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Secondary Growth

The increase in thickness or girth due to the activity of the cambium and the cork cambium is known as secondary growth.

(1) Secondary growth in stem : On the basis of the activities of cambium and cork-cambium, secondary growth in stem can be discussed under the following heads :

(i) Activity of cambium               

(ii) Activity of cork-cambium

(i) Activity of cambium : The vascular cambium in between xylem and phloem is called intrafascicular or fascicular cambium which is primary in origin. At the time of secondary growth the parenchymatous cells of medullary rays between the vascular bundles become meristematic and form strip of cambium called as interfascicular cambium which is secondary in origin. Both inter and intrafascicular cambium joins together and form cambium ring which is partly primary and partly secondary in origin. By anticlinal divisions the circumference of the cambium increase. By periclinal division cambium produced the secondary xylem and phloem tissues on innerside and outerside. The amount of sec. xylem produced is 8-10 times greater than sec. phloem. The cambium has two types of cells:

(a) The fusiform initials which are elongated and form fibres, sieve cells, sieve tubes, tracheids.

(b) Ray initials which produce parenchyma cells of the rays in wood and phloem. Ray initials are much shorter than fusiform initials. Certain cells of cambium form some narrow bands of living parenchyma cells passing through secondary xylem and secondary phloem and are called secondary medullary rays. These provide radial conduction of food from the phloem, and water and mineral salts from the xylem.

• Annual rings: Activity of cambium is not uniform in those plants which grow in the regions where favourable climatic conditions (spring or rainy season) alternate regularly with unfavourable climatic conditions (cold water or dry hot summer). In temperate climates, cambium becomes more active in spring and forms greater number of vessels with wider cavities; while in winter it becomes less active and forms narrower and smaller vessels. The wood formed in the spring is known as spring wood and that formed in the dry summer or cold winter autumn wood or late wood. Both autumn and spring wood constitute a growth or annual ring. In one year only one growth ring is formed. Thus by counting the number of annual rings in the main stem at the base we can determine the age of a tree. This branch of science is known as dendrochronology. Age is determined by an instrument increment borer. Growth rings are distinct or sharply demarcated in the plants of temperate regions where as in tropical climate (near equator) they are not distinct or sharply demarcated in the trees.

(ii) Activity of cork cambium : Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellum on outerside. The cells of phellem are dead, suberized and impervious to water. Cells of phelloderm are thin walled, living and store food. Phellem, phellogen and phelloderm collectively called as periderm. Periderm is secondary protective tissue. Due to pressure of secondary xylem, epidermis raptures and cortex is largely lost after two or three years of secondary growth.

(a) Bark : All dead tissues lying outside the active cork-cambium are collectively known as bark. This includes ruptured epidermis, hypodermis and cork. When cork-cambium appears in the form of a complete ring, it is known as ring bark, e.g., Betula (Bhojpatra). If the cork cambium occurs as separate strips and the resulting bark appears in the form of scales, such a bark is known as scaly bark. e.g., Eucalyptus, Psidium guava. The outermost layer of bark is dead and called as rhytidome.

(b) Lenticels : These are aerating pores formed in the cork through which gaseous exchange takes place. They are formed as a result of the action of phellogen. A lenticel appears as a scar or protrusion on the surface of the stem and consists of a radial row of thin-walled cells, known as complementary cells or filling tissue. They are found in old dicot stem, main function is guttation.

(c) Cork : It consists of dead cells with thick walls heavily impregnated with suberin. These cells are compactly arranged in radial rows without intercellular spaces. Cork is impervious to water and prevents its loss from the plant surface. It also protects the inner tissues from the attack of fungi and insects. There is no differentiation of bark, sap wood and heart wood of Date palm.

(d) Heart wood and sap wood : In old trees, secondary wood is differentiated into a centrally situated darker and harder wood called the heart wood or duramen which are physiologically inactive (almost dead)and an outer light-coloured zone called the sap wood or alburnum which are physiologically active. Dark colour of heart wood is due to the deposition of tannins, resins, gums, essential oils, etc. in the cell walls and cell cavities. The water conduction takes place through sap wood. During the conversion of sap wood into heartwood the most important change is development of tyloses in the heart wood. Tyloses are ballon like structures, develop from xylem parenchyma. These tyloses block the passage of xylem vessels so also called as tracheal plug. The heart wood is commercially used as wood. When the plant is made hollow, it will not die because the water conduction takes place through sap wood. The heart wood is well developed in Morus alba (Mulberry). The heart wood is absent in Populus and Salix plant. As a tree grows older thickness of heartwood increases and sap wood remains same.

