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匍 supplies the sound — pú level-tone, no drift. Pleasingly,
匍 means 'to crawl, creep along the ground,' a perfect description of how a grapevine actually grows; so the phonetic doubles as a faint semantic hint.
葡萄 is a Han-era loanword from a Central Asian language (likely Bactrian), written with these two sound-tagged plant characters.