culm

noun

the hollow stem of a grass or cereal plant, especially that bearing the flower.

mid 17th century: from Latin culmus ‘stalk’.

1. a series of Carboniferous strata in south-western England, mainly shale and limestone with some thin coal seams.
2. coal dust or slack.

Middle English (in the sense ‘soot, smut’, now only Scots): probably related to coal.