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It is common that we soak raisins in water, and they acquire water by endosmosis. That means that the cells are permeable to water, right? Then why don't they lose water when kept in air? Or kept in sugar without any water?
EDIT: Also, take the example of peeled potatoes. Put them in a hypertonic solution, and they lose water, but when kept there, in air, they don't. How come?
That means that the cells are permeable to water, right?
Then why don't they lose water when kept in air?
They do, but it occurs very, very slowly. It might take a few months (or years in some areas), but if you leave raisins exposed to air they will eventually become harder until the water reaches equilibrium with the surrounding air.
This slow process is why there are tips on how to reconstitute raisins that are too hard to make them more moist and plump after getting a bad batch or finding old ones in your kitchen.
They already have! Raisins are dried-out grapes, and according to a brief search are less than 20% water by weight but over 50% sugar by weight. They've lost plenty of water already. Your average grape is probably hypertonic to most solutions and thus will not readily lose more water. Additionally, for water to be lost to air, it must be vaporized via evaporation, which is slow, inefficient from something as dry as raisins, and miniscule in effect. Sun-dried raisins already went through that process.
Sarah - Well there are two different things that are going on here. There's the argument of why plants wilt, so in the case if you have a house plant, and why if you don't water it, it wilts. I'll address that first.
Usually in plants, the cells within the plant are what are known as turgid, which means that they're absolutely stuffed full of water which keeps moving into them by osmosis. They're very rigid which is how plants are able to support themselves. But if they're not able to get enough water, water will move out of the cells and the cells become sort of floppy or what's known as flaccid and that is why the plant is no longer able to support itself and the leaves go all floppy and wilted and soft. So that's what happens if you don't get enough water, but there are actually some other reasons behind cut flowers wilting. So obviously, if you don't water them, the same thing will happen, but also, it can be because they run out of nutrients because obviously, they're no longer attached to roots that are getting any nutrients as well and there can be a build up of bacteria, and fungi, and things on the end of the cut surface.
But also, when you cut flowers, you cut them on your work top or whatever and then you put them in water. Because of the water tension within the xylem vessels (which are the vessels that go up and down a plant, carrying the water around), if you cut the stem, it sucks in a bubble of air into the xylem. If you then put the stem in water, it stops more water from flowing up the xylem. So that actually can be a real reason why they wilt. Some florists recommend that you cut the stems of flowers underwater which will keep the water just a little droplet on the end whilst you put them in the vase and will help them to stay alive for longer.
Chemical changes during freezing
Fresh fruits and vegetables, when harvested, continue to undergo chemical changes which can cause spoilage and deterioration of the product. This is why these products should be frozen as soon after harvest as possible and at their peak degree of ripeness.
Enzymes cause loss of color, flavor changes and nutrient loss
Fresh produce contains chemical compounds called enzymes which cause the loss of color, loss of nutrients, flavor changes, and color changes in frozen fruits and vegetables. These enzymes must be inactivated to prevent such reactions from taking place.
Blanch vegetables to inactivate enzymes
Enzymes in vegetables are inactivated by the blanching process. Blanching is the exposure of the vegetables to boiling water or steam for a brief period of time. The vegetable must then be rapidly cooled in ice water to prevent it from cooking. Contrary to statements in some publications on home freezing, in most cases blanching is absolutely essential for producing quality frozen vegetables. Blanching also helps to destroy microorganisms on the surface of the vegetable and to make some vegetables, such as broccoli and spinach, more compact.
Add ascorbic acid to fruit to control enzymes
The major problem associated with enzymes in fruits is the development of brown colors and loss of vitamin C. Because fruits are usually served raw, they're not blanched like vegetables. Instead, enzymes in frozen fruit are controlled by using chemical compounds which interfere with deteriorative chemical reactions. The most common control chemical is ascorbic acid (vitamin C). Ascorbic acid may be used in its pure form or in commercial mixtures with sugars.
Less effective methods of controlling enzymes
Some directions for freezing fruits also include temporary measures to control enzyme-activated browning. Such temporary measures include soaking the fruit in dilute vinegar solutions or coating the fruit with sugar and lemon juice. However, these latter methods don't prevent browning as effectively as treatment with ascorbic acid.
