vine
薟 centers on a vine, a climbing or trailing plant. The meaning is narrow and concrete.
薟 combines the grass radical 艹 with 僉, which likely provides the sound. The exact historical development is uncertain.
The grass radical 艹 sits above 僉, like a vine growing over a structure. Think of a vine climbing up and covering the shape below.
For レン, imagine a vine wrapping around a lens: lens -> レン, and the vine coils around the glass.