Emex australis

Name and classification

Emex australis Steinh.
Family Polygonaceae

Common names

three-cornered jack, doublegee, spiny emex

Disseminule

fruit

Description

Fruit an achene enclosed in persistent hardened perianth 4.5–11(12) mm long, 2.5–5.5 mm wide and thick (not including spines). Three sided, each side with 2 depressions in upper portion, and 2 depressions in lower portion; all the lower depressions together form a tapered base. Toward the apex, each edge (of outer segments) extends outward into a straight, rigid, sharp spine. Three bract-like inner segments form a connivent apex. Cross section slightly triangular to nearly round. Surface dull, light to dark straw-colored to brown; dusty appearance, smooth to rough. Scar cream-colored, irregular in shape, with 3–6 small holes. Embryo peripheral, cotyledons shorter than radicle; endosperm readily visible.

Identification considerations

The spiny fruits of Emex australis and Emex spinosa distinguish this small genus of only two species from Rumex, Fagopyrum, and Polygonum.

Similar species

Emex spinosa (L.) Campderá

Distribution

Africa: Kenya, Zambia, Zimbabwe; Asia: China, Taiwan; Oceania: Australia, New Zealand, New Caledonia; North America: Trinidad and Tobago, United States

native to temperate southern Africa

Habitat

subhumid and semi-arid tropical, subtropical, and temperate regions, mainly in sandy and loamy soils; a weed of cultivated land, roadsides, disturbed, and waste places

General information

Emex australis is a prostrate or erect, spreading annual herb, up to 40 cm tall. It is fast growing; fruit may be produced as early as 6 weeks. It is primarily a weed of cultivated areas. Infestations may be very dense. In South Africa and Australia it is a serious weed of wheat, other cereals, and vineyards. The spiny fruits are structured so that a spine is always pointing up, which aids in dispersal. However, the spines may cripple animals and injure barefoot humans. The fruits may be contaminants in dried fruit final products, such as sultanas and raisins.

fruits with persistent perianth

fruits with persistent perianth

A, fruit with persistent floral parts; B, fruit with floral parts removed; C, longitudinal section of fruit showing embryo; D, transection of fruit; drawing by Lynda E. Chandler

A, fruit with persistent floral parts; B, fruit with floral parts removed; C, longitudinal section of fruit showing embryo; D, transection of fruit; drawing by Lynda E. Chandler