Scanning electron micrograph of a germinating seed of a cabbage, Brassica oleracea v. capitata. The picture shows a young seedling with its testa, or seed coat (brown) split open to reveal paired cotyledons ( green). A radicle (pale yellow) has emerged from the base of the embryo (between the cotyledons and not visible here). The testa is the protective outer wall of the seed. The cotyledons, also known as seed leaves, contain a store of food for the embryo. The embryo starts to grow when water is absorbed by the seed. The first structure produced is the embryonic root, the radicle. This quickly develops a large number of fine hairs, as here, that anchor the seed in the ground and take in moisture from the surrounding soil. This early growth is supported by the nutrients stored in the cotyledons until finally the embryonic shoot appears, develops leaves, and photosynthesis can begin.