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Family Cactaceae

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.


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.

Text by G. J. Harden
Taxon concept:

Taxa not yet included in identification key
Cereus

 Key to the genera 
1Leaves broad and flat, alternate along woody stems, more or less persistent; flowers pedunculatePereskia
Leaves absent or reduced and more or less terete, caducous; flowers sessile2
2Ultimate segments compressed, lower stems sometimes more or less cylindrical, not regularly tuberculate, not angled or wingedOpuntia
All ultimate segments more or less cylindrical, often tuberculate, sometimes angled or deeply 3-winged
                       Back to 1
3
3Plants more or less tree-like; leaves present on new segments, terete, falling early; segments tuberculate but not regularly angled or winged; flowers usually less than 6 cm long; fruit yellow to greenish4
Plants low-growing and stems prostrate to climbing; leaves mostly absent; branches with regular longitudinal wings or angles, sometimes tuberculate or smooth; flowers mostly more than 15 cm long, rarely less than 6 cm long; fruit usually red or sometimes purplish
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5
4Spines covered with papery sheaths; areoles rounded, filled with brownish woolly hairs, spines scattered in the areoles; tubercles flattened laterallyCylindropuntia
Spines without papery sheaths; areoles elongated, filled with white woolly hairs, a stout spine in the upper broader section of the areoles and 2 or 3 smaller spines in the lower, narrowed part of the areoles; tubercles broad or flattened dorsally
                       Back to 3
Austrocylindropuntia
5Branches mostly smooth without regular longitudinal wings or angles; areoles with glochids, spines and hairs; flowers usually less than 6 cm longOpuntia
Branches mostly angled or winged; areoles without glochids, though spines and hairs usually present; flowers more than 15 cm long
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6
6Branches 4–8-angled; spines 4–9, central spine more than 20 mm long surrounded by 3–8 shorter spines; fruit 3–4 cm diam., usually spinyHarrisia
Branches deeply 3-winged; spines 1–3, mostly 2–4 mm long; fruit 10–12 cm diam., spineless
                       Back to 5
Hylocereus

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