Emex australis
（重定向自Doublegee）
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Emex australis, commonly known in English as southern threecornerjack, devil's thorn, or double gee (also doublegee, from the old Afrikaner name dubbeltge-doorn - 'devil's thorn'), or three-cornered jack, is a herbaceous plant of the Polygonaceae. It occurs indigenously in South Africa and is also an invasive species in Australia & Texas in the USA.