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83-655: See text . The Cactoideae are the largest subfamily of the cactus family, Cactaceae , and are widely distributed throughout the Americas. Cactaceae is the 5th most endangered plant or animal family evaluated globally by the International Union for Conservation of Nature. Around 80% of cactus species belong to this subfamily. The genera of the Cactoideae are characterized by microscopic foliage leaves. All photosynthesis occurs in shoot cortex cells covered by

166-453: 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

249-464: 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 )

332-547: 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

415-464: 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)

498-411: 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,

581-415: 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

664-415: 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

747-442: 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 the ground surface. Cactus stems are often ribbed or fluted with

830-409: A large oak tree can transpire 40,000 gallons (151,000 liters) per year. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 ( i.e. , crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced). Transpiration rates of plants can be measured by

913-418: 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

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996-492: 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

1079-403: A number of techniques, including potometers , lysimeters , porometers, photosynthesis systems and thermometric sap flow sensors. Isotope measurements indicate transpiration is the larger component of evapotranspiration . Recent evidence from a global study of water stable isotopes shows that transpired water is isotopically different from groundwater and streams. This suggests that soil water

1162-466: 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

1245-404: A persistent epidermis and stomata. Another important characteristic of this subfamily is ribbed stems, which enable the inner cortex to expand radially without breaking the shoot surface to absorb large quantities of water. As of August 2018, the internal classification of the family Cactaceae remained uncertain and subject to change. A classification incorporating many of the insights from

1328-655: 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

1411-685: 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 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,

1494-433: 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

1577-467: 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 a spiny plant whose identity is now not certain. Cacti occur in

1660-401: 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

1743-546: 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

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1826-482: 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

1909-419: Is lower than that in the leaf airspace of the stomatal pore, water vapor will travel down the gradient and move from the leaf airspace to the atmosphere. This movement lowers the water potential in the leaf airspace and causes evaporation of liquid water from the mesophyll cell walls. This evaporation increases the tension on the water menisci in the cell walls and decreases their radius, thus exerting tension in

1992-556: Is necessary for plants, but only a small amount of water taken up by the roots is used for growth and metabolism. The remaining 97–99.5% is lost by transpiration and guttation . Water with any dissolved mineral nutrients is absorbed into the roots by osmosis , which travels through the xylem by way of water molecule adhesion and cohesion to the foliage and out small pores called stomata (singular "stoma"). The stomata are bordered by guard cells and their stomatal accessory cells (together known as stomatal complex) that open and close

2075-464: Is not as well mixed as widely assumed. Desert plants have specially adapted structures, such as thick cuticles , reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed crassulacean acid metabolism or CAM photosynthesis, in which

2158-835: 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

2241-479: 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

2324-472: Is rapid transpiration which produces wilting. Green vegetation contributes to moderating climate by being cooler than adjacent bare earth or constructed areas. As plant leaves transpire they use energy to evaporate water aggregating up to a huge volume globally every day. An individual tree can transpire hundreds of liters of water per day. For every 100 liters of water transpired, the tree then cools by 70 kWh. Urban heat island effects can be attributed to

2407-557: 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

2490-416: 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

2573-469: 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

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2656-441: Is the process of a liquid flowing in narrow spaces without the assistance of, or even in opposition to, external forces like gravity . The effect can be seen in the drawing up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper and plaster, in some non-porous materials such as sand and liquefied carbon fiber , or in a biological cell . It occurs because of intermolecular forces between

2739-724: 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

2822-401: Is when the plant cannot supply its xylem with adequate water so instead of being filled with water the xylem begins to be filled with water vapor. These particles of water vapor come together and form blockages within the xylem of the plant. This prevents the plant from being able to transport water throughout its vascular system. There is no apparent pattern of where cavitation occurs throughout

2905-402: 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

2988-570: The diffusion of water out of stomata into the atmosphere . We can see the history of the word transpiration when we break it down into trans, a Latin preposition that means "across," and spiration, which comes from the Latin verb spīrāre, meaning "to breathe." The motion suffix adds the meaning "the act of," so we can see transpiration is, literally, "the ACT of breathing across," which clearly identifies vapor emission from plant leaves. Capillary action

3071-443: 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)

3154-441: 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

3237-484: 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

3320-403: 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

3403-413: 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

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3486-406: The cells' water. Because of the cohesive properties of water, the tension travels through the leaf cells to the leaf and stem xylem, where a momentary negative pressure is created as water is pulled up the xylem from the roots. In taller plants and trees, the force of gravity pulling the water inside can only be overcome by the decrease in hydrostatic pressure in the upper parts of the plants due to

