cau·dex 
(kôdĕks′)
(kôdĕks′)n. pl. cau·di·ces (-dĭ-sēz′) or cau·dex·es
1. The thickened, usually underground base of the stem of many perennial herbaceous plants, from which new leaves and flowering stems arise.
2. The trunk of a palm or tree fern.
[Latin caudex, tree trunk.]