1. Erodium L’Hér. ex Aiton (stork’s bill)

Plants annual (herbaceous or woody perennials elsewhere). Stems prostrate to loosely ascending at maturity, usually reddish- or purplish-tinged. Leaves basal and opposite. Leaf blades simple or pinnately compound, the main veins pinnate (sometimes appearing 3-veined in E. texanum), the veins and margins sometimes reddish- or purplish-tinged. Stipules mostly 3 at each node (by fusion of adjacent stipules on 1 side but not the other). Inflorescences axillary clusters of mostly 2–5 flowers, usually appearing umbellate. Sepals abruptly narrowed or tapered to a short awnlike extension at the tip. Stamens 5, the filaments free, gradually or abruptly broadened toward the base. Staminodes 5, scalelike, shorter than the filaments of fertile stamens. Mericarps at maturity with the stylar beaks separating from the column and curling and/or twisting outward, the basal portion narrowly ellipsoid (tapered to a sharply pointed base), usually indehiscent. Seeds ellipsoid, the surface smooth, brown. About 80 species, widespread in temperate and warm-temperate regions, especially in the Mediterranean region.

Seed dispersal in Erodium is accomplished by the shedding of intact mericarps, which split from the central column downward from the tip. The long slender stylar beak of each mericarp is hygroscopic, that is, it becomes spirally coiled as it dries and tends to uncoil when conditions become wetter or more humid. This action can effectively drill the spindle-shaped basal portion of the mericarp into the soil. The awns have been used as crude hygrometers to measure changes in atmospheric humidity.