3569-600: 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

3652-669: 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

3735-495: 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

3818-460: The evaporative demand of the atmosphere surrounding the leaf such as boundary layer conductance, humidity , temperature , wind, and incident sunlight. Along with above-ground factors, soil temperature and moisture can influence stomatal opening, and thus transpiration rate. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots. Factors that effect root absorption of water include: moisture content of

3901-404: 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

3984-452: The functional status of xylem and allows scientists to view cavitation events for the first time. Transpiration cools plants, as the evaporating water carries away heat energy due to its large latent heat of vaporization of 2260 kJ per liter. Transpirational cooling is the cooling provided as plants transpire water. Excess heat generated from solar radiation is damaging to plant cells and thermal injury occurs during drought or when there

4067-576: 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

4150-880: 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

4233-529: 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

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4316-564: 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,

4399-507: 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

4482-441: 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

4565-401: 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 )

4648-529: The internal status of the xylem during transpiration, in a non invasive manner. This method of imaging allows for scientists to visualize the movement of water throughout the entirety of the plant. It also is capable of viewing what phase the water is in while in the xylem, which makes it possible to visualize cavitation events. Scientists were able to see that over the course of 20 hours of sunlight more than 10 xylem vessels began filling with gas particles becoming cavitated. MRI technology also made it possible to view

4731-483: 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

4814-415: The liquid and surrounding solid surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and container wall act to propel the liquid. Plants regulate the rate of transpiration by controlling the size of the stomatal apertures. The rate of transpiration is also influenced by

4897-513: The molecular studies was produced by Nyffeler and Eggli in 2010. Various revisions have been published since, e.g. to the tribe Hylocereeae and the tribe Echinocereeae . Classifications remained uncertain as of March 2019. This Cactaceae article is a stub . You can help Misplaced Pages by expanding it . Cactaceae See also Classification of the Cactaceae A cactus ( pl. : cacti , cactuses , or less commonly, cactus )

4980-455: 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

5063-421: 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

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5146-449: 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"

5229-442: 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

5312-405: The plant's xylem. If not effectively taken care of, cavitation can cause a plant to reach its permanent wilting point, and die. Therefore, the plant must have a method by which to remove this cavitation blockage, or it must create a new connection of vascular tissue throughout the plant. The plant does this by closing its stomates overnight, which halts the flow of transpiration. This then allows for

5395-453: 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

5478-404: The pore. The cohesion-tension theory explains how leaves pull water through the xylem. Water molecules stick together or exhibit cohesion. As a water molecule evaporates from the leaf's surface it pulls on the adjacent water molecule, creating a continuous water flow through the plant. Two major factors influence the rate of water flow from the soil to the roots: the hydraulic conductivity of

5561-456: 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

5644-451: The process by which these xylem structures are repaired in the plant. After three hours in darkness it was seen that the vascular tissue was resupplied with liquid water. This was possible because in darkness the stomates of the plant are closed and transpiration no longer occurs. When transpiration is halted the cavitation bubbles are destroyed by the pressure generated by the roots. These observations suggest that MRIs are capable of monitoring

5727-467: The roots to generate over 0.05 mPa of pressure, and that is capable of destroying the blockage and refilling the xylem with water, reconnecting the vascular system. If a plant is unable to generate enough pressure to eradicate the blockage it must prevent the blockage from spreading with the use of pit pears and then create new xylem that can re-connect the vascular system of the plant. Scientists have begun using magnetic resonance imaging (MRI) to monitor

5810-431: 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

5893-399: The soil and the magnitude of the pressure gradient through the soil. Both of these factors influence the rate of bulk flow of water moving from the roots to the stomatal pores in the leaves via the xylem. Mass flow of liquid water from the roots to the leaves is driven in part by capillary action , but primarily driven by water potential differences. If the water potential in the ambient air

5976-558: The soil, excessive soil fertility or salt content, poorly developed root systems, and those impacted by pathogenic bacteria and fungi such as pythium or rhizoctonia . 1) An increased rate of evaporation due to a temperature rise will hasten the loss of water. 2) Decreased relative humidity outside the leaf will increase the water potential gradient . During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000–4,000 gallons (11,400–15,100 liters) of water each day, and

6059-487: 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

6142-666: 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

6225-513: 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

6308-481: 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 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 ,

6391-433: 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

6474-444: The stomata are closed during the day and open at night when transpiration will be lower. To maintain the pressure gradient necessary for a plant to remain healthy they must continuously uptake water with their roots. They need to be able to meet the demands of water lost due to transpiration. If a plant is incapable of bringing in enough water to remain in equilibrium with transpiration an event known as cavitation occurs. Cavitation

6557-493: 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

6640-410: 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

6723-422: 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

6806-460: 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

6889-436: 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 Transpiration Water

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