Editing Cactus (section)

==Morphology==
[[File:Barrel cactus with a view.JPG|thumb|upright|''[[Ferocactus]]'' species, a [[Cactoideae|cactoid]], in its native [[Arizona]] habitat]]
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 plant|succulent]] [[Plant stem|stems]] that are major [[Organ (biology)|organs]] of [[photosynthesis]]. They have absent, small, or transient [[Leaf|leaves]]. They have [[flowers]] with [[Ovary (plants)|ovaries]] that lie below the [[sepals]] and [[petals]], often deeply sunken into a fleshy [[Receptacle (botany)|receptacle]] (the part of the stem from which the flower parts grow). All cacti have [[areole]]s—highly specialized short [[Shoot (botany)|shoots]] with extremely short [[Internode (botany)|internodes]] that produce [[Thorns, spines, and prickles|spines]], normal shoots, and flowers.<ref name=EdwaDono06/>

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,<ref name=EdwaDono06/> 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, in that when mature, they have woody stems that may be covered with [[Bark (botany)|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.<ref name=EdwaDono06/>

===Growth habit===

Cacti show a wide variety of growth [[habit (biology)|habits]], which are difficult to divide into clear, simple categories.

; {{vanchor|Arborescent cacti}}
Cacti can be tree-like (arborescent), meaning they typically have a single more-or-less woody [[Trunk (botany)|trunk]] topped by several to many [[branch]]es. 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 [[opuntia]]s. Some cacti may become tree-sized but without branches, such as larger specimens of ''[[Echinocactus platyacanthus]]''. Cacti may also be described as [[shrub]]by, with several stems coming from the ground or from branches very low down, such as in ''[[Stenocereus thurberi]]''.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

; {{vanchor|Columnar cacti}}
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]]'' can be described as columnar even though it has stems growing along the ground, rooting at intervals.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

; {{vanchor|Globular cacti}}
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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

; Other forms
Other cacti have a quite different appearance. In tropical regions, some grow as forest climbers and [[epiphyte]]s. 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 {{convert|100|m|ft|sp=us}} 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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}
{{center|'''Growth habits of cacti'''}}
<gallery mode="packed" heights="155" style="font-size:95%; line-height:120%">
Pereskia aculeata5.jpg|Treelike habit (''[[Pereskia aculeata]]'')
Cardon Pachycereus pringlei.jpg|Tall treelike habit (''[[Pachycereus pringlei]]'')
Cephalocereus columna-trajani rev.jpg|Tall unbranched columnar habit (''[[Cephalocereus]]'')
Ferocactus1.jpg|Shorter clustered columnar habit (''[[Ferocactus pilosus]]'')
Ferocactus echidne var victoriensis 1.jpg|Solitary globular habit (''[[Ferocactus echidne]]'')
Rebutia flavistylus 2 rev.jpg|Clustered globular habit (''[[Rebutia]]'' species)<!--image info implies R. flavistyla, but Anderson p.603 says "usually solitary"-->
Rhipsalis paradoxa.jpg|Epiphytic cactus (''[[Rhipsalis paradoxa]]'')
Great Variety of Cacti at the Desert Botanical Garden.jpg|There is a wide variety of cacti at the [[Desert Botanical Garden]]
</gallery>

===Stems===
[[File:Succu Mammillaria longimamma 02.jpg|thumb|Stem of ''[[Mammillaria longimamma]]'', showing tubercles]]
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 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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

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 [[Stoma (botany)|stomata]] (small structures that can open and close to allow passage of gases). Cactus stems are often visibly waxy.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

===Areoles===

{{multiple image
| title             = Cactus areoles
| total_width       = 400
| perrow            = 2 / 3
| image1            = Pereskia grandifolia areole.jpg
| caption1          = Areole of ''[[Rhodocactus grandifolius]]'' showing its position relative to leaves
| image2            = Cut Cereus (brightened).jpg
| caption2          = Cross-section of ''[[Cereus (plant)|Cereus]]'' showing areoles with spines and wool
| image3            = Echinopsis candicans (3).jpg
| caption3          = Areoles of an ''[[Echinopsis]]'' species
| image4            = Astrophytum capricorne areole.jpg
| caption4          = Close-up of an areole of ''[[Astrophytum capricorne]]'' showing fine wool
| image5            = Cactus 4a - geograph.org.uk - 1446436 cleaned.jpg
| caption5          = Flowers appear from the upper part of an areole, spines from the lower (''Cereus'' species).
}}

