[proh-tuh-zahy-luhm, -lem]
noun Botany.
the part of the primary xylem that develops first, consisting of narrow, thin-walled cells.

1895–1900; proto- + xylem Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
protoxylem (ˌprəʊtəˈzaɪləm)
Compare metaxylem the first-formed xylem tissue, consisting of extensible thin-walled cells thickened with rings or spirals of lignin

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
protoxylem   (prō'tə-zī'ləm)  Pronunciation Key 
The first formed xylem that differentiates from the procambium.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
During internode elongation only protoxylem vessels stained for the presence of lignin.
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