81.The warp was always made of willow，and the most commonly used welt wassedge root—a woody fiberthat could easily be separated into strands no thicker than a thread．

(a woody fiber作sedge root的同位语)

经向篾条总是用柳条做成，使用最多的横向篾条是莎草根，这是一种能够被轻易分成不比线条粗的多股的木质纤维。

解析：

主句为两个并列句：

并列句1：The warp was always made of willow，

并列句2：the most commonly used welt wassedge root—a woody fiber

从句1：that could easily be separated intostrands．

从句2：no thicker than a thread

从句1作为同位语从句，修饰并列句2中的宾语sedge root—a woody fiber

从句2修饰从句1中的strands