Limit air during freezing
Another group of chemical changes that can take place in frozen products is the development of rancid oxidative flavors through contact of the frozen product with air. This problem can be controlled by using a wrapping material which does not permit air to pass into the product. Also, remove as much air as possible from the freezer bag or container to reduce the amount of air in contact with the product.
Added March 20, 2011
Jonathan Lavian, who writes a research paper for an education minor, writes:
|Many people try to explain the problem with physics alone with a different argument. They argue that less hot air is captured in the cup. In other words, the cup covers a volume of less dense air because the air is heated around the candle. When the air cools after the candle goes out, the pressure decreases almost entirely from less dense air cooling. |
Regardless, some may argue that the chemical aspect is very minimal because the water level sometimes rises to one third of the volume, but under perfect conditions reaction condition, the reaction chemistry can only account for a maximum ten percent water level rise. You suggest that the water level rises to one tenth of the height, however it can be much higher if more candles are used.
I agree that the chemical reaction can have an effect, but how would you rank the contribution of each? Is one effect minimal or more important? How does the size of the candle or container play a factor?
I myself did not make the experiment with several candles but I can imagine that one can boost the physics part like this: if one would take a lot of candles, burn them for a while until the air around it is hot and then place the container around it, the physics portion of the argument gets a boost. I can imagine that this can be substantial and would not be surprised to see the water level rise to 30 percent without contradicting anything said above. I myself have lighted the candles and then immediately placed it and not waited until the air around it got hot.
One could do the experiment with an other heat source which does not use any chemical processes. Then the chemistry part would be ruled out and the physics contribution alone can be measured. To completely rule out preheating, one could light the candle from inside the container. This would have to be done carefully however as gas lighters might contribute additional gas and heat for example. I think it is better to light the candle, place the candle down and then immediately place the pitcher around it. Excessive preheating is excluded like this.
The size of the pitcher certainly will have an effect. If the pitcher is too large, then both the effect of the physics as well as the chemistry will be smaller simply because only part of the room will be affected. What I liked about the experiment is that with household size objects, one can get directly to a situation where the balance between physics and chemistry is initially equal. The initial cancellation of different effects is what makes the experiment so interesting and puzzling.
How did bryophytes evolve?
It is believed that the division Bryophyte evolved from green algae on more than one occasion. Genetic analysis has shown that bryophyte species do not share the same common ancestor and in some cases they are only distantly related. Two adaptations made the move from water to land possible for bryophytes: a waxy cuticle and gametangia. The waxy cuticle helped to protect the plants tissue from drying out and the gametangia provided further protection against drying out specifically for the plants gametes. Bryophytes also show embryonic development which is a significant adaptation that links them to the vascular land plants.
Types of Leaf Forms
Leaves may be categorized as simple or compound, depending on how their blade (or lamina) is divided.
Differentiate among the types of leaf forms
- In a simple leaf, the blade is completely undivided leaves may also be formed of lobes where the gaps between lobes do not reach to the main vein.
- In a compound leaf, the leaf blade is divided, forming leaflets that are attached to the middle vein, but have their own stalks.
- The leaflets of palmately-compound leaves radiate outwards from the end of the petiole.
- Pinnately-compound leaves have their leaflets arranged along the middle vein.
- Bipinnately-compound (double-compound) leaves have their leaflets arranged along a secondary vein, which is one of several veins branching off the middle vein.
- simple leaf: a leaf with an undivided blade
- compound leaf: a leaf where the blade is divided, forming leaflets
- palmately compound leaf: leaf that has its leaflets radiating outwards from the end of the petiole
- pinnately compound leaf: a leaf where the leaflets are arranged along the middle vein
There are two basic forms of leaves that can be described considering the way the blade (or lamina) is divided. Leaves may be simple or compound.
Simple and compound leaves: Leaves may be simple or compound. In simple leaves, the lamina is continuous. (a) The banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. (b) In palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. (c) In pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). (d) The honey locust has double compound leaves, in which leaflets branch from the veins.
