Nucellus is one of the components of the ovule of a plant. As the largest part of the ovule, the nucellus contains both the embryo sac and the nutritive tissue. While the embryo can be found in the embryo sac after the process of fertilisation has been finished, it is the nutritive tissue that provides the embryo with adequate nutrition.The embryo sac is also known as the gametophyte and is known to be at the very centre of the flowering plant.