How does solid endosperm form during coconut maturation

The formation of solid endosperm in coconut proceeds via free-nuclear endosperm followed by cellularisation and maturation. Early in seed development the endosperm is free-nuclear (nuclei without cell walls) in the embryo-sac cavity. Then around ~6 months the cellular endosperm develops: a thin coating of jelly-like tissue appears then progressively walls form around nuclei to create true cells.

As the nut matures, the liquid cavity volume reduces and the solid endosperm cells accumulate oils (triacylglycerols, especially lauric acid) and storage compounds. The parenchyma cells expand radially inward from the inner surface of the endocarp, thus gradually filling the cavity with solid tissue (meat). Concurrently the liquid endosperm may decline in volume. This change allows the seed to store reserves needed for germination.