Emex spinosa

Name and classification

Emex spinosa (L.) Campderá
Family Polygonaceae

Common names

devil’s thorn, spiny emex, lesser jack

Disseminule

fruit

Description

Fruit an achene enclosed in persistent hardened perianth, 4–8 mm long, 2.4–5 mm wide and thick (not including spines). Three sided, each side with 6–10 depressions in upper portion, arranged in 2 columns, and 2 depressions in lower portion; all the lower depressions together form a tapered base. Toward the apex, each edge (of outer segments) forms a rigid recurved spine. Three inner segments form an erect, connivent apex taller than spines. Cross section triangular to nearly round. Surface dull, light to dark straw colored or reddish-brown; dusty appearance, rough. Scar variable. Embryo peripheral, J-shaped, cotyledons longer than radicle; endosperm readily visible.

Identification considerations

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

Similar species

Emex australis Steinh.

Distribution

Mediterranean countries, Australia, Kenya, and Mauritius; South America: Argentina, Brazil, Paraguay, Uruguay; United States

native to the Mediterranean

Habitat

sandy shores, disturbed ground, perennial crops, grasslands

General information

Emex spinosa is an erect, annual herb, up to 40 cm tall. This species has been found growing with E. australis in Australia. It has the potential to become a serious weed (like E. australis) by reducing the yield of cereal crops, and by causing injury to livestock with its spiny fruits.

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