Exotesta

Exotesta refers to the outermost component of the seed coat (testa) in certain plant seeds. It is produced from the outer integument of the ovule and forms the external cellular layer that encases the seed. In many seeds, the seed coat is described as exotesta (outer), mesotesta (middle), and endotesta (inner). The exotesta is typically a single layer of epidermal cells, which may be sclerenchymatous and heavily thick-walled, or more thin-walled and papery depending on the taxon.

The surface of the exotesta commonly bears ornamentation such as bumps, ridges, or hairs, contributing to the seed's external appearance and texture. This ornamentation can influence seed dispersal, adhesion to surfaces, or germination cues. Developmentally, the exotesta arises during seed development from the outer integument, and the cells often become lignified, forming a protective barrier.

In many species the exotesta remains distinct from the endotesta, whereas in others the two layers are differentiated only microscopically and the mesotesta is absent or reduced. The presence and character of the exotesta are variable across angiosperms, reflecting diverse protective roles and evolutionary histories.

Functionally, the exotesta primarily acts as a protective barrier against mechanical damage, desiccation, and pathogens. Its thickness and chemical composition can influence water uptake and dormancy, and surface features can affect interactions with dispersal agents such as animals or wind.

Because exotesta characteristics—such as thickness, cell structure, and surface ornamentation—are readily observable in seeds, they are commonly used in plant anatomy and systematic descriptions and can aid in identifying taxa at various levels.