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    Author(s): Elaine T. Howard
    Date: 1971
    Source: Wood Science, Vol. 3(3): 136-148
    Publication Series: Miscellaneous Publication
    PDF: View PDF  (2.86 MB)


    The living inner bark is composed of thin-walled elements - soeve cells, albuminous cells, longitudinal and ray parenchyma, and epithelial cells. the rhytidome or outer bark is dead and has alternating areas of distorted phloem enclosed by periderm layers. Periderms consist of phellogen and its derivative cells -- phelloderm and phellem. Phelloderm cells, to the inside of the phellogen. may be thickened or expanded. Radially elongated innermost phelloderm cells may be confused with expanded parenchyma. Phellem, to the outside of the phellogen, is comprised of bands of thin-walled suberized cork cells alternating with bands of thick-walled, unsuberized spiculate stone cells. The proportions of each tissue may vary widely within a single sample. Only the innermost periderm is alive; each periderm subsequently dies when a new periderm forms closer to the vascular cambium. Periderms do not appear to be correlated with annual growth. In the transformation from inner bark to rhytidome, sieve cells collapse and parenchyma enlarge greatly. After this obliteration process, sieve cells probably form less than 30 percent of the rhytidome volume. Sieve cells are comparable to wood tracheids in length; all other cells are much smaller. Stone cells and the outermost, thickened phelloderm cells are the only heavily lignified elements. Stone cells comprise about 10 percent or less of rhytidome, but they greatly influence density and hardness. In transverse view, longitudinal parenchyma are in discontinuous tangential rows or scattered. Elongated styloid crystals occur in lumina of many sieve cells and parenchyma. Longitudinal resin canals usually are not present; horizontal ducts occur in fusiform rays. Periderm shape and spacing varied greatly within species. Periderm color was dark in pond and loblolly pines, light in Virginia pine, and variable in all other species. Bark obliteration and thickness decreased with height in tree; no other variation with height was noted. Barks of Virginia, sand, and spruce pines were harder, more fibrous, and less obliterated than barks of the other seven species. Stone cell bands form the outer margins of spruce pine periderms; thin-walled cork cells comprise the outer portion of the periderms in longleaf, Virginia, slash, and sand pines. Structural factors such as periderm shape and spacing, amount of stone cells, tangential zones of weakness, and degree of obliteration and expansion may explain most variation in physical and mechanical properties of southern pine barks.

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    Howard, Elaine T. 1971. Bark structure of the southern pines. Wood Science, Vol. 3(3): 136-148

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