(2) Secondary growth in dicot roots : Vascular bundles in dicot roots are radial, exarch and mostly triarch. Vascular cambium is formed secondarily from conjuctive parenchyma cells lying just below each phloem strand. Thus the number of cambium strips formed equals the number of phloem strands. The cells of pericycle lying outside the protoxylem also become meristematic to form part of strips of cambium. These cambial strips join the first formed cambium strips to form complete but wavy ring of vascular cambium. This cambium ring produced secondary xylem on inner side and secondary phloem on outer side. In roots, the growth rings are not distinct because there is no seasonal variation under the soil. From the outer layers of pericycle arises the phellogen which cuts phellem (cork) on the outer side and secondary cortex or phelloderm toward the inner side.

 Important Tips

• Grew is the father of anatomy (1682) and coined the term tissue and parenchyma.
• Haberlandt proposed the names of protoderm (for dermatogen), ground meristem (for periblem) and procambium (for plerome)
• Haberlandt (1914) gave the terms lepton for soft walled conducting part of phloem and hadrom for conducting part (tracheary elements) of xylem.
• Strasburger discovered albuminous cells instead of companion cells in the phloem of non-flowering plants.
• In sugarcane there is no distinction of tunica and corpus.
• Reproductive apex is elongated in Sagittaria but it can be 400 times broad in Chrysanthemum.
• Seive cells or seive tube elements resemble RBCs in being without nucleus in the mature state.
• Cavities are of three types :

(i) Schizogenous : They are formed by enlargement of intercellular spaces or separation of cells e.g. oil of Sunflower.

(ii) Lysigenous : They are formed by degeneration of cells, e.g., oil cavity of Citrus and protoxylem lacunae or water cavity in monocot stem vascular bundles.

(iii) Schizolysigenous : They are formed partly by separation and partly by degeneration of cells. e.g., protoxylem cavity.

• Pith cavity often present in monocot stems (e.g., grass) and occassionally in dicot stems (e.g., Ricinus).
• Wood without vessels is called homoxylous, e.g., Ranales (winteraceae, tetracentraceae, trochodendraceae). Whereas with vessels is called heteroxylous.
• The wood of Tactona grandis is termite resistance.
• The bottle cork is prepared from cork of Quercus suber (Oak tree).
• Lightest wood is of Ochroma pyramidate (O.lagopus).
• Heaviest wood is of Guaiacum officinale. In India heaviest wood is of Acacia sundra.
• Most durable soft wood is of Cedrus deodara.
• Latex for chewing or chickle gum obtained from Achras sapota, Gutta percha (insulating material) from palaquium gutta alkaloid opium from Papaver sominiferum (poppy), papain (enzyme) from carica papayas, Rubber from Hevea brasiliensis, Ficus elastica.
• Reaction wood is a wood formed in bending stems. When reaction wood is formed on the lower side, it is called as Compression wood e.g., conifers. When it is formed on the upper side, it is called as tension wood e.g., Dicots.
• Wound periderm is similar to natural periderm. But it is restricted to the place of injury and is used in producing the commercial cork.
• Maceration is a method of separation of various tissues by disintegration of middle lamella.
• In some plants primary structure is abnormal such as presence of medullary bundles in pith e.g., Boerhaavia, Mirabilis, Achyranthes, Bougainvillea or presence of cortical vascular bundles (inverted) e.g., Casuarina and Nyctanthus.
• A protective tissue found in roots of some plants (Rosaceae, Myrtaceae) having alternate layers of endodermal and parenchyma cells are called periderm.
• Knots are the bases, scars/wounds of fallen branches get covered by growth of secondary tissues. They form knots in the wood.
• Abscission is a special layer of parenchymatous cells appears at the base. Abscission is premature fall of plant parts from the plant without causing the injury. A protective layer of suberised thick walled cork cells is formed below the abscission layer to prevent infection or dessication (sometimes it is corky layer).
• Metaxylem consist of two larger and rounded vessels situated on the sides with the pitted tracheids in between them.
• Protoxylem consists of two smaller vessels situated to wards the centre. The vessels of metaxylem are pitted and those of protoxylem are annular and spiral.
• Depending upon the relative position of protoxylem; xylem is of four types :

                        (i) Exarch : Protoxylem towards the outerside.

                        (ii) Endarch : Protoxylem towards innerside of metaxylem.

                        (iii) Mesarch : Protoxylem surrounded by metaxylem.

                        (iv) Centrach : Protoxylem in the centre of metaxylem.

• Endarch xylem is also called centrifugal as xylem matures from inside to outside. Similarly, exarch xylem is known as centripetal because differentiation of xylem proceeds from outside to inside e.g. roots.
• Root cap is absent in hydrophytes.
• Root hairs are found in zone of maturation.
• In the leaf, vascular bundles are found in the veins.
• An example of monocots showing secondary growth in stems is Yucca or Draceana.
• Safranine stains lignified elements of the tissue.

The longitudinal section of a root have four zones which occur in the following order (from the tip upward) : Root cap, cell division, cell enlargement, cell maturation.