In a simple leaf, such as the banana leaf, the blade is completely undivided. The leaf shape may also be formed of lobes where the gaps between lobes do not reach to the main vein. An example of this type is the maple leaf.
In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Compound leaves are a characteristic of some families of higher plants. Each leaflet is attached to the rachis (middle vein), but may have its own stalk. A palmately compound leaf has its leaflets radiating outwards from the end of the petiole, like fingers off the palm of a hand. Examples of plants with palmately compound leaves include poison ivy, the buckeye tree, or the familiar house plant Schefflera sp. (commonly called “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance the leaflets are arranged along the middle vein, as in rose leaves or the leaves of hickory, pecan, ash, or walnut trees. In a pinnately compound leaf, the middle vein is called the midrib. Bipinnately compound (or double compound) leaves are twice divided the leaflets are arranged along a secondary vein, which is one of several veins branching off the middle vein. Each leaflet is called a “pinnule”. The pinnules on one secondary vein are called “pinna”. The silk tree (Albizia) is an example of a plant with bipinnate leaves.
Why Do We Have Wrinkly Fingers After Swimming?
You&rsquove been playing in the pool for almost an hour now, practicing your best underwater somersaults. Now it&rsquos time to get out, and as you look at your hands, it&rsquos . . . it&rsquos . . . it&rsquos the attack of the wrinkly fingered monster!
Don&rsquot get frightened or run for cover under your towel yet. After spending lots of time in the water, it&rsquos totally normal for fingers (and sometimes toes) to wrinkle.
/> Why Do We Have Wrinkly Fingers After Swimming?
Even though you can&rsquot see it, your skin is covered with its own special oil called sebum. Sebum is found on the outermost layer of skin. Sebum lubricates and protects your skin. It also makes your skin a bit waterproof. That&rsquos why getting caught in the rain, hopping in the shower after a game, or washing your hands before dinner won&rsquot leave your skin soggy. Sebum is there to keep the water out.
But what happens when you spend a long time in the water? Well, there&rsquos only a particular amount of sebum on your skin at a time. Once the sebum is washed away, the water can make its way into the outer layer of skin. The water does this by osmosis. This is when water actually moves from one thing into another, from a place where there is more water to a place where there is less water. There is more water in the pool than there is in your body, so it naturally moves to the place where there is less water – you!
Although your fingers may look shriveled like raisins, they aren&rsquot really shriveled – they&rsquore actually waterlogged! The extra water in your fingers causes the skin to swell in some places but not others, and that&rsquos what causes the wrinkles.
It isn&rsquot just pool water that washes away the sebum. Sitting in the bathtub for a long soak can also wash away the sebum and leave a kid with soggy skin. Washing dishes for a long time, scrubbing and rinsing your puppy, cleaning the gravel in your aquarium – anything that keeps your hands in water long enough will give you wrinkly fingers.
Why Do We Have Wrinkly Fingers After Swimming? [Illustration by Shinod AP]
What should you do if you come out of the pool looking like a raisin? Not a thing! Having wrinkly skin after a swim or bath doesn&rsquot mean there is anything wrong, and it goes away by itself quickly. You&rsquoll have more sebum on your skin in no time. If you really can&rsquot stand the raisin look and will be doing something around the house that keeps your hands in water, you can wear rubber gloves to keep the sebum from being washed away.
Most fish exchange gases using gills on either side of the pharynx (throat). Gills are tissues which consist of threadlike structures called filaments. These filaments have many functions and "are involved in ion and water transfer as well as oxygen, carbon dioxide, acid and ammonia exchange.   Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx.
