plant which resembles wheat but has no edible grain
蘧 refers to a specific plant that looks like wheat but produces no edible grain. Its meaning is narrow and botanical, without broader abstract extensions.
蘧 combines the grass radical 艹 with 遽, which likely contributes the sound. The character was created to name a specific grass-like plant.
The grass top 艹 sits above 遽, a component associated with haste or suddenness. Imagine a plant that grows quickly like wheat but, to your surprise, yields no grain.
For ク, picture a cook mistaking this plant for wheat and trying to make flour, only to find it useless: cook -> ク, and the cook's mistake helps you remember the reading.
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