7.1: Secondary Stem
In many seed plants, secondary growth begins in their first year within the stem and continues on for many more years. These plants are classified as woody . They develop secondary tissues like periderm and wood, and even tertiary structures like bark.
The first step in producing secondary phloem and xylem (other names are metaphloem and metaxylem) is to form the vascular cambium, which involves cell division inside the vascular bundles and the parenchyma that are between the bundles (Figure \(\PageIndex{1}\)). The vascular cambium divides in two directions. The cells that are formed to the outside become the secondary phloem , and those formed to the inside are the secondary xylem (Figure \(\PageIndex{3}\)). After several years, central pith disappears under the pressure of growing wood, and only traces of primary xylem (protoxymem) can be seen under the thick secondary xylem. Altogether, these tissues (pith + primary xylem + secondary xylem) are wood (Figure \(\PageIndex{2}\)).
The secondary phloem forms outside of the vascular cambium, and traces of primary phloem (protophloem) are visible above it. It is rich in fibers, and unlike the wood, it does not form annual rings.
Most of cambium cells are fusiform initials forming axial vessel elements, while some cambium cells are ray initials cambium cells which make rays and they form rays : combinations of parenchyma cells and tracheids transporting water, minerals and sugars (because it is dark inside the stem and only respiration is possible) horizontally . Rays are visible best on the tangential section of the stem (when section plane is tangent to the stem surface); two other possible sections ( radial and transverse ) show axial components of the stem better. In the secondary phloem, rays are sometimes dilated (wedge-shaped).
The cambium usually does not work evenly all year round. In temperate climates, a ring forms for each growing season and makes it possible to determine the age by counting the growth rings. This is because at the end of season cambium makes much smaller (“darker”) tracheary elements. Trees growing in climates without well-expressed seasons will not make annual rings. To tell the age of a tree, researchers observe the number and thickness of annual rings that are formed. This is called dendrochronology .
Some trees (like oaks, Quercus ) have large vessel elements are found primarily in the wood formed early in the season (early wood); this pattern is known as ring porous . Large vessel elements of other trees (like elm, Ulmus ) occur more evenly in both early and late wood. This pattern is known as diffuse porous wood : with large vessel elements in both early and late wood .
Vesselless wood of conifers is of a simpler structure with relatively few cell types. There are simple rays and frequently resin ducts ; resin is secreted by specialized cells.
In the tree trunk, the lighter wood near the periphery is called sapwood and has functioning xylem where most of the water and minerals are transported. Darker wood closer to the center is called heartwood and is a non-functional, darkly colored xylem (Figure \(\PageIndex{4}\)). Tracheary elements are dead cells and to block them, plants uses tyloses vessel element “stoppers” which also help control winter functioning of vessels. A tylose forms when a cell wall of parenchyma grows through a pit or opening into the tracheary element; they look like bubbles.
Most liliids (for example, palms) do not have lateral meristems and true wood. Some thickening does occur in a palm but this happens at the base of the tree, as a result of adventitious roots growing. Palms may also have diffuse secondary growth which is division and enlargement of some parenchyma cells. These processes do not compensate the overall growth of plant, and palms frequently are thicker on the top than on the bottom. Few other liliids (like dragon blood tree, Dracaena ) have anomalous secondary growth which employs cambium but this cambium does not form the stable ring.
Constantly thickening stem requires constantly growing “new clothes”, secondary dermal tissue, periderm. Periderm is a part of bark. Bark is everything outside vascular cambium . It is unique structure which is sometimes called “tertiary tissue” because it consists of primary and secondary tissues together:
- \(\mbox{trunk} = \mbox{wood} + \mbox{vascular cambium (``cambium'')} + \mbox{bark}\)
- \(\mbox{wood} = \mbox{secondary xylem} + \mbox{primary xylem} + \mbox{[pith]}\)\(^{[1]}\)
- \(\mbox{bark} = \mbox{bast (primary + secondary phloem)} + \mbox{periderm} + \mbox{[cortex]} + \mbox{[epidermis]}\)
- \(\mbox{periderm} = \mbox{[phelloderm]} + \mbox{cork cambium (phellogen)} + \mbox{cork (phellem)}\)
Each year, a new layer of phellogen (cork cambium) appears from the parenchyma cells of the secondary phloem which makes bark multi-layered and uneven. On the surface of a young stem, one may see lenticels , openings in phellem layer which supply the internals of the stem with oxygen; together with rays, lenticels work as ventilation shafts. To produce lenticels, some phellogen cells divide and grow much faster which will finally break the periderm open.
Apart from the lenticels, older or winter stems have leaf scars with leaf traces on their surface. The first are places where leaf petiole was attached, and the second are places where vascular bundles entered the leaf.
The secondary structure of root reminds the secondary structure of stem, and with time, these two organs become anatomically similar.
References
1. “Optional” tissues are given in brackets, synonyms in parentheses.