Sentence:  Frequently a few isolated thick walled cells or , rarely , groups of such cells in the xylem region , were also specifically stained , but there was no such staining in epidermis , cortex , most <WSD>xylem</WSD> cells , ray cells , or pith . 
Meaning of the word marked with <WSD> tags:  the woody part of plants: the supporting and water-conducting tissue, consisting primarily of tracheids and vessels