Ontogeny of the protophloem fibers and secondary xylem fibers within the stem of Coleus. I. A light microscope study
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Abstract
The maturation of the primary phloem fibers and secondary xylem fibers in Coleus stems was studied using Nomarski optics. The phloem fiber initials differentiate from the procambium at the same time as the earliest sieve-tube elements and companion cells. Lateral expansion and intrusive growth by the fiber primordia, which obliterate most of the other primary phloem cells, continue until the young fibers begin forming secondary walls. The single nucleus divides a few times and cytokinesis occurs by the formation of septa. After a few septa are formed, cytokinesis may not follow additional mitoses. The cell wall continues to thicken, and the septa may develop secondary layers. Although there is some nuclear degeneration, the fibers remain alive after maturity. During their maturation small groups of protophloem fibers in the corner sectors become consolidated into fewer, larger groups by the crushing of intervening cells. The maturation of xylem fibers is more rapid than that of phloem fibers. Secondary wall thickening begins in the second cell from the cambium. After wall thickening ceases, the single fusiform nucleus undergoes mitosis followed by septum formation. The septate wood fiber is alive at maturity.
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