Secondary Growth in Plants PDF

Normal Secondary Growth All plant stem growth occurs at the meristems of the shoot system because this is where cell division occurs. There are two types of meristem in the plant stem: apical and lateral. Primary growth occurs at the apical meristem and allows the plant stem to increase in length....

Normal Secondary Growth All plant stem growth occurs at the meristems of the shoot system because this is where cell division occurs. There are two types of meristem in the plant stem: apical and lateral. Primary growth occurs at the apical meristem and allows the plant stem to increase in length. However, some plants need more than just growth in the length of the stem. Primary growth occurs when plants grow toward the sunlight necessary for photosynthesis and also sink roots deep into the soil to anchor them and enable them to absorb water and nutrients. This 'up and down' growth is possible due to apical meristem, stem cell like tissue that, upon division, creates an undifferentiated cell that will become either a new root or shoot tip. Secondary growth happens when stems or branches grow outward (get thicker) This type of growth is possible because some plants (like trees and shrubs) have lateral meristem, another stem cell like tissue. Instead of causing the plant to grow up or down, lateral meristematic tissue causes the plant to increase in girth by adding rings of growth. So when the width of a plant or its girth increases is called secondary growth and it arises from the lateral meristems in stems and roots. As with apical meristems, lateral meristems are regions of high cell division activity. However, the cells they make grow outward rather than upward or downward. Dicots use lateral meristems to add to their width; monocots, however, do not experience secondary growth. The lateral meristems that produce secondary growth are called cambium, which just mean a tissue layer that adds to plant growth. The two important ones for secondary growth are the vascular cambium and the cork cambium. The vascular cambium produces more vascular tissue (xylem and phloem), which provide support for the shoot system in addition to transporting water and nutrients. Because the xylem and phloem that come from the vascular cambium replace the original (primary) xylem and phloem, and add to the width of the plant, they are called secondary xylem and secondary phloem. Secondary growth is growth at the lateral meristem and increases the girth of the stem. This type of growth is only found in dicots and is not found in monocots. There are two types of vascular tissue: xylem, which moves water and dissolved minerals, and phloem, which moves food in the plant stem. In monocots and dicots, these structures are organized a bit differently. In monocots, the xylem and phloem are found in paired bundles and are scattered throughout the stem. Remember that monocots are simple flowering plants such as grasses. However, in dicots - which are more advanced flowering plants such as roses and apple trees - the xylem and phloem are found in rings with the xylem on the inside and the phloem on the outside. This organization allows for secondary growth of plant stems. Cambium Meristematic tissue responsible for lateral (outward) growth in plants is known as cambium. There are two kinds of cambium in woody plant stems, both of which increase the diameter of stems. First type of cambium is vascular cambium found in the center of the stem; its division produces the plant's secondary vascular tissue (xylem and phloem cells.) The outer ring or near the epidermis the bark of a woody plant also contains a cambium called secondary cambium or cork cambium, which creates cork cells of the outer layer and responsible to give rise the bark. Fig. 1.1: Structure of cambium The cambium layer consists of a single layer of cell and these cells divide in a direction parallel with epidermis. Each time it divides into two cells and one of the two new cells one remains meristematic and the other differentiates into permanent tissue. If the newly formed cell is near the xylem it will form secondary xylem and if newly formed cell is towards phloem it will develop in secondary phloem. The activity of cambium thus increases and the enlargement of stem takes place and the activity of cambium remains for a considerable long period of time (Fig. 1.1). NORMAL BEHAVIOUR OF CAMBIUM Cells of apical meristems divide, differentiate and develop to form primary tissues. As a result, the plant grows in length this is called primary growth. While by the activity of secondary lateral meristems, increase in the circumference/girth of the plant organs due to the formation of secondary tissues in stelar and extra stelar regions, is called as secondary growth. Normally secondary growth takes place in roots and stem of dicots & Gymnosperms. Due to lack of cambium in monocots, secondary growth is absent. But exceptionally secondary, growth takes place in some monocots such as