Cambium, vascular

Figure 1. Structural responses of the bark of Picea sitchensis to wounding and inoculation with Phaeolus schweinUzii. IW, inoculated wound SP, surface periderm NT, necrotic tissue TC, thickened cells SIT, relic of suberized impervious tissue NP, necrophylactic periderm P, phloem VC, vascular cambium.

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). 

Immediately inside the endodermis is the pericycle, which is typically one cell thick in angiosperms. The cells of the pericycle can divide and form a meristematic region that can produce lateral or branch roots in the region just above the root hairs. Radially inside the pericycle is the vascular tissue. The phloem generally occurs in two to eight or more strands located around the root axis. The xylem usually radiates out between the phloem strands, so water does not have to cross the phloem to reach the xylem of a young root. The tissue between the xylem and the phloem is the vascular cambium, which through cell division and differentiation produces xylem (to the inside in stems and older roots) and phloem (to the outside in stems and older roots). 

Wood Cell Production. The site of wood cell production, the vascular cambium, is illustrated in Figure 3B. Technically, it is a microscopic sheath of meristematic cells. However, the exact circumferential line of cambial cells is very difficult to locate precisely, particularly during the tree s growing season, because of the presence of recent xylem and phloem derivatives. Therefore, it is more common to reference this lateral meristem as the cambial zone (2). 

Causes. Different specimens of wood even from the same tree are never identical and are similar only within broad limits (2). Within the larger categories of softwoods and hardwoods, such variability is extended to different trees of the same species and to different genera and families. All of this variability occurs naturally and is the combined result of tree genetics, the environment, and the age of the vascular cambium (i.e., tree age). Observed consequences are changes in the type, number, and form of wood cells. Additionally, such changes are not infrequently accompanied by varying wood chemistry and cell wall ultrastructure. 

At times when a plant needs to slow down growth and assume a resting stage (dormant), abscisic acid is produced in the terminal bud, which slows down growth and directs the leaf primordia to develop scales that protect the dormant bud during winter. Since the hormone also inhibits cell division in the vascular cambium, both primary and secondary growth is put on hold during winter. 

Axial parenchyma cells (also called longitudinal parenchyma) are generally very abundant in hardwoods. Like vessel elements and fibres, axial parenchyma cells are derived from the axially-elongated fusiform initials of the vascular cambium but, whereas vessel elements and fibres (except septate fibres) remain unsegmented, axial parenchyma cells are formed by the transverse segmentation of the derivatives of fusiform initials. Axial parenchyma cells, therefore, tend to lie in vertical files... 

Cells that have been formed recently at the vascular cambium have only a very thin primary cell wall. Even in the fully lignified cell the primary wall is very thin (0.1 j,m) and can be hard to distinguish from or isolate from the middle lamella many studies analyse the two together (ML+P) and relate results to the compound middle lamella (CML), a term which embraces both middle lamella and primary wall. The primary wall displays both elasticity and plasticity (permanent extension) during early cell growth and extension - at this stage in tracheid cell development... 

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. 

Microscale measurements of lAA levels in tissue sections through the vascular cambium of both Pinus and Populus [199,200] have revealed the existence of a steep gradient of lAA levels across the cambium. The existence of these gradients supports the idea that auxin functions in the plant as a positional signal which can specify developmental... 

Loomis RS, Torrey JG (1964) Chemical control of vascular cambium initiation in isolated radish roots. Proc Natl Acad Sci USA 52 3-11 McCready CC, Jacobs WP (1963) Movement of growth regulators in plants IV. Relationships between age, growth and polar transport in petioles of Phaseolus vulgaris. New Phytol 62 360-366... 

Thair BW, Steeves TA (1976) Response of the vascular cambium to reorientation in patch grafts. Can J Bot 54 361-373... 

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