What is the starchy material inside seeds that provides nutrients for the embryonic plant called?

Endosperm

Explanation:

An endosperm develops in a seed after a process known as double fertilisation occurs in the ovary of a flower whereby one of the sperm nuclei inside a pollen grain reaches the interior of the embryo sac and fuses with the two polar nuclei forming a triploid nucleus which is the primary endosperm.The endosperm supports embryonic growth by supplying nutrients and acting as a mechanical barrier to protect the embryo during seed development and germination.The endosperm has living cells that regulate seed germination.The endosperm is secreted with hydrolysis enzymes through the gibberellin hormone that induces the synthesis of amylase in the aleurone layer.The hydrolysis enzyme catabolizes starch which act as energy source providing carbon and nitrogen for seed germination and seedling development.