Emex spinosa, commonly known as "devil's thorn" is an annual herbaceous plant of the Polygonaceae. It originates in the warmer parts of the old world, but now has spread with man to other places. It is common in disturbed areas, aspecially in sandy soils. It has shown some weedy behaviour in restricted areas within southern Australia.
The lesser jack tends to grow as a sprawling weed. The leaves are plain in shape, resembling spinach. The flowers of different sexes are clustered separately on the same plant. The plant produces many seeds with a hard, prickly casing. These are produced both in clusters along the branches and at the base of the stem. The root is thick and succulent. At the end of the plant's life, the root dries up and pulls the seeds at the base of the stem into the ground. The spiny, durable seeds make the plant a nuisance around human habitats, much like the similarly shaped seeds of Tribulus terrestris.
Although bitter, the root and leaves can be eaten