Fish from multiple groups can live out of the water for extended time periods. Amphibious fish such as the mudskipper can live and move about on land for up to several days, or live in stagnant or otherwise oxygen depleted water. Many such fish can breathe air via a variety of mechanisms. The skin of anguillid eels may absorb oxygen directly. The buccal cavity of the electric eel may breathe air. Catfish of the families Loricariidae, Callichthyidae, and Scoloplacidae absorb air through their digestive tracts.  Lungfish, with the exception of the Australian lungfish, and bichirs have paired lungs similar to those of tetrapods and must surface to gulp fresh air through the mouth and pass spent air out through the gills. Gar and bowfin have a vascularized swim bladder that functions in the same way. Loaches, trahiras, and many catfish breathe by passing air through the gut. Mudskippers breathe by absorbing oxygen across the skin (similar to frogs). A number of fish have evolved so-called accessory breathing organs that extract oxygen from the air. Labyrinth fish (such as gouramis and bettas) have a labyrinth organ above the gills that performs this function. A few other fish have structures resembling labyrinth organs in form and function, most notably snakeheads, pikeheads, and the Clariidae catfish family.
Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation (summertime hibernation) until water returns.
Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish, are obligated to breathe air periodically or they suffocate. Facultative air breathers, such as the catfish Hypostomus plecostomus, only breathe air if they need to and can otherwise rely on their gills for oxygen. Most air breathing fish are facultative air breathers that avoid the energetic cost of rising to the surface and the fitness cost of exposure to surface predators. 
All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Each gill is supported by a cartilaginous or bony gill arch.  The gills of vertebrates typically develop in the walls of the pharynx, along a series of gill slits opening to the exterior. Most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other, which increases the efficiency of oxygen-uptake from the water.    Fresh oxygenated water taken in through the mouth is uninterruptedly "pumped" through the gills in one direction, while the blood in the lamellae flows in the opposite direction, creating the countercurrent blood and water flow, on which the fish's survival depends. 
The gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange.  When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside. The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches, as some of their chordate relatives have more than 50 pairs of gills. 
Higher vertebrates do not develop gills, the gill arches form during fetal development, and lay the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and in mammals the malleus and incus.  Fish gill slits may be the evolutionary ancestors of the tonsils, thymus gland, and Eustachian tubes, as well as many other structures derived from the embryonic branchial pouches. [ citation needed ]
Scientists have investigated what part of the body is responsible for maintaining the respiratory rhythm. They found that neurons located in the brainstem of fish are responsible for the genesis of the respiratory rhythm.  The position of these neurons is slightly different from the centers of respiratory genesis in mammals but they are located in the same brain compartment, which has caused debates about the homology of respiratory centers between aquatic and terrestrial species. In both aquatic and terrestrial respiration, the exact mechanisms by which neurons can generate this involuntary rhythm are still not completely understood (see Involuntary control of respiration).
Another important feature of the respiratory rhythm is that it is modulated to adapt to the oxygen consumption of the body. As observed in mammals, fish "breathe" faster and heavier when they do physical exercise. The mechanisms by which these changes occur have been strongly debated over more than 100 years between scientists.  The authors can be classified in 2 schools:
- Those who think that the major part of the respiratory changes are pre-programmed in the brain, which would imply that neurons from locomotion centers of the brain connect to respiratory centers in anticipation of movements.
- Those who think that the major part of the respiratory changes result from the detection of muscle contraction, and that respiration is adapted as a consequence of muscular contraction and oxygen consumption. This would imply that the brain possesses some kind of detection mechanisms that would trigger a respiratory response when muscular contraction occurs.
Many now agree that both mechanisms are probably present and complementary, or working alongside a mechanism that can detect changes in oxygen and/or carbon dioxide blood saturation.
Bony fish Edit
In bony fish, the gills lie in a branchial chamber covered by a bony operculum. The great majority of bony fish species have five pairs of gills, although a few have lost some over the course of evolution. The operculum can be important in adjusting the pressure of water inside of the pharynx to allow proper ventilation of the gills, so that bony fish do not have to rely on ram ventilation (and hence near constant motion) to breathe. Valves inside the mouth keep the water from escaping. 
The gill arches of bony fish typically have no septum, so that the gills alone project from the arch, supported by individual gill rays. Some species retain gill rakers. Though all but the most primitive bony fish lack a spiracle, the pseudobranch associated with it often remains, being located at the base of the operculum. This is, however, often greatly reduced, consisting of a small mass of cells without any remaining gill-like structure. 
Marine teleosts also use gills to excrete electrolytes. The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids, so saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of seawater and excrete the salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills. 
In some primitive bony fishes and amphibians, the larvae bear external gills, branching off from the gill arches.  These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods. 
Cartilaginous fish Edit
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active pelagic sharks.  While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species. 
The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the shark pumps blood to its gills via the ventral aorta artery where it branches into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta. The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From there blood enters the heart ventricle and the cycle repeats. 
Sharks and rays typically have five pairs of gill slits that open directly to the outside of the body, though some more primitive sharks have six or seven pairs. Adjacent slits are separated by a cartilaginous gill arch from which projects a long sheet-like septum, partly supported by a further piece of cartilage called the gill ray. The individual lamellae of the gills lie on either side of the septum. The base of the arch may also support gill rakers, small projecting elements that help to filter food from the water. 
A smaller opening, the spiracle, lies in the back of the first gill slit. This bears a small pseudobranch that resembles a gill in structure, but only receives blood already oxygenated by the true gills.  The spiracle is thought to be homologous to the ear opening in higher vertebrates. 
Most sharks rely on ram ventilation, forcing water into the mouth and over the gills by rapidly swimming forward. In slow-moving or bottom dwelling species, especially among skates and rays, the spiracle may be enlarged, and the fish breathes by sucking water through this opening, instead of through the mouth. 
Chimaeras differ from other cartilagenous fish, having lost both the spiracle and the fifth gill slit. The remaining slits are covered by an operculum, developed from the septum of the gill arch in front of the first gill. 
Lampreys and hagfish Edit
Lampreys and hagfish do not have gill slits as such. Instead, the gills are contained in spherical pouches, with a circular opening to the outside. Like the gill slits of higher fish, each pouch contains two gills. In some cases, the openings may be fused together, effectively forming an operculum. Lampreys have seven pairs of pouches, while hagfishes may have six to fourteen, depending on the species. In the hagfish, the pouches connect with the pharynx internally. In adult lampreys, a separate respiratory tube develops beneath the pharynx proper, separating food and water from respiration by closing a valve at its anterior end. 
The circulatory systems of all vertebrates are closed, just as in humans. Still, the systems of fish, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system. In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. The heart of fish is therefore only a single pump (consisting of two chambers). Fish have a closed-loop circulatory system. The heart pumps the blood in a single loop throughout the body. In most fish, the heart consists of four parts, including two chambers and an entrance and exit.  The first part is the sinus venosus, a thin-walled sac that collects blood from the fish's veins before allowing it to flow to the second part, the atrium, which is a large muscular chamber. The atrium serves as a one-way antechamber, sends blood to the third part, ventricle. The ventricle is another thick-walled, muscular chamber and it pumps the blood, first to the fourth part, bulbus arteriosus, a large tube, and then out of the heart. The bulbus arteriosus connects to the aorta, through which blood flows to the gills for oxygenation.
In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.
Jaws allow fish to eat a wide variety of food, including plants and other organisms. Fish ingest food through the mouth and break it down in the esophagus. In the stomach, food is further digested and, in many fish, processed in finger-shaped pouches called pyloric caeca, which secrete digestive enzymes and absorb nutrients. Organs such as the liver and pancreas add enzymes and various chemicals as the food moves through the digestive tract. The intestine completes the process of digestion and nutrient absorption.
In most vertebrates, digestion is a four-stage process involving the main structures of the digestive tract, starting with ingestion, placing food into the mouth, and concluding with the excretion of undigested material through the anus. From the mouth, the food moves to the stomach, where as bolus it is broken down chemically. It then moves to the intestine, where the process of breaking the food down into simple molecules continues and the results are absorbed as nutrients into the circulatory and lymphatic system.
Although the precise shape and size of the stomach varies widely among different vertebrates, the relative positions of the oesophageal and duodenal openings remain relatively constant. As a result, the organ always curves somewhat to the left before curving back to meet the pyloric sphincter. However, lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomach at all, with the oesophagus opening directly into the intestine. These animals all consume diets that either require little storage of food, or no pre-digestion with gastric juices, or both. 
