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 xylem cells , ray cells , or <WSD>pith</WSD> . 
Meaning of the word marked with <WSD> tags:  soft spongelike central cylinder of the stems of most flowering plants