Kew recognises 8 genera but does not specify the number of species.
Mabberley has 8 genera with 660 species.
Others have 7 to 9 genera with 350 to over 700 species.
The variations are due to which genera such as Isotrema are induced.
Australia has 8 or 10 species with 4 in S. E. Queensland.
Most are twining or scrambling lianas (woody vines) or vines.
They commonly grow from rhizomes (underground stems) with tubers.
There are a few trees, shrubs and herbs.
Stems, up to 30 m long are mostly unbranched.
The alternately arranged leaves are in 2 ranks (distichous).
The leaves are on a petiole that may have a psuedostipule at the base.
The pseudostipule is the first leaf of an undeveloped axillary shoot.
Most are entire but a few have 3 to 7 lobes and may resemble a fiddle.
The linear to ovate blades are commonly heart-shaped (cordate).
The lower lobes may point down (sagittate) or outwards (hastate).
Inflorescences can be terminal or axillary.
They are commonly on old wood opposite the leaf bases.
They can be a single flower or 1 or more racemes.
(Racemes are a spike with flowers, on a pedicel opening from the base first.
The bisexual flowers, on a pedicel are almost all zygomorphic (bilaterally symmetrical).
In almost all species flowers have no petals but the sepals may be petal-like.
Sepals can be pink, red, purple or yellow or a mix of these.
The sepals are fused into a tube with 1 to 3 (4, 6 or 6) lobes.
The tube is often bent into an ‘S’ shape when viewed sideways.
Fused to the top of the ovary the tube usually pouches out around the style.
It then constricts at the throat then widens as a tube or funnel into the lobes.
The tube can be short or long with 3 equal lobes or 1 or 2 lips.
There are downward pointing hairs inside the narrow section.
Aristolochia and Isotrema trap insects and use them for pollination.
Attracted by their smell of rotten meat, insects move down the tube into the swollen base.
Pollen from another flower is deposited on the stigma.
Stiff downward pointing hairs prevent the insects from escaping.
Anthers then mature and pollen is transferred to the insect.
Flowers wither after a day or so, the hairs become less rigid and the insects escape.
There is usually nectar for the insects at the bottom of the swollen base.
There are 6 to many stamens on short, thick filaments, in 1 to 4 whorls.
They may lie free around the ovary or be fused to the base of the style.
When fused the column formed is called a gynostegium.
The anthers may be free or attached to the style or the 3 or 6 stigmatic lobes.
The 4 pollen sacs open outwards through long slits.
The inferior ovary typically has 3 (4) or 6 fused carpels and as many locules.
Each locule has numerous ovules in 1 or 2 vertical rows.
The styles (or the free tips) are fused into a short thick column.
There are 3 to 6 (9 or 12) stigmatic lobes.
Fruit are typically a woody capsule that opens into the 6 chambers in various ways.
A few are a berry or a schizocarp that breaks into 6 separate segments.
The flattened seeds may have a wing around the edge.
J.F.
