Opuntia monacantha Haw.

• an upright, fleshy, tree-like plant usually growing 2-3 m tall.
• its stems consist of a series of very flattened fleshy segments.
• these spiny stem segments are hairless and some of them usually droop towards the ground.
• its showy flowers (3-6 cm across) are yellow with reddish-coloured markings on their outer 'petals'.
• its fleshy fruit (4-7.5 cm long and 2-5 cm wide) turn purplish-red as they mature.

[From: Environmental Weeds of Australia]