type of grain, polygala japonica
葽 is a rare kanji for a specific plant: a type of grain or Polygala japonica. Its meaning is narrow and botanical, with no broad conceptual center.
葽 combines the grass radical 艹 with 要, which likely contributes a phonetic element. The exact historical development is uncertain.
The grass radical 艹 sits above 要, like a plant that is essential or important. Picture a field of grain that is a vital crop.
For ヨウ, imagine a yellow grain field: yellow -> ヨウ, and the essential grain grows under the sun.
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