Secondary Growth of Stems
An aquatic plant is buoyed by the water in which it grows, and its structural needs are simple. Land plants, however, require a structural support system. During the course of evolution when plants developed the ability to synthesize lignin—the polysaccharide that gives rigidity to the cell walls of wood—large, erect bodies were achievable, and their possessors became highly successful in colonizing the land. In modern plants, lignified wood cells are the secondary xylem cells. Most of the primary tissues outside of the vascular cambium are destroyed by the sideways push of the new cells, and a new group of secondary tissues—the bark—replace them.
Formation of the secondary plant body
During formation of the primary body, many plants retain meristematic tissues among differentiated ones. When stimulated to divide, these meristems, called cambia, produce new cells that, together with the remaining primary tissues, form the secondary (woody) plant body.
The vascular cambium lies between the primary xylem and phloem. It consists, accurately, of only one layer of cells, but the first cells it produces cannot be distinguished from cambial cells so the narrow area is sometimes referred to as the “cambium” or the “cambial zone.”
Two kinds of meristematic cells, called initials, are recognizable in the cambium: fusiform and ray initials. The fusiform initials are elongated vertically in the stem and have tapering ends. They divide to produce the conducting cells of both the xylem and the phloem (xylem toward the inside of the stem, phloem toward the outside).
Considerably more xylem cells than phloem cells always are produced. The ray initials are smaller, more cuboidal and produce parenchyma in rows radiating out from the center of the stem. The bands of parenchyma, called rays (vascular rays), conduct water and dissolved materials laterally in the stem.
Wood: Secondary xylem
The structure of wood varies from species to species and between major groups. A common categorization separates the softwoods of gymnosperms from the hardwoods produced by angiosperms. (These are not very good descriptive terms because of the great variability in density among species in both groups, but the groups do differ in the kinds of cells in their wood.)
Gymnosperm wood. Softwood lacks vessels and is composed almost entirely of tracheids. The rays are ribbon‐like structures of parenchyma, one cell wide and only a few cells deep. Vertical resin ducts or canals are characteristic of gymnosperms. The ducts intercellular spaces lined with parenchyma tissue, the cells of which secrete resin into the cavity in response to wounding.
Angiosperm wood. Hardwoods are harder than most softwoods because of the numerous fibers present. The usual conducting cells (tracheids and vessel segments), scattered parenchyma, and ray parenchyma are present in the wood. Some dicot (eudicot) species have resin and resin ducts, but other substances—latex (rubber), for example—are more commonly secreted in angiosperms in response to wounding. The rays of the hardwoods usually are multiseriate (many cells in width) and hundreds of cells deep.
Growth rings. In climates that alternate favorable with unfavorable seasons for plant growth, the xylem cells produced by the cambium vary in size throughout the growing season, resulting in rings with discernible differences. If there is one growing season per year, the rings are annual rings and a simple count gives the age of the tree. Other circumstances may cause the cambium to stop and start growth—forest fires, volcanic eruptions, defoliating caterpillar outbreaks, or extreme drought, for example—with false annual rings the result. The science of dendrochronology is the study of growth rings to date past events and climates.
Characteristics of wood. Many descriptive names are commonly applied to easily seen features of wood. Wood in the center of trees is called heartwood and it often is discolored by accumulations of tannins, gums, and oils carried there and stored in balloon‐like outgrowths called tyloses that fill and plug the vessels. Sapwood is the youngest, last‐formed xylem. All of the heartwood can decay, leaving a hollow trunk, and the tree will remain alive and healthy if the sapwood is intact.
All tissues from the vascular cambium outwards collectively are the bark of a woody plant. Almost all are secondary in origin and are produced after divisions in the vascular cambium start.
Phloem. Secondary phloem cells are produced by the vascular cambium at the same time as secondary xylem cells, but in fewer numbers. Their outward growth pushes the primary phloem cells against the cortex, breaking most and leaving only the thicker‐walled fibers as remnants. Ray parenchyma cells initiated by the cambium give rise to phloem rays and, towards the center of the stem, xylem rays. The rays are the primary avenues of lateral movement of materials from the vascular vertical conduits that lie close to the cambium.
Periderm. Changes occurring in the cortex and the epidermis replace the protective layers of primary tissues with periderm, which consists of three tissues: the cork cambium (phellogen), cork (phellem), and the phelloderm. Both cork and phellloderm arise in the cork cambium, but differ structurally and functionally. Cork, with heavily suberized, tightly packed, dead‐at‐maturity cells, lies to the outside of the cork cambium and serves to waterproof, insulate, and protect the underlying stem tissues. Phelloderm, in rows to the inside of the cork cambium, remains alive and carries on routine metabolic functions (including photosynthesis in some green‐barked trees). Lenticels, small openings in the bark, permit diffusion of gases. They are raised areas filled with loose parenchyma cells that are in direct contact with the atmosphere on one side and with cortical tissues on the other.