Sentence:  The <WSD>stele</WSD> may have&mdash;in different structures&mdash;one to many protoxylem (primitive wood) groups, and is accordingly described as monarch...diarch...triarch...tetrarch...polyarch. 
Meaning of the word marked with <WSD> tags:  The central core of a plant's root and stem system, especially including the vascular tissue and developed from the plerome.