The small intestine is the part of the digestive tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In fish, the divisions of the small intestine are not clear, and the terms anterior or proximal intestine may be used instead of duodenum.  The small intestine is found in all teleosts, although its form and length vary enormously between species. In teleosts, it is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epithelium. 
There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This valve greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.  In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions. 
The large intestine is the last part of the digestive system normally found in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body.  In fish, there is no true large intestine, but simply a short rectum connecting the end of the digestive part of the gut to the cloaca. In sharks, this includes a rectal gland that secretes salt to help the animal maintain osmotic balance with the seawater. The gland somewhat resembles a caecum in structure, but is not a homologous structure. 
As with many aquatic animals, most fish release their nitrogenous wastes as ammonia. Some of the wastes diffuse through the gills. Blood wastes are filtered by the kidneys.
Saltwater fish tend to lose water because of osmosis. Their kidneys return water to the body. The reverse happens in freshwater fish: they tend to gain water osmotically. Their kidneys produce dilute urine for excretion. Some fish have specially adapted kidneys that vary in function, allowing them to move from freshwater to saltwater.
In sharks, digestion can take a long time. The food moves from the mouth to a J-shaped stomach, where it is stored and initial digestion occurs.  Unwanted items may never get past the stomach, and instead the shark either vomits or turns its stomachs inside out and ejects unwanted items from its mouth. One of the biggest differences between the digestive systems of sharks and mammals is that sharks have much shorter intestines. This short length is achieved by the spiral valve with multiple turns within a single short section instead of a long tube-like intestine. The valve provides a long surface area, requiring food to circulate inside the short gut until fully digested, when remaining waste products pass into the cloaca. 
Regulation of social behaviour Edit
Oxytocin is a group of neuropeptides found in most vertebrate. One form of oxytocin functions as a hormone which is associated with human love. In 2012, researchers injected cichlids from the social species Neolamprologus pulcher, either with this form of isotocin or with a control saline solution. They found isotocin increased "responsiveness to social information", which suggests "it is a key regulator of social behavior that has evolved and endured since ancient times".  
Effects of pollution Edit
Fish can bioaccumulate pollutants that are discharged into waterways. Estrogenic compounds found in pesticides, birth control, plastics, plants, fungi, bacteria, and synthetic drugs leeched into rivers are affecting the endocrine systems of native species.  In Boulder, Colorado, white sucker fish found downstream of a municipal waste water treatment plant exhibit impaired or abnormal sexual development. The fish have been exposed to higher levels of estrogen, and leading to feminized fish.  Males display female reproductive organs, and both sexes have reduced fertility, and a higher hatch mortality. 
Freshwater habitats in the United States are widely contaminated by the common pesticide atrazine.  There is controversy over the degree to which this pesticide harms the endocrine systems of freshwater fish and amphibians. Non-industry-funded researchers consistently report harmful effects while industry-funded researchers consistently report no harmful effects.   
In the marine ecosystem, organochlorine contaminants like pesticides, herbicides (DDT), and chlordan are accumulating within fish tissue and disrupting their endocrine system.  High frequencies of infertility and high levels of organochlorines have been found in bonnethead sharks along the Gulf Coast of Florida. These endocrine-disrupting compounds are similar in structure to naturally occurring hormones in fish. They can modulate hormonal interactions in fish by: 
How is Water Gained and Lost from the Body or Fluid Balance in the Body
Water balance in the body is a meticulously regulated process where losses and gains are balanced to achieve a proper &lsquohomeostasis&rsquo. During this process, the osmotic pressures within the cells and in the spaces between cells are regulated without deviating much from the norm. However, an imbalance between the water gained and the water lost may lead to a derangement in the osmotic pressure and therefore a derailment in the cellular and organ functions. Thus, this article will discuss the ways in which the body gains water, as well as how it loses the same depending on the body state and the environmental factors.
How does the body gain water?
