Bordered pit membranes

Figure 10. View of a bordered pit membrane with the dome-shaped pit border removed. The dark central portion is the torus. The stringlike microfibrils radiating from the torus constitute the margo portion of the pit membrane. Water flows freely from cell to cell through the openings between the margo microfibrils. 3,000X...

Figure 14. Surface view of an aspirated bordered pit membrane. The imprint of the pit aperture through the torus is the result of an extremely tight seal. 6,200X...

In the standing, living tree the bordered-pit membranes between softwood fibers act as valves to prevent the spread of air or bubbles into sap-filled cells in the event of tree injury and potential rupture to vertical water columns. Unfortunately, they perform a similar function in the processing of wood into commercial products. For example, during wood drying, substantial capillary and surface tension forces are developed upon water retreat from the fiber lumens through the pits, and the membranes move effectively (particularly in earlywood) to seal the apertures in the direction of water... 

Figure 21. Softwood bordered-pit membranes of western hemlock. (Reproduced from Ref 39. Copyright 1982, American Chemical Society.) (A) SEM of unaspirated pit in earlywood. Note porous periphery of the membrane. Rodlike bacteria are also present here, apparently filtered out onto the membrane during sample preparation split wood radial surface. (B) Light micrograph of aspirated pits (AP) in latewood cross section. (C) Light micrograph y aspirated pit and unaspirated pit (UP) in earlywood cross section. (D) Fully aspirated pit in earlywood. Note the reduction in porosity upon aspiration. (Compare to A above.)...

Figure 9. SEM photo showing bordered pit membranes from recent Douglas fir sample. Little difference could be seen between the structures shown here and those in Figure 10. Both had well-preserved margos (M) and tori (T).

Figure 13. Cross-sectional view of an aspirated bordered pit-pair. The pit membrane has moved to the border and sealed a pit aperture with the torus. In this condition, liquid flow no longer occurs between contiguous cells. 5,000X...

When vessels end, they rarely do so in isolation but rather among a group of vessels. Translocation continues into the adjacent vessels via the intervessel pits. These pits differ from softwood bordered pits in that they lack a torus and openings large enough to be readily detected with an electron microscope. Figure 17 depicts a typical intervessel pit membrane. Different arrangements of intervessel pits can be detected and are useful in the identification of hardwood species. 

Figure 17. Pit membrane from an intervessel bordered pit, Note the absence of a torus and detectable openings in the membrane. 2,400X...

It should be pointed out that most of the research on chemical composition of the pit membrane has been limited to the bordered pits. The chemical composition of the simple pit membranes in the ray parenchyma cells may or may not be the same. However, since the parenchyma cells produce the precursors for the formation of polyphenolic compounds, it is anticipated that the membrane occlusions would be similar. 

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. 

Copyright 1982, American Chemical Society.) (A) SEM of interfiber pits in earluwood as seen on the wood radial face. Note the donut-shaped borders. (B and C) SEM of pit pairs between adjacent fibers cross-sectional suiface. (D) SEM ofbordered-pit membranes (PM) in face view of a split wood radial surface. (E) Light micrograph of pit pairs as seen in cross section with a light microscope. Key PM, pit membranes PB, pit border and PA, pit aperture. 

The moisture content of heartwood in softwood trees is reduced to a level much lower than that of normal sap-wood (2, 24), During the moisture reduction period, the membranes of bordered pits in sapwood fibers have a strong tendency to become aspirated. This situation, together with that of pit membrane incrustation, greatly reduces the natural permeability of heartwood tissue to liquids and gases. 

Figure 1.8. Pit membrane of an intertracheid bordered pit in Phyllocladus glaucus with the central toms and the microfihrils of the margo through which sap passes, x 3000.

Figure 1.9. Detail of the margo and toms of Figure 1.10. A surface view of an aspirated a pit membrane in Pinus radiata showing pit membrane in Agathis australis. The pit the open texture of the microfihrils in the membrane has aspirated hack against the pit margo. x 12 500. border of the underlying tracheid. x 450.

Sideways movement of sap between adjaeent vessels oeeurs though the intervessel pits (Figure 1.25). These resemble softwood intertracheid bordered pits, but lack the separation of the pit membrane into a torus and margo. The small size of the pores in these membranes permits lateral water flow between adjacent vessels but probably prohibits air or vapour embolisms spreading to the adjacent, sapconducting vessels. 

In sapwood, the pit membrane of the bordered pit is suspended in the centre of the pit ehamber between two over-arching pit borders (Figures 1.7 to 1.10). The membrane is composed of a cobweb-like network of microfibrils, known as the margo, with a central thickened area, known as the torus. In a living tree these pits proteet the continuity of the ascending sap stream. Where the sap column is... 

The pit chambers are reduced to what ontogenetically is called the initial pit chamber, and the pit membranes are dissolved or detached. Fengel (15) reports pit membranes in fossil spruce to be quite resistant. However, in tissues where all secondary cell walls have been degraded we have not seen any pit membranes in bordered pits. 

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