[[Areole]]s 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).{{sfnp|ps=none|Anderson|2001|p=566}} 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, [[Plant stem|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 ([[trichome]]s) that give the areole a hairy or woolly appearance, sometimes of a distinct color such as yellow or brown.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

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]]''.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

===Leaves===

The great majority of cacti have no visible [[leaf|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 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 [[Paraphyly|paraphyletic]], forming two taxonomic [[clade]]s. 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 the growing season and then lost (as in many species of ''[[Opuntia]]'').{{sfnp|ps=none|Anderson|2001|pp=15–37}} The small genus ''[[Maihuenia]]'' also relies on leaves for photosynthesis.{{sfnp|ps=none|Anderson|2001|p=398}} The structure of the leaves varies somewhat between these groups. Opuntioids and ''Maihuenia'' have leaves that appear to consist only of a midrib.{{sfnp|ps=none|Mauseth|2007|p=845}}

Even those cacti without visible photosynthetic leaves do usually have very small leaves, less than {{convert|0.5|mm|in|abbr=on|2}} long in about half of the species studied and almost always less than {{convert|1.5|mm|in|abbr=on|2}} long. The function of such leaves cannot be photosynthesis; a role in the production of plant hormones, such as [[auxin]], and in defining [[axillary bud]]s has been suggested.<ref name=Maus07/>

===Spines===

Botanically, "[[Thorns, spines, and prickles|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]]''.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

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]]'').{{sfnp|ps=none|Anderson|2001|pp=15–37}}

In addition to normal-length spines, members of the subfamily Opuntioideae have relatively short spines, called [[glochid]]s, 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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}
{{center|'''Cactus spines'''}}
<gallery mode="packed" heights="160" style="font-size:95%; line-height:120%">
Ferocactus1001.jpg|Varied spines of a ''[[Ferocactus]]''
Hakendornen IMGP7026 WP.jpg|Hooked central spine (cf. ''[[Mammillaria rekoi]]'')
Toumeya papyracantha fh 087 3 AZ BB.jpg|Unusual flattened spines of ''[[Sclerocactus papyracanthus]]''
Opuntia microdasys 01 ies cropped.jpg|Glochids of ''[[Opuntia microdasys]]''
</gallery>

===Roots===

Most ground-living cacti have only [[Root#Specialized|fine roots]], which spread out around the base of the plant for varying distances, close to the surface. Some cacti have [[taproot]]s; 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.<ref name=dalhousie/> Climbing, creeping and epiphytic cacti may have only [[Root#Specialized|adventitious roots]], produced along the stems where these come into contact with a rooting medium.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

===Flowers===
[[File:Echinopsis multiplex cropped.jpg|thumb|upright|Large flowers of an ''[[Echinopsis]]'']]
[[File:Stamen of Fish hook Cactus.jpg|thumb|Style and stamens of ''[[Mammillaria beneckei]]'' flower]]

Like their spines, cactus [[flower]]s are variable. Typically, the [[Ovary (botany)|ovary]] is surrounded by material derived from stem or [[Receptacle (botany)|receptacle]] tissue, forming a structure called a [[Hypanthium|pericarpel]]. Tissue derived from the [[petal]]s and [[sepal]]s 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 [[areole]]s that produce wool and spines. Typically, the tube also has small scale-like [[bract]]s, 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 "[[tepal]]s").{{sfnp|ps=none|Anderson|2001|pp=15–37}} Some cacti produce floral tubes without wool or spines (e.g. ''[[Gymnocalycium]]''){{sfnp|ps=none|Anderson|2001|pp=347–348}} or completely devoid of any external structures (e.g. ''[[Mammillaria]]'').{{sfnp|ps=none|Anderson|2001|pp=15–37}} Unlike the flowers of most other cacti, ''[[Pereskia]]'' flowers may be borne in clusters.{{sfnp|ps=none|Anderson|2001|p=566}}

Cactus flowers usually have many [[stamen]]s, but only a single [[Gynoecium|style]], which may branch at the end into more than one [[Stigma (botany)|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.{{sfnp|ps=none|Anderson|2001|pp=15–37}}

The flower as a whole is usually radially symmetrical ([[Floral symmetry#Actinomorphic|actinomorphic]]), but may be bilaterally symmetrical ([[Floral symmetry#Zygomorphic|zygomorphic]]) in some species. Flower colors range from white through yellow and red to magenta.{{sfnp|ps=none|Anderson|2001|pp=15–37}}