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次生木质部分散的导管分子周围出现了薄壁细胞的脱分化—-副形成层—-向内产生三生术质部，向外产生具大量薄壁细胞的三生韧皮部，这种三生结构的薄壁细胞中贮藏着营养物质。

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甘薯

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次生木质部木质化、薄壁细胞少——-不可形成 副形成层

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甜菜