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 , <WSD>ray</WSD> cells , or pith . 
Meaning of the word marked with <WSD> tags:  a branch of an umbel or an umbelliform inflorescence