The green tissue (that not the tissue pointed out by the arrow) in this angiosperm seed is called Endosperm
The endosperm plays the important role in the supporting embryonic growth and by the supplying nutrients, protecting the embryo and also controlling the embryos growth by the acting as the mechanical barrier and during the seed development and the germination
Endosperm is formed when the two sperm of nuclei inside the pollen grain reach to the interior of the female gametophyte (sometimes it called the embryo sac) rich in the cellular and reserves, and they are compactly arranged without the intercellular spaces..
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