Stem secondary growth is due to the activity of the vascular cambium meristem and cork meristem. It is a growth in thickness, not in length. Secondary growth is a feature of gymnosperms and most dicot plants (dicot woody plants). Only a few monocot plants show secondary growth and none pteridophytes (ferns and the likes).
During the primary to secondary growth transition, the first thing to happen is the formation of the vascular cambium meristem, differentiated from both procambium (or fascicular cambium) and interfascicular parenchyma (Figure 1). The fully develop vascular cambium is a cylinder that by proliferation and differentiation gives secondary phloem outward and secondary xylem inward. In this way, the previous primary vascular tissues, primary xylem and primary phloem, are pushed away from each other, and remain as small groups of cells at the surfaces of the secondary vascular tissues, sometimes as nail-like structures.
The vascular cambium activity leaves growing ring every year, which are inner as the meristem moves away from the central axis of the stem. This mechanism produces the growing in thickness of the stem. The older differentiated cells from the vascular cambium are the innermost cells of the stem, whereas the more recent differentiated cells are those closer to the meristem. In woody stems, another meristem known as phellogen or cork cambium produces the periderm or bark, which replaces the epidermis.
In herbaceous dicot plants is usual that the vascular cambium does not form tissues homogeneously organized along the whole circumference of the meristem. In these species, it is the procambium (fascicular cambium) which form the vascular bundles, whereas the interfascicular cambium gives a specialized cell type known as fibers. Hence, the secondary xylem is not a continuous ring of tissue but an alternation of vascular tissue and supporting tissue. In other species like vine, the interfascicular cambium produces parenchyma.
The following structures are observed in a stem undergoing secondary growth, from outside to inside:
Periderm. Periderm differentiates from the lateral meristem phellogen or cork cambium. In some species, both vascular cambium and cork cambium appear at the same time at the beginning of the secondary growth, but cork cambium usually appears after the vascular cambium. The outer part of periderm is the cork or phellem, which becomes part of the bark of the stem. Periderm is the outer part of the stem and functions as a protective structure. The other component of the periderm is the phelloderm, a layer of tissue inner to the cork cambium
The bark of the trees includes all those tissues more superficial than the vascular cambium, including phloem (vascular tissue) and periderm. Thus, the bark results from the activity of the two meristems: vascular cambium and cork cambium. The cork cambium is a transient meristem in many trees since it disappears and is generated again in each cycle of growing of the periderm. The successive periderms that had been formed and stay in the stem is known as ritidome (Figure 2). Hence, the bark is formed of phloem, periderm and ritidome. There is no ritidome in those species that keep the same cork cambium during their whole lives.
The bark may account for 9 to 15 % of the stem volume. It performs many functions, such as storing water or organic substances, protection against fast temperature variations, protection against mechanical damages, herbivores, pathogens and dehydration, Sun irradiation, strong wind, flooding, and many other potential damages.
The first periderm only remains a few years in most plant species of warm climate regions. New inner periderms are then produced. If the periderm is continuous all along the stem perimeter, the surface of the stem looks smooth. In other species, periderm shows shell-like or lens-like structures partially overlapped between each other. In these cases, the cork cambium develops from cortical parenchyma. The grooves and cracks observed in the ritidome are consequence of the mechanical pressure produced by the radial growing, which in turn is the result of the vascular cambium activity.
Some species develop stem bark with significant commercial interest, such as cork oak (Quercus suber). This species show a stem without ritidome, that is, the cork cambium is permanent. Not having layers, it produces a homogeneous material. On the other hand, trees having ritidome show alternate layers of parenchyma and phloem, so that the bark is ground and used as agglomerated. The cork oak is an interesting species because once the periderm is removed, the stem produces a new cork cambium with similar features than the previous one. That is why, cork can be obtained during all the lifetime of the tree.
Secondary phloem is produced by the vascular cambium toward the outer surface. The older secondary phloem is always more superficial that the younger one. The older secondary phloem progressively degenerated and is included in the stem bark. New secondary phloem is closer to the vascular cambium and it is composed of parenchyma cells, sieve tubes and companion cells.
The vascular cambium is a lateral meristem responsible for the formation of secondary xylem and secondary phloem. It is composed of undifferentiated cells organized in rows parallel to the surface of the shoot.
The secondary xylem is the tissue that forms the wood and the growth rings of the stems. The inner part of growth rings usually shows larger cells than the outer part. They are known as early and late wood, respectively. In warm weather regions, where seasons are clearly different, growth rings are annual. However, in tropical regions, the growth rings correspond to rainy periods, and are not much influenced by temperature or the photoperiod. Hence, there is no direct correspondence between the time and the growth rings. Some events like fire or lost of leaves may produce more than one growth ring per year in species of warm whether regions. These are known as false rings. Regarding the cell morphology, there are ring-porous stems if the growth rings have larger cells in the inner part and smaller cells in the outer one, and diffuse-porous stems when all cells show similar size. There are examples with intermediate features between both types. Ring-porous feature is thought to be more evolved than the diffuse-porous organization. In gymnosperm, there is non-porous wood because they have no tracheae, but tracheids.
Secondary xylem differentiates from the vascular cambium toward the interior of the stem. Secondary xylem of angiosperms is composed of traqueae, tracheids, sclerenchyma fibers and parenchyma cells. The new xylem, closer to the meristem, contains living cells that conduct substances. The inner part of the secondary xylem is dead and does not conduct anything. As the secondary xylem layers get old, parenchyma cells die and the chemical composition of the cell walls of the xylem cells changes, that is, the wood is modified. Storing substances are removed and cells are infiltrated with oils, tannins or resins. Sapwood is the region that performs conduction and heartwood is the non-conducting wood, which represents the majority of the wood in thick stems. The thickness of sapwood depends on the plant species. For example, maples and ash trees show wide sapwood, whereas in yew is thinner. Sometimes, it is difficult to distinguish the border between sapwood and heartwood. Waste products are progressively accumulated in the sapwood. In angiosperms, but not in gymnosperms, parenchyma cells outgrowth cytoplasm expansions into the interior of the tracheae for storing waste substances, which will be part of the heartwood. These expansions are known as tyloses. The cell expansions enter through the cell wall pores and a trachea may have been invaded by several parenchyma cells. These parenchyma cell store abundant waste products and eventually die.
Pith or medulla is the tissue found in the inner part of the stem, composed of parenchyma cells. The pith is surrounded by primary xylem. In woody stems, it is death tissue and is a few milimeters in diameter. However, many plant species show vascular tissue in the inner part of the stem, so they lack pith.