A seed is an embryo plant wrapped in a protective covering of maternal tissue (the testa). It is generally provided with a supply of nutrients contained in a separate tissue (the endosperm), though in many cases all the nutrients are absorbed by the seed leaves (the cotyledons) during the course of development. The primary function of the seed is reproduction. This does not necessarily result in an increase in numbers of the species. In a stable population, each adult is eventually replaced by another adult. This is achieved by the production of large numbers of offspring, most of which will die before reaching maturity. A seed therefore has several functions in addition to multiplication. Its small size (at least in comparison with its parent) renders it well suited for dispersal and the colonization of new areas. In addition, many seeds can withstand a much wider range of environmental conditions than the adult plants, especially extremes of drought and temperature. Their ability to undergo a period of arrested development and persist in a state of diapause is important as a means of persistence for many species, but it is especially crucial for annual plants that do not survive as adults during periods of unfavourable conditions such as seasonal cold or drought.

Sexual vs. asexual reproduction in plants

An important feature of seeds is their genetic variability. This derives from the fact that (except in the case of apomicts, mentioned below) they are the products of sexual reproduction.