Sentence:  Frequently a few isolated <WSD>thick</WSD> 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 xylem cells , ray cells , or pith . 
Meaning of the word marked with <WSD> tags:  not thin; of a specific thickness or of relatively great extent from one surface to the opposite usually in the smallest of the three solid dimensions