Emex australis Steinh.

Family: Polygonaceae

Disseminule:  Fruit

Common names:  three-cornered jack, doublegee, spiny emex

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 remarks

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

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