the.com/granum

means A granum is a stack of flattened, coin-like membrane discs (thylakoids) inside a chloroplast, piled up like plates to pack more light-catching chlorophyll into a small space.

from Straight from Latin 'granum,' meaning 'grain' or 'seed' — the same root that gives us 'grain,' 'granule,' and 'garnet.' Early microscopists peering into chloroplasts saw these dense little stacked bodies and, lacking a better word, reached for the Latin term for a small, compact kernel — a granule-like speck within the larger structure. The plural, 'grana,' is still used for the whole collection of stacks inside one chloroplast.