A seed is a mature ovule containing an embryo enclosed within protective seed coats (testa = outer coat, tegmen = inner coat). The main components are: Embryo: the young plant in miniature, consisting of embryonic axis (hypocotyl-radicle axis) and one (monocot) or two (dicot) cotyledons. Endosperm: food storage tissue (when present) derived from double fertilisation (triploid 3n). Seed coats (testa + tegmen): derived from integuments of the ovule. Seeds are classified based on whether endosperm persists in the mature seed: Albuminous/endospermic seeds: endosperm present and prominent as the main food reserve in the mature seed. Non-albuminous/exalbuminous seeds: endosperm absent or vestigial in the mature seed, with food stored in the cotyledons instead.

Albuminous seeds are those in which the endosperm persists as the primary food storage tissue in the mature seed. The cotyledons are typically thin, paper-like structures that serve primarily to transfer stored nutrients from endosperm to embryo during germination rather than storing nutrients themselves. Monocot examples (virtually all monocots are albuminous): Maize (Zea mays): the entire large starchy part of the kernel is endosperm; only a small region at the base is the embryo containing the single cotyledon (scutellum), which absorbs starch from the endosperm during germination. Wheat, rice, barley, and other cereals: similar organisation with prominent starchy endosperm. Coconut: the white "meat" is solid endosperm (coconut water is liquid endosperm). Dicot examples of albuminous seeds: Castor (Ricinus communis): persistent, oily endosperm rich in fats and the toxic protein ricin; thin papery cotyledons. Sunflower: persists with some endosperm. Neem, Rubber plant also retain some endosperm.

Non-albuminous seeds are those in which the endosperm is completely absorbed by the developing embryo's cotyledons during seed maturation, so no endosperm remains in the mature seed. The cotyledons become engorged and fleshy, serving as the food storage organs. These are predominantly dicot seeds: Legumes: pea (Pisum sativum), bean (Phaseolus vulgaris), gram (Cicer arietinum), groundnut (Arachis hypogaea), soybean (Glycine max), lentil (Lens culinaris). These seeds are rich in protein and starch in their cotyledons. Brassica: mustard (Brassica juncea/nigra), radish — the cotyledons store oils. Cucurbit seeds: pumpkin, cucumber — oily cotyledons. The economic importance of non-albuminous seeds is enormous — most legume crops used for human and animal protein nutrition are non-albuminous, storing protein in the cotyledons (the edible "pulses" of dried legume seeds are primarily cotyledon material).

Endosperm formation in angiosperms is a consequence of double fertilisation — a process unique to flowering plants (angiosperms) where both sperm cells brought by the pollen tube contribute to fertilisation events. First fertilisation: one sperm cell (n) fuses with the egg cell (n) → zygote (2n) → embryo. Second fertilisation: the other sperm cell (n) fuses with the diploid secondary nucleus or two polar nuclei (n + n = 2n) in the central cell of the embryo sac → primary endosperm nucleus (3n, triploid) → endosperm through repeated mitotic divisions. This double fertilisation event simultaneously initiates both embryo development (the future plant) and endosperm development (the food storage tissue), ensuring that food reserves are produced specifically when and where they are needed — in seeds containing embryos. The triploid (3n) nature of endosperm has been exploited commercially in the production of seedless watermelons and certain other seedless fruit varieties, and the endosperm-specific gene expression that directs carbohydrate and protein accumulation in cereal seeds has been intensively studied for agricultural improvement.