Euphorbia mammillaris is a short-stemmed dioecious succulent shrublet that forms dense clusters of many erect deep green stems with "spines," which are solitary sterile peduncles. The stems are club-like, ribbed, up to 14 inches (35 cm) tall, and up to 2.4 inches (6 cm) in diameter. The spines are occasionally present and scattered, thick, whitish, and up to 0.4 inches (1 cm) long. Leaves are small, green, and ephemeral. The plant produces yellow solitary cyathia at the tip of each stem in late winter to early summer.
Source: World of Succulents