Atlas of plant and animal histology

In plants undergoing secondary growth, the procambium develops into vascular cambium, giving secondary phloem outward and secondary xylem inward. Cells of secondary xylem and secondary phloem are mainly oriented vertically (parallel to the main axis of the organ), except for a group of parenchyma cells that are arranged horizontally to form the axial rays (Figure 1).

Secondary growth is typical of dicot plants and gymnosperms. These two groups of plants can be distinguished by the distinct features of their secondary xylem. In gymnosperms (Figure A, above), secondary xylem is composed of tracheids as the only conductive element. Conifers, such as pines, are trees that possess softwood because they lack sclerenchyma fibers in their xylem. Most tracheids show areolate pits to communicate with neighboring cells. In dicot plants (Figure B, above), vessel elements are the main conducting cells, with sclerenchyma fibers also presetn among the vessel elements. Parenchyma cells forms the horizontal parenchyma that it is organized as axial rays, consisting of uniseriate cell rows in gymnosperms and multiseriate cell rows in dicotyledon plants (Figure 1).

In the secondary phloem, sieve tubes are not easily distinguished from companion cells, and horizontal parenchyma cells are also difficult to distinguish since the phloem cells become tight and form compact groups. That is why, in the figure above, we enclose the phloem as a whole, without labeling the cells.

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