Allocasuarina

Allocasuarina is a genus of trees in the flowering plant family Casuarinaceae. They are endemic to Australia, occurring primarily in the south. Like the closely related genus Casuarina, they are commonly called sheoaks or she-oaks.

Wilson and Johnson[3] distinguish the two very closely related genera, Casuarina and Allocasuarina on the basis of:
Casuarina: the mature samaras being grey or yellow-brown, and dull; cone bracteoles thinly woody, prominent, extending well beyond cone body, with no dorsal protuberance;
Allocasuarina: the mature samaras being red-brown to black, and shiny; cone bracteoles thickly woody and convex, mostly extending only slightly beyond cone body, and usually with a separate angular, divided or spiny dorsal protuberance.

They are trees or shrubs that are notable for their long, segmented branchlets that function as leaves. Formally termed cladodes, these branchlets somewhat resemble pine needles, although sheoaks are flowering plants. The leaves are reduced to minute scales encircling each joint. Fallen cladodes form a dense, soft mat beneath sheoaks, preventing the development of undergrowth (a phenomenon known as allelopathy) and making sheoak woods remarkably quiet.

Another characteristic feature are the spiny “cones”, about the size of an acorn but with a texture more resembling a conifer cone. However, sheoak “cones” are a woody fruit. Male specimens bear no fruit and are sometimes colloquially referred to as a “heoak”[citation needed].

As with legumes, sheoak roots possess nodules containing symbiotic nitrogen fixing bacteria; together with their highly drought-adapted foliage, this enables sheoaks to thrive in very poor soil and semi-arid areas. However, sheoaks are much less bushfire-tolerant than eucalypts.

Fossils of closely related species have been found dating back to the time of Gondwana.