In castor seed testa and tegmen are united together. Seed coat is tough and bright due to scleroprotein. Over narrower end a brownish pad is found which is called caruncle. Caruncle is carbohydrate in nature.

This protects micropyle and develops as an integumental outgrowth after fertilisation.Below seed coat a very thin membrane is found over kernel and called perisperm (the persistant nucellus).Below perisperm there is a large, white, swollen and oily mass called endsoperm.