Editing Cactus (section)

==Adaptations for water conservation==
{{anchor|Adaptations to dry environment}} <!-- old heading, may still be linked -->

All cacti have some adaptations to promote efficient water use. Most cacti—[[opuntia]]s and [[Cactoideae|cactoids]]—specialize in surviving in hot and dry environments (i.e. are [[xerophytes]]), but the first ancestors of modern cacti were already adapted to periods of intermittent drought.<ref name=EdwaDono06/> A small number of cactus species in the tribes [[Hylocereeae]] and [[Rhipsalideae]] have become adapted to life as climbers or [[epiphyte]]s, 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 plant|succulent]] in many species.<ref name=EdwaDono06/>  Other species of cactus with long-lasting leaves, such as the opuntioid ''[[Pereskiopsis]]'', also have succulent leaves.{{sfnp|ps=none|Anderson|2001|p=572}} 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.

[[Thorns, spines, and prickles|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.{{sfnp|ps=none|Mauseth|2007|p=845}} Spines provide protection from [[herbivore]]s 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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

===Stems===
[[File:Starr 030202-0037 Cereus uruguayanus.jpg|thumb|Stem of young ''[[Cereus hildmannianus]]'' subsp. ''uruguayanus'', showing ribbing and waxy coating]]

The majority of cacti are [[stem succulent]]s, 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.{{sfnp|ps=none|Anderson|2001|pp=15–37}} A mature saguaro (''[[Carnegiea gigantea]]'') is said to be able to absorb as much as {{convert|200|USgal|l impgal|sp=us}} of water during a rainstorm.<ref name=nps/> The outer layer of the stem usually has a tough [[Plant cuticle|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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

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|Metabolism]].

===Roots===

Many cacti have roots that spread out widely, but only penetrate a short distance into the soil. In one case, a young saguaro only {{convert|12|cm|in|abbr=on}} tall had a root system with a diameter of {{convert|2|m|ft|abbr=on|0}}, but no more than {{convert|10|cm|in|abbr=on|0}} deep.<ref name=dalhousie/> 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.<ref name=GibsNobe90/> 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 {{convert|7|mm|in|abbr=on|1}} of rainfall, becoming fully hydrated in a few days.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

Although in most cacti, the stem acts as the main organ for storing water, some cacti have in addition large [[taproot]]s.{{sfnp|ps=none|Anderson|2001|pp=15–37}} These may be several times the length of the above-ground body in the case of species such as ''[[Copiapoa atacamensis]]'',{{sfnp|ps=none|Anderson|2001|pp=15–37}} which grows in one of the driest places in the world, the [[Atacama Desert]] in northern Chile.{{sfnp|ps=none|Anderson|2001|p=174}}

===Metabolism===
[[Photosynthesis]] requires plants to take in [[carbon dioxide]] gas ({{CO2}}). As they do so, they lose water through [[transpiration]]. Like other types of [[Succulent plant|succulents]], cacti reduce this water loss by the way in which they carry out photosynthesis. "Normal" leafy plants use the [[C3 carbon fixation|C<sub>3</sub> mechanism]]: during daylight hours, {{CO2}} 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 [[carbohydrate]]s. The access of air to internal spaces within a plant is controlled by [[stoma]]ta, which are able to open and close. The need for a continuous supply of {{CO2}} during photosynthesis means the stomata must be open, so water vapor is continuously being lost. Plants using the C<sub>3</sub> mechanism lose as much as 97% of the water taken up through their roots in this way.<ref name=RaveEdwa01/> A further problem is that as temperatures rise, the [[enzyme]] that captures {{CO2}} starts to capture more and more oxygen instead, reducing the efficiency of photosynthesis by up to 25%.<ref name=Shar88/>

{{multiple image
|direction = vertical
|width = 260
|header = Schematic illustration of CAM
|image1 = CAM_schema_night.svg
|alt1 =
|caption1 = '''Night''': stomata open; {{CO2}} enters and is stored as malic acid; water vapor is able to escape.
|image2 = CAM_schema_day_cropped.svg
|alt2 =
|caption2 = '''Day''': stomata close; malic acid is converted back to {{CO2}} and used to make carbohydrate; water vapor is confined.
}}
[[Crassulacean acid metabolism]] (CAM) is a mechanism adopted by cacti and other succulents to avoid the problems of the  C<sub>3</sub> mechanism. In full CAM, the stomata open only at night, when temperatures and water loss are lowest. {{CO2}} enters the plant and is captured in the form of organic acids stored inside cells (in [[vacuole]]s). The stomata remain closed throughout the day, and photosynthesis uses only this stored {{CO2}}. CAM uses water much more efficiently at the price of limiting the amount of carbon fixed from the atmosphere and thus available for growth.<ref name=KeelRund03/> CAM-cycling is a less water-efficient system whereby stomata open in the day, just as in plants using the C<sub>3</sub> mechanism. At night, or when the plant is short of water, the stomata close and the CAM mechanism is used to store {{CO2}} produced by [[Cellular respiration|respiration]] for use later in photosynthesis. CAM-cycling is present in ''Pereskia'' species.<ref name=EdwaDono06/>

By studying the ratio of <sup>14</sup>C to <sup>13</sup>C incorporated into a plant—its [[isotopic signature]]—it is possible to deduce how much {{CO2}} 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.<ref name=EdwaDono06/> ''Pereskia'' leaves are claimed to only have the  C<sub>3</sub> mechanism with CAM restricted to stems.{{sfnp|ps=none|Anderson|2001|p=37}} More recent studies show that "it is highly unlikely that significant carbon assimilation occurs in the stem"; ''Pereskia'' species are described as having "C<sub>3</sub> with inducible CAM."<ref name=EdwaDono06/> Leafless cacti carry out all their photosynthesis in the stem, using full CAM. {{As of|2012|February}}, it is not clear whether stem-based CAM evolved once only in the core cacti, or separately in the opuntias and cactoids;<ref name=EdwaDono06/> CAM is known to have [[convergent evolution|evolved convergently]] many times.<ref name=KeelRund03/>

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 [[stoma]]ta 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 "[[Ground tissue|chlorenchyma]]" – a plant tissue made up of relatively unspecialized cells containing [[chloroplast]]s, arranged into a "spongy layer" and a "[[Palisade cell|palisade layer]]" where most of the photosynthesis occurs.{{sfnp|ps=none|Edwards|Nyffeler|Donoghue|2005|p=1184}}
{{Clear}}