Cactaceae - Taxonomy and Classification - Classification

Classification

In 1984, it was decided that the Cactaceae Section of the International Organization for Succulent Plant Study should set up a working party, now called the International Cactaceae Systematics Group (ICSG), to produce consensus classifications down to the level of genera. Their system has been used as the basis of subsequent classifications. Detailed treatments published in the 21st century have divided the family into around 125–130 genera and 1,400–1,500 species, which are then arranged into a number of tribes and subfamilies. The ICSG classification of the cactus family recognizes four subfamilies, the largest of which is divided into nine tribes. The subfamilies are:

• Subfamily Pereskioideae

The only genus is Pereskia. It has features considered closest to the ancestors of the Cactaceae. Plants are trees or shrubs with leaves; their stems are smoothly round in cross section, rather than being ribbed or having tubercles. Two systems may be used in photosynthesis, both the "normal" C₃ mechanism and crassulean acid metabolism (CAM)—an "advanced" feature of cacti and other succulents that conserves water.

• Subfamily Opuntioideae

Some 15 genera are included in this subfamily. They may have leaves when they are young, but these are lost later. Their stems are usually divided into distinct "joints" or "pads" (cladodes). Plants vary in size from the small cushions of Maihueniopsis to treelike species of Opuntia, rising to 10 m (33 ft) or more.

• Subfamily Maihuenioideae

The only genus is Maihuenia, with two species, both of which form low-growing mats. It has some features that are primitive within the cacti. Plants have leaves, and crassulean acid metabolism is wholly absent.

• Subfamily Cactoideae

Divided into nine tribes, this is the largest subfamily, including all the "typical" cacti. Members are highly variable in habit, varying from tree-like to epiphytic. Leaves are normally absent, although sometimes very reduced leaves are produced by young plants. Stems are usually not divided into segments, and are ribbed or tuberculate. Two of the tribes, Hylocereeae and Rhipsalideae, contain climbing or epiphytic forms with a rather different appearance; their stems are flattened and may be divided into segments.

Molecular phylogenetic studies have supported the monophyly of three of these subfamilies (not Pereskioideae), but have not supported all of the tribes or even genera below this level; indeed, a 2011 study found only 39% of the genera in the subfamily Cactoideae sampled in the research were monophyletic. Classification of the cacti currently remains uncertain and is likely to change.