Editing Cactus (section)

===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/>