What is Secondary Growth explain briefly?
Secondary Growth in Stems : Young developing shoots increase in height by adding new cells, which are produced by the apical meristem located in the terminal bud. Angiosperm plants like dicots also increase in girth (diameter and circumference) because of new secondary tissues that are grown by another group of cells that become meristematic. This secondary growth is what we recognize as woody tissues in trees (seen at right) and shrubs. Secondary tissues come from cells that were originally procambium, as well as from parenchyma cells of ground meristem origin. These cells form a cylinder of meristematic vascular cambium just inside the bark layer.
While secondary growth is typical for dicots, some monocots like the Palm seen at left, also produce secondary growth, but from a different meristematic tissue, and not vascular cambium.
In dicots, the cylindrical layer of vascular cambium generates new cells to the to the outside, which develop into phloem tissue just inside the bark, and new cells to the inside, which develop into xylem tissue (what we know as wood). Phloem transports photosynthetic products to the rest of the plant form the leaves, and xylem transports water, nutrients and minerals from the roots to the shoot system. Phloem and xylem are clustered in bundles called vascular bundles. Viewed in cross section (as in the cut stump of a tree trunk) the vascular bundles in dicots are arranged in a circular pattern (see figure below). In monocots, the vascular bundles are randomly arranged pattern that is scattered (see figure below).
As new cells are generated toward the center of the stem, forming more wood, the stem increases in girth, and the vascular cambium gets carried out toward the bark. In order for the cambium to keep pace with the increasing circumference of the stem, the cambium also divides sideways, or laterally, creating more cambium to fill in the spaces. The actual cells of the cambium are called fusiform initials. Smaller cambium cells called ray initials produce radially oriented cells that cut across the stem, providing for lateral, or sideways transport of water.
The vascular cambium consists of a thin layer of cells producing xylem to the inside and phloem, which becomes part of the bark, to the outside. Cambium can be easily destroyed by removing the protective bark of a tree stem. A tree can be killed by cutting away the bark all the way around a tree trunk, which effectively removes all the cambium. This procedure is known as girdling. Without cambium, no new xylem and phloem can be added to the stem to carry out transport within the plant.
Another type of cambium referred to as cork cambium, forms at the same time as vascular cambium to make cork, which is found in the bark layer of a tree stem. Cork is produced in an outward radiating direction, effectively pushing the cork cambium inward. Cork cells become impregnated with a waterproofing substance called suberin.
The job of cork cells is to protect the delicate tissues of the stem, and cork cells are actually dead at functional maturity. The waterproofing suberin helps the plant retain precious water as well. Cork produces a layer of protection that keeps out most animals like insects, bacteria, fungi, and viruses. It also guards against physical damage such as neighboring tree branches which may be leaning, scraping, or even falling from above. In other cases, bark protects the tree from fire damage.
Sometimes another tissue called the phelloderm is produced toward the inside of the stem from the cork cambium. This phelloderm tissue, along with the cork and cork cambium is collectively referred to as the periderm. The periderm encases the stem as well as the much of the root exterior. Since the periderm represents an almost impenetrable barrier, gases would not be able to find their way to the living cells of cork cambium and phelloderm were it not for lenticels. Lenticels are patches on the bark that consist of loosely packed cells in the periderm that allow for gas exchange.
Bark is the common name that describes the tissues located outside of the vascular cambium in mature stems and roots. Therefore, bark includes the secondary phloem as well as the primary phloem, as well as the phelloderm and the cork and cork cambium. The primary phloem tissue actually gets crushed between the secondary phloem and the outer periderm.
Wood is the common name given to what botanists refer to as secondary xylem. That is, xylem produced by the vascular cambium, which then differentiates by laying down thick secondary cell walls of cellulose impregnated with a hardening substance called lignin. These become empty vessel elements at maturity and conduct water, nutrients, and minerals. You may be familiar with the annual rings that are so noticable in the cut stem of a woody tree.
Each annual ring actually corresponds to one year's growth of xylem. If you examine the rings in cross section under high magnification, you will see that the "rings" appear separated from each other because of a difference in vessel diameter. In other words, the smaller vessels produced at the end of the year-in late summer are located near the outer perimeter of the ring, and the larger diameter ones produced earlier in the spring make up the vessels near the inner perimeter. So a "ring" is defined by the contrast between the line of smaller, darker appearing vessels of the previous year next to larger, lighter vessels made the following year. It is thought that the larger vessels are formed in the spring in response to the greater availability of water and a slower growth rate. Smaller vessels are formed in the autumn when there is less available water and a shorter growing season. Scientists are even able to tell rainy years from drier years by the size of the vessels, and thereby study climactic conditions over the years.
Therefore, wood consists of rings of xylem that correspond to years of growth, and the age of a tree can be accurately determined by counting the number of annual rings in a tree stump, or by extracting a core sample. In older stems of some species, the center of the tree stem may contain dark colored wood, which is called "heartwood." Heartwood comes about when chemical substances like gums and resins impregnate the xylem vessels, making for very hard and dense wood. Sapwood, on the other hand, refers to the younger xylem vessels recently formed that conduct the water, nutrients and minerals to other parts of the tree.