The Christmas Star or Poinsettia, from Mexico and South America is widely cultivated as a garden or pot plant.
It has numerous synonyms including Euphorbia poinsettii, Euphorbia pulcherrima f. lutea,
Poinsettia pulcherrima and Poinsettia pulcherrima var. albida.
They are shrubs or small trees up to 4 m high with a round crown.
There is a short trunk with erect or almost erect stems that have few branches.
The woody stems have prominent leaf scars and no hairs.
Damaged tissues produce a sticky white sap.
The deciduous leaves are alternate and on reddish petioles up to 5 cm long.
There are tiny gland-like stipules that fall off early.
The leaf blades are up to 20 (25) cm long and 10 (12) cm wide.
They can be oblong, ovate to elliptic or lanceolate with a pointed tip that may be acute or
shallow and a wedge-shaped base.
The edge can be entire, irregular or have large or small triangular lobes or teeth.
The upper surface is dark green with few or no hairs while the paler lower surface may have more hairs.
The blade is puckered between the many pinnate veins.
The branched terminal inflorescences are surrounded by up to 7 bright red leaf-like bracts.
The cyathia consist of an involucre with one (2) gland, some male flowers and a single female flower.
The small yellow cyathia and the red bracts are together commonly referred to as the flower.
The narrow elliptic bracts, on petioles are up to 7 cm long.
Each cyathium, on a 3 to 4 (6) mm long peduncle, can be nearly 10 mm long and 8 mm wide.
On the rim of the pale green, urn-shaped involucre are 5 triangular lobes with red fringed edges.
There is one (2) large nectar gland on the side of the involucre.
It is elliptic, about 5 mm long and 3 mm wide with a thick rim.
As the flower grows it changes from green to a bright yellow.
Flowers in the cyathium have no sepals or petals and are unisexual.
The male flowers are reduced to a stamen with a red filament and a 2-lobed anther.
There are hairy linear bracteoles around the stamens.
The yellow pollen is released as single grains.
The central female flower is on a pedicel which elongates before pollination.
It has a hairless ovary with 3 locules and 3 bifid styles whose bases are fused.
The stigmas are markedly 2 lobed.
The round, 3-lobed capsules are 1.5 to 2 cm with smooth grey seeds.