tall bamboo
簹 centers on a specific type of tall bamboo. The meaning is narrow and concrete, referring to the plant itself.
簹 combines 竹 (bamboo) with 當, which likely contributes the sound トウ. The character was created to name a specific kind of tall bamboo.
The top is bamboo 竹, and the bottom 當 looks like a tall, structured plant. Picture a towering bamboo grove with this character as its nameplate.
For トウ, imagine a tall bamboo tower: tower -> トウ, and the bamboo grows as high as a tower.