Sentence:  Frequently a few isolated thick walled <WSD>cells</WSD> or , rarely , groups of such cells in the xylem region , 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:  (biology) the basic structural and functional unit of all organisms; they may exist as independent units of life (as in monads) or may form colonies or tissues as in higher plants and animals