Opuntioideae

Synapomorphies of Opuntioideae include small deciduous, barbed spines called glochids born on areoles and a bony aril surrounding a campylotropous ovule (inverted and curved, such that the micropyle almost meets the funiculus). Other prominent morphological characters for this subfamily are presence of cylindrical, caducous leaves that tend to be shed by maturity and the sectioning of the stem into joints or pads known as cladodes. Opuntioideae are unique among cacti for lacking in the stem a thick cortex, an extensive system of cortical bundles, collapsible cortical cells, and medullary bundles. Typically, the epidermis consists of a single layer of irregularly shaped cells, a cuticle at least 1-2 microns thick, and long, uniseriate trichomes in the areoles. Opuntioideae have a hypodermis of at least one layer, very thick walls, and druses (aggregations of calcium oxalate crystals), and their cortical cells have enlarged nuclei; the reason for this is unknown. They also possess mucilage cells. Notably, their lack of collapsible cortical cells, ribs, and tubercles mean that they cannot absorb water or transfer it intercellularly as easily as the other cacti, so this may place evolutionary constraints on the aridity of habitats and maximum adult size. One adaptation around this problem is the evolution of flattened cladodes that allow opuntioids to swell up with water, increasing in volume without an increase in surface area risking water loss. Opuntioids also lack fiber caps to their phloem bundles, which in other cacti protect against sucking insects and stiffen developing internodes. == Taxonomy ==

A major challenge in Opuntioideae classification is that the subfamily is known to hybridize (particularly the Opuntieae tribe), which complicates identifying species. Genera exhibit variation in morphology, which makes using genetic analysis more important in determining relationships, since physical characteristics used to define genera may be unseen in some of them. In 2009, a study by Griffith and Porter, based on ribosomal intergenic spacer analysis defined four clades within the Opuntioideae: In 2010, Nyffeler and Eggli, as part of a larger overhaul of Cactaceae systematics, proposed recognizing only Cylindropuntieae and Opuntieae as true tribes of Opuntioideae, since they were the tribes generally agreed to be monophyletic. All other genera would be placed in one polyphyletic basal group until further research produced more definitive answers to their phylogeny. A 2020 study using chloroplast genome sequences found a much clearer division into three monophyletic tribes. Their maximum likelihood cladogram is shown below (the genus Punotia was not included in the study). '' Cylindropuntieae – round stems '' • Cylindropuntia (Engelm.) F.M.Knuth • Grusonia Rchb.f. ex Britton & Rose • Micropuntia Daston • Pereskiopsis Britton & Rose • Quiabentia Britton & Rose Opuntieae – flattened stems • Airampoa Frič (synonym Tunilla) • Brasiliopuntia (K. Schum.) A. Berger • Consolea Lem. • Miqueliopuntia Fric ex F. Ritter • Opuntia Mill. • Salmonopuntia P.V.Heath • Tacinga Britton & Rose Tephrocacteae – round stems • Austrocylindropuntia Backeb. • Cumulopuntia F.Ritter • Maihueniopsis Speg. • Pterocactus K.Schum. • Punotia D.R.Hunt • Tephrocactus Lem. Intergeneric hybrids • × Opulea (Consolea × Opuntia) ==See also==