Xylem tracheid

Xylem Tracheids, vessel elements, Conduction of water and minerals ... 

II. Dependence upon basipetal transport. Physiol Plant 29 288-292 Wodzicki TJ, Wodzicki AB (1980) Seasonal abscisic acid accumulation in stem cambial region of Firms silvestris and its contribution to the hypothesis of a late-wood control system in conifers. Physiol Plant 48 443 47 Wodzicki TJ, Wodzicki AB (1981) Modulation of the oscillatory system involved in polar transport of auxin by other phytohormones. Physiol Plant 53 176-180 Wodzicki TJ, Zajqczkowski S (1974) Effect of auxin on xylem tracheid differentiation in decapitated stems of Firms silvestris L. and its interaction with some vitamins and growth regulators. Acta Soc Bot Pol 43 129-148 Wodzicki TJ, Wodzicki AB, Zajqczkowski S (1979) Hormonal modulation of the oscillatory system involved in polar transport of auxin. Physiol Plant 46 97-100 Wodzicki TJ, Rakowski K, Starck Z, Porandowski J, Zajqczkowski S (1982) Apical... 

Larson (4,5) fed 14C02 photosynthetically to Pinus resinosa, divided the differentiating xylem into several fractions, and counted the radioactivity of each cell wall component. From these studies, it was concluded that as tracheid maturation occurred, xylose deposition increased, whereas mannose remained relatively constant, and both arabinose and galactose decreased considerably. 

Special characteristics Mainly in warm and arid regions Stems often sympodial Stems joined at the nodes Xylem with vessels Tracheids and fibres Hairs... 

Figure 2. Transverse section of coalified Persea secondary xylem for comparison with Figure I. At least three coalu products are shown dark colored cell inclusions, vessel wall derivatives, and pber-tracheid wall derivatives. 304X...

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Fig. 1-2. Transverse section of xylem and phloem of red spruce (P/cea rubens). CZ, cambial zone DP, differentiating phloem MP, mature phloem with sieve cells (sc) and tannin cells (tc) DX, differentiating xylem with ray cells and tracheids (tr) MX, mature xylem, earlywood (EW) with resin canals (rc), lined with epithelial cells (ec) LW, latewood. Note that each ray continuous from the xylem, through the cambial zone, and into the phloem. Light micrograph by L. W. Rees. Courtesy of Dr. T. E. Timell.

Accuracy of measurements in the middle lamella depends not only on Nb, but also on the value of Nd for this region. Donaldson (1985a) measured the refractive index of the middle lamella in differentiating tracheids before lignifica-tion and in primary xylem, where the middle lamella is unlignified, and obtained... 

The movement of water and nutrients from the soil to the upper portions of a plant occurs primarily in the xylem. The xylem sap usually contains about 10 mol m-3 (10 mM)2 inorganic nutrients plus organic forms of nitrogen that are metabolically produced in the root. The xylem is a tissue of various cell types that we will consider in more detail in the final chapter (Section 9.4B,D), when water movement in plants is discussed quantitatively. The conducting cells in the xylem are the narrow, elongated tracheids and the vessel members (also called vessel elements), which tend to be shorter and wider than the tracheids. Vessel members are joined end-to-end in long... 

Besides vessel members and tracheids, parenchyma cells and fibers also occur in the xylem (see Fig. 1-3). Xylem fibers, which contribute to the structural support of a plant, are long thin cells with lignified cell walls they are generally devoid of protoplasts at maturity but are nonconducting. The living parenchyma cells in the xylem are important for the storage of carbohydrates and for the lateral movement of water and solutes into and out of the conducting cells. 

For application of Poiseuille s law to a complex tissue such as the xylem, care must be taken to ensure that particular vessel elements or tracheids are conducting (e.g., not blocked by embolisms), the actual radii must be determined (note the r4 dependence iiiEq. 9.11a), and corrections may be necessary for lumen shape, tracheid taper, and cell wall characteristics including pits (Calkin et al., 1986 Schulte et al., 1989a). For instance, if the lumen is elliptical with major and minor axes of a and b, respectively, then r4 in Equation 9.11a should be replaced by a3b3/ (8a2 + 8b2). 

Xylem is that part of a fibro-vascular bundle that contains wood cells and fibers. It may also contain tracheae, tracheids, seldom sieve tubes. 

Secondary Phloem Soft Bast—phloem cells and sieve tubes. Cambium—active layer giving rise to secondary phloem on outer and secondary xylem or inner face, and adding to depth of med. rays. Secondary xylem—wood fibers, pitted vessels, tracheids. 

The three principal portions of a tree are the wood or xylem, the inner bark or phloem, and the outer bark. During the growing season, xylem is laid down on the inside, and phloem on the outside, of the vascular cambium. In the wood of the Gymnospermae (softwoods), all of which are arborescent and which began to develop some 300 million years ago, the principal wood element is the tracheid, whereas the 100-million-years younger, arborescent Angiospermae (hardwoods) are characterized by the presence of fibers and vessels. Both woods also contain parenchyma cells, especially in the rays. 

---