Broken stems emit a milky sap. Leaves are oppositely arranged, usually have hairs, and often have at least one dark red spot on the upper surface, especially with age. Lower leaves may sometimes be alternately arranged. Leaves at the ends of stems near the flowers often appear whorled. Occur in clusters at the ends of branches.
Flowers are relatively inconspicuous and have no petals.
A capsule that is yellowish-green in color and smooth. Each capsule normally contains 3 seeds. Opposite, lanceolate leaves with dark red markings and milky sap emitted by all parts of the plant.