stalks of grasses, etc.
莛 refers to the slender, upright stalks or stems of grasses and similar plants.
莛 combines the grass radical 艹 with 廷, which likely provides the sound. The character is used for grass stalks.
The grass top 艹 sits above 廷, like a court official standing tall and straight. Picture a row of grass stalks standing upright like courtiers in a hall.
For テイ, imagine a tea stalk: tea -> テイ, and the stalk is a slender grass stem.