Xylem cells

The border between wood and bark is the cambium, which consists of only one layer of cells. This living cell layer produces xylem cells towards the inside of the stem and phloem cells towards the outside (Figure 1). In all the wood species studied, common characteristics of the cambium include a high content of pectins and the absence or low content of lignin (39, 40,... 

The induction of PAL activity at the onset of vascular differentiation can be shown by the use of plant tissue cultures (37-39). Xylem cells with secondary and lignified walls are differentiated over a time course of 3-14 days by the application of the plant growth factors naphthylene acetic acid (NAA) and kinetin in the ratio 5 1 (1.0 mg/liter NAA, 0.2 mg/liter kinetin) to tissue cultures of bean cells (Phaseolus vulgaris) (37,40). The time for differentiation varies with the type of culture, solid or suspension, and with the frequency and duration of subculture, but for any one culture it is relatively constant (37,41,42). At the time of differentiation when the xylem vessels form, the activity of PAL rises to a maximum. The rising phase of the enzyme activity was inhibited by actinomycin D and by D-2,4-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide (MDMP) applied under carefully controlled conditions (42). This indicated that both transcription and translation were necessary for the response to the hormones. Experiments using an antibody for PAL and a cDNA probe for the PAL-mRNA have also shown that there is an increase in the amount of transcript for PAL during the formation of lignin when Zinnia mesophyll cells are induced to form xylem elements in culture (Lin and Northcote, unpublished work). 

Bark is the multi-layered outer portion of the stems and roots of woody plants. While the word is used most often in referring to just the epidermis of a stem, bark actually includes all layers of the plant from the outside down to and including the vascular cambium. The vascular cambium is the only part of a stem that grows. Its undifferentiated cells divide rapidly, producing secondary phloem cells toward the outside of the plant and secondary xylem cells toward the inside (Figure 4.3). 

Radial growth begins in the cambium which is composed of a single layer of thin-walled living cells (initials) filled with protoplasm (cf. Fig. 1-2). The cambial zone consists of several rows of cells, which all possess the ability to divide. On division the initial cell produces a new initial and a xylem mother cell, which in its turn gives rise to two daughter cells each of the latter is capable of further division. More cells are produced toward the xylem on the inside than toward the phloem on the outside phloem cells divide less frequently than xylem cells. For these reasons, trees always contain much more wood than bark. 

The rings in trees are not always annual. In many desert species a ring forms when large xylem cells are produced after a suitable rainy period followed by smaller cells, and this can occur more than once or sometimes not at all in a particular year. Moreover, trees from the wet tropics can have no annual rings. 

Such a capillary rise would account for the extent of the ascent of water in small plants, although it says nothing about the rate of such movement (to be considered in Chapter 9, Section 9.4D). For water to reach the top of a 30-m-tall tree by capillary action, however, the vessel would have to be 0.5 pin in radius. This is much smaller than observed for xylem vessels, indicating that capillary rise in channels of the size of xylem cells cannot account for the extent of the water rise in tall trees. Furthermore, the lumens of the xylem vessels are not open to air at the upper end, and thus they are not really analogous to the capillary depicted in Figure 2-3. 

Before discussing the characteristics of flow in the xylem, we will briefly review some of its anatomical features [see Chapter 1, Section 1.1C (e.g., Fig. 1-3) for an introduction to the xylem]. In general, the conducting xylem elements have thick, lignified secondary cell walls and contain no protoplasts. Indeed, the xylem cells serve their special function of providing the plant with a low-resistance conduit for water flow only when they are dead Because these conducting cells are essentially membraneless hollow pipes, water in... 

Figure 9-16. Pressure drops across a hypothetical xylem cell that is 1 mm long, 40 pm in inside diameter, and has an axial flow rate of 1 mm s-1 (a) artificially surrounded by a plasma membrane, (b) artificially having a cell wall 1 pm thick across each end, and (c) realistically consisting of a hollow tube in which Poiseuille flow occurs. Allows are placed at the main barriers to flow in each case. See text for calculation details.

C. Suppose that the root xylem is arranged concentrically in a ring 500 pm below the root surface. If the average conductivity of the epidermis, cortex, endodermis, and xylem cell walls in the root is like that of the dry soil, what is the decrease in hydrostatic pressure across them ... 

Xylem cells (wood parenchyma) Tracheae (ducts)... 

A radial fibro-vascular bundle of many alternating xylem and phloem patches and hence polyarch. The phloem tissue consists of phloem cells and sieve tubes. The xylem is composed of xylem cells, tracheae and wood fibers. 

Radial fibro-vascular bundle of four, rarely two or three or five or six phloem patches alternating with as many xylem arms. Not uncommon to find bast or phloem fiber along outer face of each phlUem patch. Xylem has spiral tracheae, internal to these a few pitted vessels, then, as- root ages, more pitted vessels, also xylem cells and wood fibers make their appearance. ... 

FIGURE 10 Cross section of partially burned particles from the Cretaceous-Tertiary boundary layer, as seen by reflected light microscopy (Kruge et al., 1994). Particles show typical plant xylem cell structures. 

Though either GA3 or BA was able to promote PAL activity, and thus reverse its decrease that is caused by excision drop, only GA3 was effective in bringing about near-normal lignification in the putative xylem cells. BA-induced PAL increase could thus be non-specific, and may not be directed to any particular cell type. However, these xylem cells appear to be specifically the target of GA3. 

In combination, BA + GA3 additively stimulated PAL increase and BA augmented G A3 s effect in bringing about normal level of lignification in the xylem cells (Table 5). BA has been shown to stimulate the formation of xylem elements in the base of preformed haustorial mounds in Cuscuta campestris [29]. Taken together. 

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