Sentence:  The specific staining by both direct and indirect methods showed that WTV antigen was concentrated in the pseudophloem tissue and in a few thick walled cells in the xylem region , but was not detectable in any other <WSD>tissues</WSD> of the root and stem tumors . 
Meaning of the word marked with <WSD> tags:  part of an organism consisting of an aggregate of cells having a similar structure and function