2.5: Secondary tissues

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Secondary tissues are produced in woody plants. Secondary xylem and secondary phloem are produced from a cylinder of meristematic tissue within the woody stems and roots. This cylinder of meristematic tissue is the vascular cambium. The secondary xylem provides additional structural support and additional water conduction tissue in shrubs and trees. The secondary phloem replaces the primary phloem.

Similarly, as the trunk of a woody plant gets larger, the dermal tissue need to be expanded and replaced. New dermal tissue is produced by the cork cambium, which lies beneath the bark.

Figure 2.18. Stem of a dicotyledon plant with only primary tissues. The ground tissue system includes the pith and cortex and the vascular tissue system is in discrete bundles. (Background image, Berkshire Community College Bioscience Image Library, public domain. Labels Sean Bellairs).

Figure 2.19. Stem of a dicotyledon plant with secondary tissues. The pith has been compressed into the centre of the stem by the expanding secondary xylem. The secondary xylem now forms a complete ring, later the pith will be completely compressed and no longer be present. (Background image, Berkshire Community College Bioscience Image Library, public domain. Labels Sean Bellairs).

Figure 2.20. The locations of the secondary xylem, vascular cambium, secondary phloem, cortex, cork cambium and bark. The interior of the stem is the left and the exterior is to the right. (Background image, Berkshire Community College Bioscience Image Library, public domain. Labels Sean Bellairs).

Three year old woody stem of Tilia — Figure 2.21. Three year old woody stem of *Tilia* showing annual formation of xylem growth rings in the stem. Pith is still present in the middle of the stem. (Berkshire Community College Bioscience Image Library, public domain).

Figure 2.22. Four year old woody stem of Liriodendron showing the pith in the centre and purple stained xylem tissue. Darker purple lines of cells are ray parenchyma. (Berkshire Community College Bioscience Image Library, public domain).

Figure 2.23. Magnified view of the ray parenchyma. The lines of parenchyma cells continue out into the green stained phloem tissue. Xylem vessel cells are large at the beginning of the growth season and become smaller at the end of the growth season. (Berkshire Community College Bioscience Image Library, public domain).