paper-plant, aralia papyrifera
蓪 refers to a specific plant used for making paper, the Aralia papyrifera, also known as the paper-plant. Its meaning is narrowly botanical.
蓪 is a phono-semantic compound: the grass radical 艹 indicates a plant, while 通 (トウ) provides the sound. The character was created to name the paper-plant Aralia papyrifera.
The grass radical 艹 sits on top of 通, like a plant that 'passes through' (通) to become paper. Picture the paper-plant growing up through the soil, its fibers ready to be made into paper.
For トウ, think of the paper-plant's tall, towering stalk: 'tall' sounds like トウ, and the plant towers over the grass.