The commonest and the most significant method of gaining water into the body is through drinking water and other fluids. Thus, researchers have calculated the water gain through such means as amounting to around 60% of the total fluid gain. The second most significant method of gaining water into the body is through eating moist food and the amount that is gained accounts for around 30% of the total fluid gain. It should also be said that the moisture present in foods, as well as the water taken orally is absorbed into the body through the small and the large intestines. Thirdly, it is recognized that the cells produce certain amount of water, which accounts for around 10% of the total fluid gain through cellular respiration. During this process, the cells utilize oxygen and glucose to produce energy while releasing carbon dioxide and water as by products.
How does the body lose water?
Among the ways in which the body loses water, the amount of water excreted as urine is the most significant. The water loss that takes place through urination amounts to around 60% of the total fluid loss from the body. At the same time, a diminished urine output is considered a possible sign of dehydration, a dysfunction in the ability to excrete urine collecting in the bladder (e.g. a posterior urethral valve, hypertrophic prostate or a bladder dysfunction) or else a functional deficit in the filtering process taking place in the kidneys (e.g. acute or chronic renal failure).
Apart from the losses taking place as urine, some of the other methods of losing water from the body include sweating, through feces and through evaporation. The amount of water loss that takes place through feces is around 6% of the total fluid loss and the amount of fluid loss that take place through sweating is almost the same. However, humans tend to lose around 28% of the total fluid loss through evaporation, which take place during breathing, or else through the skin.
When looking at these methods of gaining and losing water to and from the body, it is apparent that enough water should be consumed as oral fluids, as well as through moist food in order to compensate the mostly uncontrollable fluid losses. It is not desirable nor advisable to prevent the water loss that takes place naturally, because each and every mechanism has its own purpose.
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 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 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 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.
They can be tree-like (arborescent), meaning they typically have a single more-or-less woody trunk topped by several to many branches. In 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 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, the "columns" may be horizontal rather than vertical. Thus, Stenocereus eruca 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 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, almost leaf-like in appearance, with fewer 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 the ground. 
Tall unbranched columnar habit (Cephalocereus)
Shorter clustered columnar habit (Ferocactus pilosus)
Solitary globular habit (Ferocactus echidne)
Clustered globular habit (Rebutia species)
The leafless, spiny stem is the characteristic feature of the majority of cacti (and all of those 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 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 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 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 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 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 groups of cacti. All 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. Many cacti in the opuntia group (subfamily Opuntioideae, opuntioids) also have visible leaves, which may be long-lasting (as in Pereskiopsis species) or be produced only during the growing season and then be 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 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 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 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. 
Hooked central spine (cf. Mammillaria rekoi)
Unusual flattened spines of Sclerocactus papyracanthus
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 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 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 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 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. they are xerophytes), but 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.
Leaves and spines
The absence of visible leaves is one of the most striking features of most cacti. Pereskia (which 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 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 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 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 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) 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 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 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 of as little as 7 mm (0.3 in) of rainfall, becoming fully hydrated in a few days. 
Although in most cacti, 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 the way in which they carry out photosynthesis. "Normal" leafy plants use the C3 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 a continuous supply of CO
2 during photosynthesis means the stomata must be open, so water vapor is continuously being lost. Plants using the C3 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) is a mechanism adopted by cacti and other succulents to avoid the problems of the C3 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 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 C3 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 is present in Pereskia species. 
By studying the ratio of 14 C to 13 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 the C3 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 "C3 with inducible CAM."  Leafless cacti carry out all their photosynthesis in the stem, using full CAM. As of February 2012 [update] , it is not clear whether stem-based CAM evolved once only in 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 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 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" 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 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 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 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 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 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: 
- Subfamily Pereskioideae K. Schumann
- Subfamily OpuntioideaeK. Schumann
- Subfamily MaihuenioideaeP. Fearn
- Subfamily Cactoideae
Molecular phylogenetic studies have supported 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) 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.
A 2011 study using fewer genes but more species also found that Pereskia s.l. was divided into the same clades, but was unable to resolve the members of the "core cacti" clade. It was accepted that the relationships shown above are "the most robust to date." 
