This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 莔 is uncertain. It visually combines 艹 (plant) with 囧, but the original meaning and component function are unclear.
The top 艹 suggests a plant, and the bottom 囧 looks like a window or enclosure. Picture a rare, unnamed plant growing inside a hidden garden window.
For ボウ, imagine a bow tied around the mysterious plant: bow -> ボウ, and the bow marks the unknown plant.