type of water plant
蓴 refers to a specific type of water plant, likely a kind of edible aquatic herb. Its meaning is narrow and botanical, without broad abstract extensions.
蓴 combines the plant radical 艹 with 專, which likely provides the sound. The character was created to name a specific water plant, but the exact historical development is uncertain.
The top 艹 shows it's a plant, and the bottom 專 looks like a specialized tool or container. Imagine a rare water plant grown in a dedicated pond, marked with a special sign.
For シュン, picture a water plant shooting up quickly in spring: "shoot" -> シュン, as the plant shoots up from the water.
sea lettuce (Ulva spp.)