Leuenbergeria species (Pereskia s.l. Clade A) always lack two key features of the stem present in most of the remaining "caulocacti": like most non-cacti, their stems begin to form bark early in the plants' life and also lack stomata—structures that control admission of air into a plant and hence control photosynthesis. By contrast, caulocacti, including species of Rhodocactus and the remaining species of Pereskia s.s., typically delay forming bark and have stomata on their stems, thus giving the stem the potential to become a major organ for photosynthesis. (The two highly specialized species of Maihuenia are something of an exception.)  
The first cacti are thought to have been only slightly succulent shrubs or small trees whose leaves carried out photosynthesis. They lived in tropical areas that experienced periodic drought. If Leuenbergeria is a good model of these early cacti, then, although they would have appeared superficially similar to other trees growing nearby, they had already evolved strategies to conserve water (some of which are present in members of other families in the order Caryophyllales). These strategies included being able to respond rapidly to periods of rain, and keeping transpiration low by using water very efficiently during photosynthesis. The latter was achieved by tightly controlling the opening of stomata. Like Pereskia species today, early ancestors may have been able to switch from the normal C3 mechanism, where carbon dioxide is used continuously in photosynthesis, to CAM cycling, in which when the stomata are closed, carbon dioxide produced by respiration is stored for later use in photosynthesis. 
The clade containing Rhodocactus and Pereskia s.s. marks the beginnings of an evolutionary switch to using stems as photosynthetic organs. Stems have stomata and the formation of bark takes place later than in normal trees. The "core cacti" show a steady increase in both stem succulence and photosynthesis accompanied by multiple losses of leaves, more-or-less complete in the Cactoideae. One evolutionary question at present unanswered is whether the switch to full CAM photosynthesis in stems occurred only once in the core cacti, in which case it has been lost in Maihuenia, or separately in Opuntioideae and Cactoideae, in which case it never evolved in Maihuenia. 
Understanding evolution within the core cacti clade is difficult as of February 2012 [update] , since phylogenetic relationships are still uncertain and not well related to current classifications. Thus, a 2011 study found "an extraordinarily high proportion of genera" were not monophyletic, so were not all descendants of a single common ancestor. For example, of the 36 genera in the subfamily Cactoideae sampled in the research, 22 (61%) were found not monophyletic.  Nine tribes are recognized within Cactoideae in the International Cactaceae Systematics Group (ICSG) classification one, Calymmantheae, comprises a single genus, Calymmanthium.  Only two of the remaining eight – Cacteae and Rhipsalideae – were shown to be monophyletic in a 2011 study by Hernández-Hernández et al. For a more detailed discussion of the phylogeny of the cacti, see Classification of the Cactaceae.
No known fossils of cacti exist to throw light on their evolutionary history.  However, the geographical distribution of cacti offers some evidence. Except for a relatively recent spread of Rhipsalis baccifera to parts of the Old World, cacti are plants of South America and mainly southern regions of North America. This suggests the family must have evolved after the ancient continent of Gondwana split into South America and Africa, which occurred during the Early Cretaceous, around 145 to 101 million years ago .  Precisely when after this split cacti evolved is less clear. Older sources suggest an early origin around 90 – 66 million years ago, during the Late Cretaceous. More recent molecular studies suggest a much younger origin, perhaps in very Late Eocene to early Oligocene periods, around 35–30 million years ago.   Based on the phylogeny of the cacti, the earliest diverging group (Leuenbergeria) may have originated in Central America and northern South America, whereas the caulocacti, those with more-or-less succulent stems, evolved later in the southern part of South America, and then moved northwards.  Core cacti, those with strongly succulent stems, are estimated to have evolved around 25 million years ago.  A possible stimulus to their evolution may have been uplifting in the central Andes, some 25–20 million years ago, which was associated with increasing and varying aridity.  However, the current species diversity of cacti is thought to have arisen only in the last 10–5 million years (from the late Miocene into the Pliocene). Other succulent plants, such as the Aizoaceae in South Africa, the Didiereaceae in Madagascar and the genus Agave in the Americas, appear to have diversified at the same time, which coincided with a global expansion of arid environments.