Double fertilization is the fusion of two male gametes brought by a pollen tube to two different cells of the same female gametophyte in order to produce two different structures. It is found only in angiosperms where it was first discovered by Nawaschin in 1898 in Fritillaria and Lilium. Out of the two male gametes one fuses with egg or cosphere to perform generative fertilization. Generative fertilization is also called syngamy or true fertilization. It give rise to a diploid zygote or cospore. the second male gamete fuses with the two haploid polar nuclei or diploid secondary nucleus of the central cell to form a triploid primary endosperm nucleus (PEN). This second fertilisation is called vegetative fertilisation.

In coconut (Cocos nucifera), the surrounding white kernel called coconut meal is cellular endosperm and the coconut water (also called coconut milk) in the centre is free nuclear endosperm made up of thousands of nuclei.

Endosperm represents the food storing tissue of a seed. It is produced as a result of double fertilisation in angiosperms. In most monocots and some dicot seeds, the food reserve remains in the endosperm. They are called endospermic or albuminous seeds, e.g., maize, wheat, castor bean, coconut, barley, rubber. However, in the majority of dicot seeds(e.g., orchids, Sagittaria), the endosperm is consumed during seed development and the food is stored in cotyledons and other regions. They are called non endospermic or exalbuminous seeds.

During the development of dicot embryo, initially the dicot embryo is globular and undifferentiated. Early embryo with radial symmetry is called proembryo. It is transformed into embryo with the development of radicle, plumule and cotyledons. Two cotyledons differentiate from the sides with a faint plumule in the centre. At this time the embryo becomes heart-shaped. Part of embryo axis between the plumule and cotyledonary node is epicotyl (above the level of cotyledons) while the part between radicle and cotyledonary node is called hypocotyi (below the level of cotyledons). The single cotyledon of monocotyledonous seed {e.g. maize grain) is called scutellum. It occupies the major portion of the embryo regions of grain.

Given figure represents the monocotyledonous embryo of a grass.

The given figure represents the castor seed. It is an oblong mottled brown endospermic and dicotyledonous seed. A thick hard but brittle testa covers the seed. A thin perisperm lies below it and around the kernel. A white oily endosperm lies below the perisperm. It stores food as oil drops and proteins. The endosperm is the source of castor oil. Embryo lies in the centre of the seed. It consists of a short embryo axis bearing two thin semitransparent oval cotyledons, a small indistinct plumule and a knob-shaped radicle.

In some seeds, remains of nucellus persist. This residual nucellus which persists in the seed is called perisperm, e.g., black pepper, coffee, castor, cardamum, Nymphaea.

In angiosperms. Endosperm is a triploid (3n) structure
∴ 3n = 24, then n = 8
Root represents is the diploid structure of a plant, therefore 2n = 16

In angiosperms, endosperm is a triploid structure.
∴ 3n = 24 (given), then n = 8
As gametes are haploid structures, therefore number of chromosomes in gametes is 8

Nuclear endosperm (X) is the most common type of endosperm. It is named so because it contains free nuclei in the beginning. The primary endosperm nucleus divides of free nuclei. Meanwhile central vacuole appears in the central cell and pushes the cytoplasm containing the nuclei to the periphery. The cytoplasm thickens so that the vacuole decreases in size. It ultimately disappears with the exception of a few cases. The multinucleate cytoplasm undergoes cleavage and gives rise to a multicellular tissue, e.g., maize, wheat, rice, sunflower, Capsella bursa-pastoris.

There are several records of very old yet viable seeds. The oldest one is of a lupine (Lupinus arcticus), excavated from Arctic Tundra. Seeds of Lupinus arcticus, at least 10,000 years old were found in lemming burrows deeply buried in permanently frozen silt of Pleistocene age in unglaciated central Yuzon. They readily germinated in the laboratory and have since grown into normal healthy plants.

 In many aquatic plants with emergent flowers, pollination occurs by wind or insects, e.g., lotus, water lily, water hyacinth.

After fertilisation, the integuments of the ovule form the protective coats (seed coats). Outer integument develops into testa and the inner one develops into tegmen.