Used, in Amos 9:9, of a small stone or kernel; in Matthew 13:31, of an individual seed of mustard; in John John 15:37, of wheat. The Hebrews sowed only wheat, barley, and spelt; rye and oats are not mentioned in Scripture.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Grain'". "Easton's Bible Dictionary".