A rounded or oval mass of parenchymatous tissue in an ovule, containing the embryo sac. Its size and shape may be used as a diagnostic character. It is almost totally surrounded by the integuments except for a small channel, the *micropyle
, through which the pollen tube may grow prior to fertilization. At fertilization the nucellus may be reabsorbed as the embryo develops or it may persist to form a nutritive *perisperm
in some seeds.