stem, stalk
莖 centers on the main supporting axis of a plant: the stem or stalk that holds up leaves and flowers.
莖 combines the grass radical 艹 with 巠, which likely contributes the sound and may relate to the idea of a vertical line or stalk. The exact historical development of 巠 is uncertain.
The top 艹 is grass, and the bottom 巠 looks like a vertical line with a crossbar, like a plant's stem. Picture a sturdy stalk rising through the grass.
For ケイ, imagine a plant stem shaped like the letter K: K -> ケイ, and the stem stands tall like a capital K.