water-chestnuts, caltrop
萕 refers to water-chestnuts or caltrop, aquatic plants with edible corms or spiky fruits. The two meanings are closely related as different names for the same plant or its parts.
萕 combines the grass radical 艹 with 斉, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a plant name written with a phonetic component.
The top grass radical 艹 hints at a plant, and the bottom 斉 looks like a neat arrangement. Imagine water-chestnuts growing in orderly rows in a pond, their spiky caltrop fruits floating among the grass.
For セイ, picture a calm pond where water-chestnuts grow, and you say 'say' as you point them out: say -> セイ.