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pro·to·xy·lem | \ ˌprō-tə-ˈzī-ləm 
, -ˌlem \
, -ˌlem \Definition of protoxylem
: the first-formed xylem developing from procambium and consisting of narrow cells with annular, spiral, or scalariform wall thickenings
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The first known use of protoxylem was in 1887
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