Secondary Growth In Stem

Secondary Growth In Stem

Many species of plant complete their life cycle  using only the primary tissues generated by primary meristems. These are the herbaceous species described in Topic C3. In many other species, production of primary tissue and elongation growth are followed by the deposition of secondary tissue. These tissues give increases in diameter and the tissues formed are strengthened in comparison to primary tissues by deposition of extensive secondary walls and lignin, a poly￾meric phenolic compound, in the cell wall making them woody. Live for many years plants show secondary growth, the wood making them resistant to damage by herbivores and weather. Monocot do not normally generate secondary tissues, but some, like palm trees, undergo additional primary growth to form thick stems. Some palms also continue cell division in older parenchyma tissue to give what is known as diffuse secondary growth.

Vascular Cambium and Cork Cambium

The vascular cambium is a narrow band of cells between the primary phloem and xylem (Fig. 1), which remains a meristem. This tissue goes on dividing indefinitely with active growth in spring and early summer in temperate trees, with the new cells being formed to the outside of the cambium to form phloem and the inside to form xylem (Fig. 2). The newly formed secondary phloem and xylem function like the primary vascular tissue in transport. As xylem cells are deposited on the inner face, the vascular cambium moves outwards. In temperate regions early in the growing season, the cambium produces xylem cells of large diameter and these get progressively smaller as the season progresses. In consequence, growth appears as more and less dense bands of cells, the familiar annual rings observed in cross section of a tree trunk. The cork cambium arises within the stem cortex and generates cuboid cells that quickly become filled with the waxy substance suberin (also found as water-proofing in the root endodermis; Topic I1). The dead cells remain as a protective layer (Fig. 3) but the subernized cells die, required because the original epidermis can no longer function. Cork tissue forms part of the bark of the tree (the remainder being secondary phloem) and replaces the epidermis. Its character and thickness varies from species to species. Some gas exchange to the stem occurs through lenticels, pores remaining through the bark.

Wood anatomy 

After the studied of transverse section of a tree trunk (Fig. 3), a number of features appear.

Concentric circles of annual growth rings are evident in most trees from seasonally varying environments. The center of the trunk of many species is the heartwood, where the vascular system no longer functions. It provides struc￾tural support and is often darker and impregnated with tannins (Topic J5), but may be completely removed without killing the tree. Around the heartwood lies the sapwood that contains functional phloem and xylem. Vascular rays radiate from the center of the trunk.

These are formed of long lived parenchyma cells which conduct nutrients and water across the trunk, crossing both phloem and xylem, and are responsible for secretions into the heartwood. The outermost layers of the trunk are known as bark. Bark includes the corky tissues produced by the cork cambium and the underlying layers outside the vascular cambium, including the secondary phloem.