sedges, rushes
茢 refers to grass-like wetland plants: sedges and rushes. The meaning is narrow and botanical, without a broader conceptual center.
茢 combines the grass radical 艹 with 列, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a plant name.
The top 艹 shows it is a plant, and the bottom 列 (line up) suggests these plants grow in dense rows or clusters like sedges and rushes in a marsh.
For レツ, imagine a row of sedges lined up like a regiment: the word 'regiment' starts with 're' and ends with 'ts' sound, cueing レツ.