Cell/fiber secondary wall

Fig. 4.2.4. a-d Changes in the UV absorption spectra ol various cell wall layers of hybrid poplar during differentiation, showing the initial (/), middle (2), and later (5) stages of lignification. V-SIV Secondary wall of vessel F-SW secondary wall of fiber FF-CC cell corner of middle lamella between fibers VF-CC cell corner of middle lamella between vessel and fiber, e UV absorption spectra of mature fiber secondary wall (F-SIV) and mature cell corner of middle lamella between fibers (FF-CC) obtained from the same poplar fiber. The UV spectrum shown by (FF-CC)-(F-SW) is the difference spectrum between the cell corner middle lamella and fiber secondary wall. (Takabe et al. 1987)... 

Sculpturing. Pits. Softwood and hardwood fibers have closed ends, but a special wall feature facilitates movement of the tree s sap stream from one fiber to another, from fibers to vessel elements, and from fibers to ray cells. This special feature is a small opening or recess in the fiber secondary wall known technically as a pit. 

Lignin in the true middle lamella of wood is a random three-dimensional network polymer comprised of phenylpropane monomers linked together in different ways. Lignin in the secondary wall is a nonrandom two-dimensional network polymer. The chemical structure of the monomers and linkages which constitute these networks differ in different morphological regions (middle lamella vs. secondary wall), different types of cell (vessels vs. fibers), and different types of wood (softwoods vs. hardwoods). When wood is delignified, the properties of the macromolecules made soluble reflect the properties of the network from which they are derived. 

The process of differentiation that transforms primary cells into secondary cells is only partially understood. In such thick-walled cells as tracheids, fibers, and vessels in mature tissues, the greater part of the wall is made up of secondary thickening deposited after expansion of the primary wall has stopped. In cells that show localized growth (for example, tip growth), the secondary wall may well start to form in areas removed from the growing zone, while the latter is still active.2... 

Water conduction in a tree is made possible by pits, which are recesses in the secondary wall between adjacent cells. Two complementary pits normally occur in neighboring cells thus forming a pit pair (Fig. 1-5). Water transport between adjacent cell lumina occurs through a pit membrane which consists of a primary wall and the middle lamella. Bordered pit pairs are typical of softwood tracheids and hardwood fibers and vessels. In softwoods the pit membrane might be pressed against the pit border thus preventing water transport, since the torus is impermeable. The pits connecting tracheids, fibers, and vessels with the ray parenchyma cells are half-bordered. Simple pits without any border connect the parenchyma cells with one another. 

Fig. 7-3. UV absorbance (222 nm, 0.5 p,m section thickness) by various morphological regions of spruce fibers delignified to various lignin contents by the kraft and acid sulfite method (Wood and Goring, 1973). S, secondary wall P, primary wall CCP, primary wall at the cell corner.

FIGURE 5.17 Cellulose diffraction patterns. Top left synchrotron radiation x-ray diffraction pattern for cotton fiber bundle. The fiber was vertical and the white circle and line correspond to a shadow from the main beam catcher and its support. (Credit to Zakhia Ford.) Top right electron diffraction pattern of fragments of cotton secondary wall. The much shorter arcs in the top right figure are due to the good alignment and small number of crystallites in the electron beam. (Credit to Richard J. Schmidt.) Bottom a synthesized powder pattern for cellulose, based on the unit cell dimensions and crystalline coordinates of Nishiyama et al. [209]. (Credit to Zakhia Ford.) Also shown are the hkl values for the Miller indices. The 2-theta values are for molybdenum radiation instead of the more commonly used copper radiation. 

Kim, H.J. and Triplett, B.A., Regulation of gene expression in the transition from cell elongation to secondary wall formation in cotton fiber. Proceedings of the Beltwide Cotton Conferences, National Cotton Council, Memphis, TN, 2005, 1043. 

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