sugar cane
蔗 refers specifically to sugar cane, the tall grass from which sugar is extracted.
蔗 combines 艹 (grass/plant) with 庶, which likely contributes the sound. The character was created to name the sugar cane plant.
The grass radical 艹 sits above 庶, like tall sugar cane growing in a field. Think of the plant itself.
For シャ, imagine a sugar cane field under a sharp sun: sharp sun -> シャ, and the cane grows tall.