Pencil tree or Milk bush are a semi-succulent shrub or sometimes a tree up to 12 or 14 m high.
Large old trees have grey or brown bark on a woody trunk that can be more than 20 cm across.
Leafy side branches each typically have up to around 6 branchlets in a whorl at the tip.
Each of these then branch in similar whorls giving rise to dense foliage.
The smallest pale green cylindrical branchlets are only 7 to 8 mm thick.
These younger branchlets have pale longitudinal grooves containing pores (stomata) for gas exchange.
Their tips may have a few curly brown hairs that are soon lost.
All parts of the plant exude a white latex when damaged.
The few simple alternate leaves on young branchlets, with a 1 to 2 mm petiole fall off early.
The almost linear leaves, up to around 2.5 cm long and a few mms wide have a few crinkly hairs.
The 2 tiny brown glandular stipules at the leaf base remain on either side of the leaf scars.
Flowers are unisexual with male and female on different plants.
Inflorescences, on the branch tips consist of a tight cluster of a few cymes.
Each cyme has up to 4 levels of branching with branches under 1 mm long.
There are bracts at the base of each branch and there may be stipular glands at their base.
There are short brownish hairs on the branches and the edges of the bracts.
Each terminal branch of the cyme has a false flower called a cyathium.
Each cyathium has one or more unisexual flowers surrounded by a cup-shaped whorl of bracts – the involucre.
There are 5 green, yellow or orange petal-like nectary glands on the rim of the involucre.
Between these are small 0.5 mm long lobes with hairs on the edge.
Flowers have no sepals or petals.
Male cyathia have dense male flowers and linear bracteoles.
Each male flower has a pedicel holding 1 stamen around 4.5 mm long.
There is occasionally a female in the centre but it aborts.
Female cyathia have 1 to a few female flowers surrounded by a few linear bracteoles.
Flowers consist of a hairy ovary 0.5 mm high with 3 locules.
There are 3 fused styles and 3 bifid stigma lobes that curl back.
Initially directly attached the ovary develops a stalk up to 1 cm long as the fruit develops.
The fruit are an almost spherical septicidal capsule on a pedicel with the nectaries at the base.
The slightly 3-lobed green than pink capsules are around 8.5 mm wide.
The pale brown speckled seeds have a dark line down them.