Editing Cactus (section)

===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}}