Top grass radical — the standard 3-stroke compressed form of 艸. Indexes 茨 in the plant family. The original meaning was a thorny plant (the puncture vine) and, by extension, the rough thatched roofing made from such weedy growth.
Bottom 次 supplies the sound with a tone shift: cì → cí. Same phonetic powers 资 capital, 姿 posture, 瓷 porcelain — the cí/zī family. Pure phonetic; the "next, sequence" meaning of 次 plays no part in 茨.
