The seed coat prevents destruction of the seed by dehydration or predation. In bitegmic seeds the testa is derived from the outer integument, and the inner integument forms the tegmen (Fig. 6.1). In unitegmic seeds the term ‘‘testa’’ applies to the entire seed coat. Seed coats are multilayered tissues; they generally
include a hard, protective mechanical layer that is formed from all or part of the testa or tegmen23. In exotestal seed coats the mechanical layer is derived from the outer epidermis of the outer integument, whereas in endotegmic seed coats it is derived from the inner epidermis of the inner integument. In some species the mechanical layer consists of one or more rows of elongated, often with characteristic papillate or striate surface sculpturing(Fig. 6.2). Some seeds possess epidermal trichomes; for example, the seed coat hairs of Gossypium (cotton) are an important source of textile fibres.
Seed-coat vasculature usually consists of a single bundle passing from the raphe to the chalaza, but this can vary in extent and degree of branching. Many seed coats possess specialized structures that are related to dispersal. For example, some wind-dispersed seeds possess wings, and some animal-dispersed seeds are fleshy
The fleshy part of the seed coat, termed the sarcotesta, is most commonly formed from part of the outer integument. Arils are fleshy outgrowths of the funicle. Some plants, especially parasitic or mycoheterotrophic plants such as Monotropa or orchids, produce large numbers of highly reduced ‘‘dust seeds’’ from each ovary; these minute seeds can be blown over long distances. The seed coat of many orchids lacks vasculature entirely (Fig. 6.3).