Palm, Yucca, Dracaena, Smilax, Agave, Coconut etc. Secondary growth in stelar region begins earlier than the extra stelar region. It starts from the development of cambial ring and the detailed activities occur during this is given below (Fig. 1.2, 1.3): Fig. 1.2 Showing primary growth and secondary growth Formation of ring of vascular cambium Vascular bundle comprises xylem and phloem in a bundle and in case of dicot stem these are conjoint, collateral and open type i.e. cambium cells are present in between xylem and phloem cells. A cambium which is present inside the vascular bundle is called intrafascicular cambium. This is a type of primary meristem. When plants become mature then the secondary growth starts and the first step of secondary growth is the formation of cambial ring. For this first of all the cells of medullary rays present in between the vascular bundles become meristematic to form interfascicular cambium this is secondary lateral meristem. Interfascicular cambium is the meristematic cells present outside the vascular bundle and these cells are developed from the medullary cells. Intrafascicular and interfascicular cambia are collectively known as vascular cambial ring. Vascular cambium is formed in the form of a complete ring which is made up of single layer of cells. In dicot stem some part of vascular cambium is primary and some part is secondary. Two types of cells are found in the ring of this vascular cambium. (i) Fusiform initials (ii) Ray initials Fig. 1.3: A. T.S. of stem before secondary growth; B. T.S. of stem after secondary growth Fusiform initials are long with pointed ends, while ray initials are spherical (oval). Amount of fusiform initials is more in vascular cambium. Continuous periclinal divisions or tangential division takes place in fusiform initials. The plane of division in periclinal divisions is parallel to longitudinal axis of a cell. Through this type of activity few cells are formed towards the radius (periphery) and these cells differentiate into secondary phloem or bast and some of the cells are formed towards the central axis and these cells are differentiated into secondary xylem or wood. Now the complete cambial ring starts producing cells towards inside and outside by division. Normally more secondary xylem is formed as compared to secondary phloem due to unequal distribution of hormones. (secondary xylem is formed 8-10 times more as compared to secondary phloem). By the pressure of secondary phloem; primary phloem is pushed towards the outside and gets crushed. By the pressure of secondary xylem, all the primary tissues such as primary xylem, pith and old secondary xylem degenerates in the center of the stem. Due to this central part of the stern becomes woody. These activities keep going on continuously in plants throughout. Before secondary growth the sequences of cells from center towards outside remains pith, primary xylem, cambium, primary phloem, pericycle and endodermis. But due to secondary growth the sequence of the vascular bundle from center changes to primary xylem, secondary xylem, cambium, secondary phloem, primary phloem and then endodermis. Pith crushes due to the pressure created by the newly formed secondary xylem. Secondary xylem forms in the plant regularly and primary tissues degenerate continuously. This new secondary xylem also degenerates the old secondary xylem. Waste materials are formed in the stem such as lignin, suberin, tannin, resin-gums etc. due to degeneration of the cells. All these waste materials are filled in the lumen (cavity) of tracheids and vessels of secondary xylem. Because of this, wood in the central region of the stem becomes dark colored (Black brown). It is called heart wood or Duramen. The peripheral or outer wood which looks light in color is known as Sap wood or Alburnum. As a result of growing of secondary xylem, the diameter of heart wood increases. Physiologically active wood is sapwood and the main function of sap wood is water conduction. Heart wood provides maximum mechanical strength to stem. Conduction of water is not carried by heart wood because: Cavities of tracheids and vessels are progressively filled by waste materials. The bladder like in growth of parenchyma cells which enter the lumen of vessels (mainly) and tracheids through the pits in their wall. Such bladder like in growth is called tyloses. Tyloses block the conduction of water. In Gymnosperms tylosoids are formed in place of tyloses. If the heart wood is destroyed in any stem, then there will be no effect on plants (any vital function is not effected), but if the sap wood is destroyed, then the plant will die because conduction of water will be blocked. Heart wood provides stiffness to the stem. The waste materials of heart wood are antiseptic in nature. Heart wood is resistant to bacteria and fungus. Heart wood has a power of repelling insects- so it is resistant to the termites and in rainy season it does not imbibe water. Thus it is the best quality of wood. Study of wood is known as xylotomy.