Sentence:  Frequently a few isolated thick walled cells or , rarely , groups of such cells in the xylem <WSD>region</WSD> , were also specifically stained , but there was no such staining in epidermis , cortex , most xylem cells , ray cells , or pith . 
Meaning of the word marked with <WSD> tags:  a part of an animal that has a special function or is supplied by a given artery or nerve