Sentence:  `` Now '' , said Arlene , eventually , making them both sit in formation on a big root of a live oak , the sort of <WSD>root</WSD> that divided itself and made their bottoms sag down and feel comfortable . 
Meaning of the word marked with <WSD> tags:  (botany) the usually underground organ that lacks buds or leaves or nodes; absorbs water and mineral salts; usually it anchors the plant to the ground