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 <WSD>epidermis</WSD> , cortex , most xylem cells , ray cells , or pith . 
Meaning of the word marked with <WSD> tags:  the outer layer of the skin covering the exterior body surface of vertebrates