In addition to conducting cells, phloem tissue also contains unspecialized parenchyma, the phloem parenchyma, and often strengthening cells such as phloem fibers. When sieve tube members are present, they are always accompanied by companion cells (FIG. 7.15), specialized parenchyma cells that develop from the same procambial initial (mother cell) as the associated sieve tube member.
figure 7.14 Longitudinal section of Zea mays xylem showing annular (A) and helical thickenings (B) (Extant). Bar = 60 pm.
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