Key Concepts:

Figure 1. Diagram of a section of stem during its third year of secondary growth. (a) The three planes (transverse, radial and tangential) that are used to examine the organization of the cambium and its derivatives are indicated. Transverse section from the cambium from (b) Scots pine and (c) silver birch at ×200 and ×400 original magnification, respectively. The cambial zone (indicated by the bracket) separates the blue staining xylem on the right from the phloem. The rays (r) are clearly visible running through the cambium, xylem and phloem. The xylem of the pine is formed of similar-sized tracheids, whereas the birch xylem is composed of large vessel members and fibres, both of which arise in the same file of cells and are consequently derived from the same cambial initial. Radial ((d) and (f)) and tangential ((e) and (g)) sections of Scots pine and silver birch, respectively. Radial sections show the files tracheids and vessels that arise in the xylem from the long thin fusiform initial (f) and fusiform initials that form the rays running horizontally (r). Sections from the pine phloem (e) and pine xylem (g) show the simple patterns of uniseriate rays in pine and more complex multiseriate rays in birch. Original magnifications for radial sections (pine and birch) and tangential were ×100, ×400, ×200 and ×200, respectively. (h) Radial section through the phellogen of silver birch. The cells are clearly arranged in files and the phellem cells (ph) near the outside of the stem are heavily suberinized. (i) Transverse poplar stem section with vascular cambium (vc) and interfascicular cambium (ifc) labelled.

Figure 2. PXY is required for setting the plane of cell division. (a) Diagrammatic representation of the cambium an ordered cell division down the long axis of a dividing cell (dotted line). The ordered nature of the division leads to organized vascular tissue in wild-type plants (b). In pxy mutants, cell divisions in the cambium are not ordered (dotted lines) (c), leading to tissue that lacks order (d). Scales are 50 m; ph is phloem, c the cambium and x the xylem.

Figure 3. Transverse sections through soc1-3 ful-2 plants. (a) Section through lateral branch of five-month-old double mutant with secondary xylem stained dark blue and secondary phloem stained pink. Inset shows a wild-type stem at the same magnification. (b) Magnified cross-section through branch of a five-month-old double mutant. Files of xylem and phloem derived from secondary cambium shown between yellow arrowheads. (c) Transverse section through base of a 6-cm wild-type inflorescence with no secondary growth in marked area. (d) soc1-3 ful-2 inflorescence at the same stage with secondary xylem and phloem in marked area. Scale bars in (a) 500 mm, (c) and (d) 100 mm. SP, secondary phloem; SX, secondary xylem. Reprinted by permission from Macmillan Publishers Ltd, Melzer et al. (2008).

Figure 4. (a) and (b) Ethylene stimulated cambial growth and wood formation in wild-type poplar. (c) and (d) The response was reduced by the expression of the dominant-negative mutant allele Atetr1-1. Horizontal lines indicate the position of ethylene application. The arrows indicate the approximate position after which wood was formed under the influence of ethylene. Width of sections is 2 mm. Reprinted by permission from National Academy of Sciences, Love et al. (2009).