The warp was always made of willow，and the most commonly used welt was sedge root—a woody fiber that 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 was sedge root—a woody fiber
从句1：that could easily be separated into strands.
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
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