There are 125 – 130 genera and around 1750 (1200 – 1800) species almost all from the Americas.
Most are from desert areas.
Genera that have been introduced into Australia are Acanthocereus, Epiphyllum, Eriocereus,
Hylocereus, Nyctocereus, Opuntia, Peniocereus and Pereskia.
They are mostly herbs, shrubs or trees with sizes ranging from 1 cm to nearly 20 m.
The stems can be branched and tree-like, columnar, shrubby or globular in shape.
The stems are typically succulent being used for water storage.
As the amount of water varies so does the shape of the stem.
Stems may be ribbed when there is little water and smooth when full of water.
Stems are various shades of green and often have a waxy coating.
In almost all there are no visible leaves with photosynthesis carried out by the stems.
In some genera the stems are jointed being divided into flattened leaf-like sections (cladodes in Opuntia).
A few plants have spirally arranged leaves resembling those of other families (Pereskia).
The stems have small specialised areas called areoles.
They are hairy and of various shapes and sizes.
Leaves, when present, arise from the areoles as do the spines, bristles and flowers.
Spines vary in length, colour, hardness, thickness and can be straight, curved or hooked.
Each areole can have 1 to 5 or more spines and woolly hairs.
Spines usually persist but sometimes fall off when young.
Subfamily Opuntioideae have specialised deciduous barbed spines called glochids.
Inflorescences, from the areoles, are typically lateral (axillary).
They often consist of a single flower but may be a cluster.
Flowers have no stalk but arise directly from the stem (Pereskia have stalks).
The mostly bisexual flowers can be radially or bilaterally symmetric.
The 6 to many spirally arranged sepals and petals are typically undifferentiated tepals.
Their bases are fused to form a hypanthium (some Opuntioideae have no hypanthium)
This can be short and cup-like or long and tubular.
At the base of the hypanthium is the short pericarpel.
This consists of the ovary surrounded by tissue from the top of the stem (the receptacle).
The receptacular tissue can be differentiated from the hypanthium by its histology including
the presence of vascular bundles.
Many people use alternate terms for the hypanthium such as floral, calyx or perianth tube.
(For further information on these terms see the glossary in Section 6b.)
The outer surface usually has spirally arranged areoles with woolly hairs and spines.
There may also be green leaf-like sepaloids, bracts or bracteoles.
The tepals range from outer green to inner white, cream, red, pink, yellow or magenta ones.
Apart from Pereskia the ovary is inferior or part-inferior.
It consists of 3 to many fused carpels with 1 locule having 3 to many placentas.
There are numerous stalked ovules with parietal placentation (basal in Pereskia).
There is a single style that may branch into up to 100 stigmas.
There is a nectiferous disc at the base of the tube.
The fruit are usually fleshy berries and rarely a capsule.
They can be smooth or scaly and have hairs, bristles, spines or scales.
They are currently divided into 4 subfamiles.
The bulk of species are in the subfamilies Cactoideae and Opuntioideae whose members are typical
cacti with fleshy stems, small or no leaves, areoles producing spines and inferior ovaries.
1. Opuntioideae has 15 genera and around 235 species in 5 Tribes.
- These have flattened stems segmented into cladodes with nodes or joints between them.
- Areoles have tiny barbed hairs or spines (glochids) and there may also be large spines.
- The cup or funnel-shaped flowers are mostly yellow with some white or pink.
- The cup-like pericarpel around the inferior ovary is small and there is no hypanthium (tube).
- Pear-shaped fruit are green or red to purple and have spines.
- The well known Opunita aciculata or prickly pears have yellow to brown spines and bristles
on the flat, oblong to round cladode segments and yellow or red flowers.
2. Cactoideae has around 80 % of all the species in 9 Tribes.
Their form varies greatly. The features of a few found in Australia are listed below.
- Acanthocereus in Tribe Pachycereeae.
- The 6 species of bushy shrubs or sprawling plants have stems a few metres long.
- The usually thin, sometimes segmented stems have up to 5 ribs and short, thick spines.
- The nocturnal, white flowers are up to 25 cm long and 12 cm across.
- The long tubular hypanthium and the short pericarpel have scales and spines.
- The red or green, globular to pear-shaped fruit may or may not have spines.
- Cereus in Tribe Cereeae.
- Around 33 species typically of tall, columnar cacti up to 15 m high.
- Others are shrubby or tree-like and the stems are angled or have up to 10 distinct ribs.
- There are usually spines on the large areoles.
- The funnel-shaped, mostly white flowers are nocturnal and up to 30 cm long.
- The spineless fruit, up to 13 cm long, are red or yellow with large, black seeds.
- Epiphyllum is detailed later.
- Peniocereus = Nyctocereus in Tribe Pachycereeae.
- Around 18 species of cacti with thick roots and inconspicuous stems.
- The thin stems may be prostrate or climbing and up to 4.5 m long.
- The white or occasionally red flowers, usually on the sides of the stems, open day or night.
- The spines on the pericarpels are lost as the fruit matures from green to red.
- Selenicereus in Tribe Hylocereeae.
- This genus has about 28 species with some similar to Epiphyllums.
- Thin stems, up to 5 m long clamber over other vegetation aided by the aerial roots.
- The stems can be round, flat or have up to 12 ribs.
- A few have flat, deeply lobed stems resembling some Epiphyllums.
- Spines on the areoles can be absent or 1 mm to 1.5 cm long.
- The nocturnal, usually terminal, mostly white flowers are up to 39 cm long and 30 cm wide.
- Most have a long thin hypanthium with hairs, scales (bracts), bristles or spines.
- Hairs on the pericarpel are sometimes black.
- The spines remain on the fruit.
The other 2 subfamiles only have 1 genus each.
3. Pereskioideae with the Pereskia genus is the most primitive of all the Cactaceae.
They appear to be normal shrubs and trees with woody stems and persistent leaves but they all have areoles.
Their flowers may be in clusters and the ovary is superior.
4. Maihuenioideae has only 2 species in the Maihuenia genus.
They form low-growing cushions with prominent leaves.