In most of the Dicotyledons and Gymnosperms after the completion of primary growth of axis in length and diameter, further enhance in diameter takes place due to the formation of secondary tissues.
In the stelar region a cambium ring is produced which gives rise to secondary xylem and phloem, and in extrastelar region the cork cambium forms secondary cortex and cork, resulting in the enhance in thickness. Thus, the secondary growth can be defined as the increase in thickness because of the formation of secondary tissues by the activity of cambium and cork cambium.
1. Activity of the Vascular Cambium:
The meristem producing the secondary xylem and phloem is known as the vascular cambium. Since it occupies the lateral position in the axis, it is known as lateral meristem. The cambial cells differ from normal meristems in being highly vacuolated.
These exist in two produces, the fusiform initial which is much longer than wide, and the ray initial which is almost isodiametric. The fusiform initials constitute the axial system and the ray initials produce the radial system of zone of cambial initials. The former produce axial system of secondary xylem and the latter to the radial system.
a. Formation of Cambium Ring:
At the time of secondary growth, the cells of medullary rays, in a line with fascicular cambium (cambium of the vascular bundle), become meristematic and produce new strips of secondary meristems, known as interfascicular cambium. These strips join the strips of fascicular cambium on both the sides and form a complete ring, called cambium ring (Fig. 13.2).
In certain dicotyledonous stems e.g.,Linum, and Tilia, the primary xylem and phloem are not visible as separate vascular bundles. They look like a close cylinder. In these cases the interfascicular cambium, forms the entire cambium ring and there is no scope for the formation of interfascicular cambium.
b. Formation of Secondary Tissues:
At the beginning of cambial development divisions starting the cambium within the vascular bundles frequently precede those appearing in the interfascicular regions. If these regions are wide, their initial cambial divisions can start next to the fascicular cambium and spread tangentially.
Then the cambium ring as entire forms new cells on both the sides (external and internal) by tangential divisions (Fig 13.3). The activity is more on the inner side, and these cells are gradually modified into secondary xylem cells.
The secondary xylem includes wood vessels, wood fibres, tracheids, and wood parenchyma. The cells produced on the outer surface of cambium get modified into secondary phloem, which consists of sieve tube, sieve plate, companion cells, bast fibres and phloem parenchyma.
As the activity of cambium is more on the inner side, more of secondary xylem is produced. At maturity, this secondary xylem forms the main bulk of the stem. By its pressure the cambium and phloem are pushed outward. As a result, the primary phloem is generally crushed. The primary xylem, specially the metaxylen, however does not lose its identity for a considerable time period. It is usually recognized as the conical protuberances in the pith.
When quantity of secondary xylem increases too much, the pith and primary xylem become deshaped, and the primary xylem tissue becomes nonfunctional.
The secondary phloem produced on the outside also forces the primary phloem to outside where the later becomes non-functional. The cortex cells divide anticlinally for certain time so as to increase the circumference to keep pace with the increasing width of secondary tissue. After some time, when it does not cope with the developing secondary xylem, it gets crushed or broken here and there.
The epidermis also enhances in circumference by the anticlinal divisions for some time and then it breaks up. The large amounts of secondary xylem also exert a pressure on the developing secondary phloem which is accommodated in the increasing quantity of parenchyma and is not crushed.
At certain places adjacent to the rays (ray initials) the cambium forms narrow bands of radially elongated, parenchymatous cells both on the inner and outer side instead of xylem and phloem. These produce the secondary medullary rays, and are one to few cells in thickness and one to many cells in height.
In a good number of stems e.g., Tilia, the cells of secondary medullary ray divide and grow in the phloem region, so as to produce dilated masses of parenchyma between two phloem patches.
As the secondary xylem cylinder expands, there must be a compensating expansion of the cambium, in order to maintain the unbroken character of its cylinder. Such an enhance in the circumference of the cambial ring is accomplished by an in total number of cambial cells due to occasional radial divisions in addition to the usual tangential divisions.
The cambial cells can also divide by an oblique radial wall (pseudo-transverse wall), and the daughter cells move apart one another until the two initials reach normal length and lie side by side tangentially. The number of these divisions is generally sufficient to make up the number of cells enough to maintain the enlarging circumference of the cambium ring. This method is termed dilatation.
