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 , <WSD>cortex</WSD> , most xylem cells , ray cells , or pith . 
Meaning of the word marked with <WSD> tags:  the tissue forming the outer layer of an organ or structure in plant or animal