesis requires plants to take in carbon dioxide gas (CO 2 ). As they do so, they lose water through transpiration . Like other types of succulents , cacti reduce this water loss by
7387-433: The stems from which spines emerge. Flowers are also produced from areoles. In the genus Leuenbergeria , believed similar to the ancestor of all cacti, the areoles occur in the axils of leaves (i.e. in the angle between the leaf stalk and the stem). In leafless cacti, areoles are often borne on raised areas on the stem where leaf bases would have been. Areoles are highly specialized and very condensed shoots or branches. In
7476-493: The subfamily Opuntioideae have relatively short spines, called glochids , that are barbed along their length and easily shed. These enter the skin and are difficult to remove due to being very fine and easily broken, causing long-lasting irritation. Most ground-living cacti have only fine roots , which spread out around the base of the plant for varying distances, close to the surface. Some cacti have taproots ; in genera such as Ariocarpus , these are considerably larger and of
7565-410: The temperature of the surface of the cactus, also reducing water loss. When sufficiently moist air is present, such as during fog or early morning mist, spines can condense moisture, which then drips onto the ground and is absorbed by the roots. The majority of cacti are stem succulents , i.e., plants in which the stem is the main organ used to store water. Water may form up to 90% of the total mass of
7654-422: The top of the stem then produces flowers, the other part spines. Areoles often have multicellular hairs ( trichomes ) that give the areole a hairy or woolly appearance, sometimes of a distinct color such as yellow or brown. In most cacti, the areoles produce new spines or flowers only for a few years and then become inactive. This results in a relatively fixed number of spines, with flowers being produced only from
7743-485: The tribe. Subsequent studies suggested a number of changes, for example including Acanthocereus in the tribe and excluding Strophocactus (which had been sunk into Selenicereus ). It was also shown that Hylocereus was nested within Selenicereus . A major molecular phylogenetic study of the tribe was published in 2017. The lefthand cladogram below shows the relationships obtained (using the authors' circumscriptions of
7832-460: The way in which they carry out photosynthesis. "Normal" leafy plants use the C 3 mechanism : during daylight hours, CO 2 is continually drawn out of the air present in spaces inside leaves and converted first into a compound containing three carbon atoms ( 3-phosphoglycerate ) and then into products such as carbohydrates . The access of air to internal spaces within a plant is controlled by stomata , which are able to open and close. The need for
7921-448: Was basal within the Cactaceae, but confirmed earlier suggestions it was not monophyletic , i.e., did not include all the descendants of a common ancestor. The Bayesian consensus cladogram from this study is shown below with subsequent generic changes added. Pereskia s.l. Clade A → Leuenbergeria Pereskia s.l. Clade B → Rhodocactus + Pereskia s.s. Opuntioideae Maihuenia Plant See text Plants are
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