Tissues called apical meristems are responsible for the growth of the plants. The primary structure to the plant body is given by apical meristems. These tissues increase the thickness of stems and roots. They also increase the height and weight of the plant. This growth in thickness is called secondary growth. This type of growth takes place in gymnosperms and dicotyledonous plants and not in monocotyledonous plants.
The increase in thickness takes place in two stages; namely—
(i) