3.1 Stems

Stems are produced by the primary apical meristem in but may be increased in girth in woody plants due to secondary growth. Secondary growth is produced by lateral meristems in the woody stems and roots of 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 3.1 Stem of a dicotyledon plant with only primary tissues
Figure 3.1 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.
Figure 3.2. Stem of a monocotyledon plant with primary tissues. The vascular bundles are scattered throughout the stem with the xylem to the middle and the phloem towards the outside. Some monocotyledons stems are hollow.
Figure 3.3. Stem of a dicotyledon plant with secondary tissues.
Figure 3.3. 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.
Figure 3.4. Vascular cambium
Figure 3.4. 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.
Figure 3.5. Tilia 3 growth rings
Figure 3.5. 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.
Figure 3.6. Liriodendron stem
Figure 3.6. 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.
Figure 3.7. Liriodendron stem
Figure 3.7. 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.

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