Jesus said unless what falls to the ground and dies, it remains only a seed?

Answer: A kernel of wheat

Jesus said a kernel of wheat that dies bears much fruit, as the hour came for the Son of Man to be glorified.

“But Jesus replied, “The hour has come for the Son of Man to be glorified. Truly, truly, I tell you, unless a kernel of wheat falls to the ground and dies, it remains only a seed. But if it dies, it bears much fruit.”— John 12:23-24 (BSB)