NEW SOUTH WALES FLORA ONLINE

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Description: Herbs or shrubs with succulent ± spiny stems, modified into cladodes; small leaves often present on new growth but falling very early, rarely conspicuous and persistent; ultimate branches called segments (pads or joints); segments cylindric or flattened, often tuberculate, ridged or winged. Spines and barbed bristles (called glochids), restricted to areoles, each areole representing a modified axillary bud or short branch with its leaves or bud scales modified into spines.

Flowers mostly actinomorphic, bisexual, usually solitary at the areoles, short-lived, often large and showy. Hypanthium cup-shaped to elongate. Perianth segments generally numerous and spirally arranged, inner segments often showy and petaloid, sometimes basally fused. Stamens numerous. Ovary inferior, 1-locular, style simple, stigma lobed.

Fruit usually a fleshy inferior berry.

Illustration M. Flockton

Distribution and occurrence: World: 75–220 genera, c. 2000 species, temperate & tropical regions of North & South America, mainly in hot dry areas. Australia: 8 genera, c. 32 species (naturalized), all mainland States.

External links:
Angiosperm Phylogeny Website (Family: Cactaceae, Order: Caryophyllales)
Wikipedia

The number of recognized genera and species is variable. Many species are cultivated as ornamentals, some for their edible fruit; some species are naturalized, and a few have become pests in areas of Australia, especially in inland areas of Qld and N.S.W. Cochineal, the dye used to colour soldiers' coats, was obtained from the Cochineal insects (Dactylopius species) that live on species of Opuntia and Nopalea. The insects, along with the host plants, were introduced into N.S.W. prior to 1839 and the plants subsequently spread and densely infested large areas used for grazing and farming. Most species readily propagate from seed and by vegetative means when segments, fragments of segments or fruit are detached and roots develop from the areoles. This makes control of the plants difficult once they have become established in an area. Many species covered large areas of N.S.W. in pest proportions; the denser infestations were in NWS, CWS, NWP, also in the Hunter Valley and smaller areas SW of Sydney. Populations are still widespread but they have been controlled to a great extent by the Cactoblastis insects. Note: young plants often differ markedly in spine and other characters from the adult of the same species.