'Endosperm' is a food-laden tissue, produced inside the seeds of most of the flowering plants following fertilization. It surrounds the 'embryo' on one side or on all sides. An 'embryo' is a stage of development of a multicellular organism. The 'endosperm' provides nutrition in the form of starch though it can also contain oil and protein. Depending on its presence or absence in the mature seed, seeds are of two types: 1) non-'endospermic' or 'exalbuminous' seeds: in these 'dicots' seeds like gram, pea, groundnut, sunflower, the 'endosperm'' is completely consumed by the 'embryos'; 2) 'endospermic' or 'albuminous' seeds: in 'monocots' like wheat, barley, maize, corn and 'dicot' castor beans (exception) do not consume all 'endosperm'. So it persists in the mature seed.

Hence, 'castor bean' with others, are called 'endospermic' or 'albuminous' seeds. In wheat the 'endosperm' is selectively retained in food processing (commonly called wheat flour) and the 'embryo' (germ) and seed coat (bran) are removed, while castor bean is the source of castor oil (uses in medicine). These seeds contain between 40% and 60% oil that is reach in triglyceride ester derived from glycerol and three fatty acids and recinolein.

More Info: en.wikipedia.org