Importance of Xylem and Phloem Tissues
The primary xylem and primary phloem tissues are pushed inward into the] pith and outwards respectively by the continuous production of secondary tissues cut I off by the cambium ring. The primary xylem is gradually pushed inward and is found at the centre of the axis; whereas, the primary phloem, being soft in nature, gets completely crushed. These activities in the stelar region exert a great pressure outwardly. The cortex cells, the pericycle and the epidermis divide anticlinally to cope with the production of tissues in the stelar region.
Formation of annual ring or growth ring The activity of the cambium ring is under the control of series of physiological and environmental factors. For example, in spring the cambium becomes more active and forms a greater number of vessels with wider cavities. In winter, the temperature is low due to which the cambium also becomes less active and forms narrow pitted vessels, tracheids and wood fibers. The xylem (wood) formed during the spring is known as spring wood or early wood and which is formed in winter is called autumn wood or late wood.
The spring wood is lighter in color and exhibits low density where as the autumn (or winter) wood is darker and has higher density. These two kinds of wood appear together, in a transverse section of the stem, as a concentric ring known as the annual ring or growth ring. Successive annual rings are formed year after year by the activity of the cambium. Each annual ring corresponds to one year’s growth.
Thus one can estimate the age of plant to some degree of accuracy by counting the total number of annual rings. Annual rings are readily seen with naked eye in the logs of a tree trunk.
Heart-wood and sap-wood: In old trees, the greater part of the secondary wood is filled up with tannins resins, gums, essential oils, etc. , which make it hard and durable. It looks dark or brown. This region is known as heart-wood. The heart wood gives mechanical support to the stem. The outer region of the secondary wood is of lighter color and is known as sap-wood or alburnum. It helps in conduction of water and mineral salts from the root to the leaf. (B) Secondary growth in extra-stelar region Secondary growth in the extra stellar region occurs to cope with the addition of tissues in the stellar region.
It occurs in the cortex and helps in the formation of periderm. It occurs by the formation and activities of a secondary meristem called cork cambium or phellogen. Origin and activity of the cork-cambium or phellogen Due to the addition of secondary phloem and secondary xylem elements, the outermost layer of the cortex becomes highly stretched and may crack open. During this process a few layers of meristematic tissue arise in the cortex. This is called the cork cambium or phellogen.
The nature of cambium is secondary. Commonly it arises in the outer layers of the cortex i. . hypodermis. It may also arise in the epidermis itself, or in the inner layers of the cortex, or even in the pericycle. The cork combium consists of narrow, thin-walled and roughly rectangular cells. It is of few layers in thickness. The cork cambium cuts of cells on both the sides and secondary cortex on the inner. The cells of the secondary cortex are parenchymatous in nature and often contain chloroplast. Cork The new cells cut off by the cork-cambium on its outer side are rough rectangular and soon become suberized. They form the cork of the plant.
Cork cells are dead, suberized, thick-walled and brown in color. They are arranged in radial rows being suberized, cork is impervious to air and water. Functions of Crock (i)It acts as a waterproof covering to the stem (ii) It also protects the plant] against the attacks of parasitic fungi, (iii) Cork cells, being dead and empty, containing air only, are bad conductors of heat.
This being so, a sudden variation in outside] temperature does not affect the internal tissues of the plant; (IV) Cork is also made use of by the plant for the healing of wounds. The bottle cork is obtained from the cork] cells of Quercus suber. ) Bark In restricted sense, all the dead tissues lying outside the active cork cambium constitute the bark of the plant. It, therefore, includes the epidermis, the lenticels and cork, and sometimes also hypodermis and a portion of the cortex depending on the position of the cork-cambium. Thus the deeper the origin of the cork-cambium, the thicker would be the bark. The term bark in a wider since is used to describe all tissues outside the vascular cambium of the stem.
Phellem, phelloderm and phellogen layers are collectively called periderm. Thus periderm is a protective multilayered structure of secondary origin. On the basis of function, two types of barks are distinguished- ring barks and scale bark. When the cork cambium appears in the form of a complete ring the bark that is formed comes away in a sheet; such a bark is known as the ring bark as in Betula. When the cork-combium appears in strips the resulting bark comes away in the form of scales; such a bark is known as the scale bark as in (guava) Psidium. Function of bark
The bark protects the inner tissues (i) against the attack of fungi and insects (ii) against loss of water by evaporation and (iii) against variation of external temperature. Lenticels These are small aerating pores formed in the bark of stems through which gaseous exchange takes place. Externally they appear as scars or raised portions on the surface of stems. Lenticels are first formed below the stomata. A section through one of the scars shows that the lenticels consists of a loose mass of small thin walled cells called complementary cells or filling tissue.
