Board Thread:General Discussion/@comment-28128661-20160404143233/@comment-27701762-20160418055537

GetYourFix wrote:

Alphys took the first flower that bloomed when Toriel left (or something to this end) and injected it with tons of DETERMINATION. Nothing happened. I glossed over this detail until recently - Alphys did not know the Flowey that we know; Asriel had not been induced into the vessel in the True Lab. The lore makes more sense if we go at it knowing this. Why would Alphys return an Asriel-flower to ASGORE's garden? That's messed up. She did not do this and instead returned what she thought to be a failed experiment that showed no signs of life.

The determined vessel was replanted into the dust-riddled garden and absorbed Asriel's essence, giving it his memories and consciousness. True Lab, Entry 18: "the flower's gone."