The 8 species in, Family Apocynaceae, were previously in the Vinca genus which has similar plants.
Both have dehiscent fruit, Vinca stems form stolons, Catharanthus do not.
In Vinca the anthers are free, in Catharanthus they form a cone around the style head.
Catharanthus roseus, the Pink, Cape or Madagascar periwinkle, is a common garden plant.
It is naturalised, and a weed, in Queensland.
They are perennial herbs or small shrubs with a woody older stems.
They can be erect and up to 60 cm high or branches can almost rest on the ground.
The young green to reddish stems, with narrow wings, can be smooth or hairy.
All parts contain a milky sap.
The simple leaves are opposite and decussate.
On stalks around 1 cm long the blades are up to 10 cm long by 4 cm wide.
The blade can be elliptic, ovate to obovate with a wedge-shaped base.
The tip can be rounded, pointed or have a mucro (a short, abrupt point).
They are a shiny green above with a pale midrib and pale green underneath.
Both sides can be smooth or hairy.
There are axillary glands.
Inflorescences of 1 or 2 flowers are in the terminal leaf axils.
Flowers, on a 2 mm long stalk, have parts in 5’s.
The green sepals, around 4 mm long, have the bases fused.
The 2 to 3 cm long, slender corolla tube is swollen at the top around the stamens.
It is smooth or hairy outside and has hairs inside at the throat and below the stamens.
The flaring obovate corolla lobes are up 3 cm long and often have a pointed tip.
There are short hairs near the bases of the lobes.
Flowers, present most of the year, are pale pink to red with a darker throat.
Cultivars come in white, mauve, deep pink or red with centres in red, cream or white.
The stamens, on short filaments, are inserted just below the top of the corolla tube.
The anthers form a cone around the style head but are free of it.
The superior ovary of 2 free carpels has a single style up to 2 cm long.
The style head has hairs at the top and on a long frill at the base.
The stigmatic area in between is smooth and shiny.
There are 2 long and narrow nectary glands.
The fruit are erect, paired, ribbed, hairy follicles up to 4 cm long and 3 mm wide.
Each has up to 20 black seeds in 2 rows.
The 2.5 mm long, oblong seeds are grooved on one side.
The surface is covered in small bumps, raised lines and tiny pits.