In herbaceous stems the cambium ring remains active only for one season after which the aerial stem dies, but in the woody plants it remains active for several years continuously producing xylem and phloem.
c. Formation of Annual Rings:
The activity of the cambium fluctuates with the fluctuations of the atmosphere. In spring season, the cambium forms xylem vessels with wide cavities. This is known as spring wood or early wood. In winters, i.e., the inactive period of vegetative growth of the plant, small diametered vessels are produced by the cambium.
This is called autumn wood or late wood. The two kinds of wood together form and annual ring and represent one year’s growth. Every year, after the winter season, there comes the spring season, followed by another period of inactive growth. Therefore, we get alternate layers of xylem vessels with wide cavities and those with smaller cavities.
The contrast between the two is so sharp that the successive rings are distinct even to the naked eye. In the transverse section of a stem we may find several concentric rings of vessels with wider and smaller cavities i.e., annual rings. As one ring is formed in a year, the age of the plant can be calculated by counting the number of rings. Determination of the age of a tree by counting the number of annual rings is known as dendrochronology. Sequoia dendron tree sections at the base (5-6 meter in diameter) have revealed their age to be upto about 3,500 years.
In certain cases in one year two annual rings are formed. It is probably due to interpolation of dry period in the middle of a growing season. In these cases dry period xylem resembles the summer wood. In these cases it is not possible to get correct age of the tree by counting the number of annual rings.
d. Heart-Wood and Sap-Wood:
In older stems, where sufficient quantity of secondary growth has taken place, the secondary wood loses the power of conduction and its cells are filled with tannin and other substances. It becomes hard and durable and is blackish in colour.
This region is called heart-wood or duramen and its function is to give mechanical support to the plant. The outer region of secondary wood, which includes younger xylem cells, is yellow in colour and is called sap-zuood or alburnum. It does the function of conduction.
2. Activity of the Cork Cambium:
a. Formation of Periderm:
The secondary tissues produced by the activity of the cambium exert pressure on the outer tissues and the epidermis can be ruptured. The hypodermis and cortex are also adversely affected. To replace this peripheral protective tissue, new secondary tissues are developed and called periderm.
It includes three parts:
(i) Cork cambium or phellogen,
(ii) Secondary cortex or phelloderm.
(iii) Phellem or cork and,
(i) Cork Cambium:
It originates usually in the outer layers of hypodermis cells. Sometimes it may arise from the epidermis e.g., willows, or form the cortical cells e.g., Clemates. It includes a row of narrow, thin-walled and roughly rectangular cells which are living and active. The cork cambium becomes meristematic and produces new cells on both the sides.
(ii) Secondary Cortex:
The newly produced cells of the cork cambium on the inner side get converted into parenchyma and form the secondary cortex or phelloderm. The cells usually bear chloroplast and add to the primary cortex. Sometimes, the cells can become thick walled.
(iii) Cork:
The new cells produced on the outside of the cork cambium lose their contents, becomes filled with air and are arranged in rows at right angles to the surface forming a dead imperious layer. The cell wall becomes suberised and the intercellular spaces are lost. This dead tissue acts as a protective coating and is called cork or phellem.
b. Formation of Bark:
The cork cells being suberized prevent the outward passage of the water. The outer tissue becomes dead and acts as the bark. Therefore, the bark includes all the dead tissues lying outside the cork cambium. It also includes the living parts inside. It can include epidermis, hypodermis, a part of cortex, secondary cortex, phloem and cork. The deeper is the seat of cork cambium, the thicker will be the bark.
When the cork cambium appears in strips, the bark is eliminated as scales d is known as scale bark e.g., Eucalyptus and Guava etc. In case it comes away as a sheet, it is known as ring bark e.g., Betula.
c. Function of Cork and Bark:
(i) Protection is the major function of cork and bark,
(ii) The lack of air spaces and suberization in the cork help in checking evaporation of water, and
(iii) It prevents the invasion of living tissues by fungi and bacteria.
d. Formation of Lenticels:
By the formation of periderm the stomata are closed and also the cuticular transpiration is prevented. To replace this, some aerating pores are formed in the bark known as lenticels. Through these pores exchange of gases and evaporation of water can take place. They appear as small perforations on the stem surface.
The phellogen at certain places on the outside, instead of cork, gives rise to a loose tissue composed of thin-walled cells, which are rounded, have air-filled spaces and are known as complementary cells. The epidemic above it is ruptured. This structure is known as lenticel.
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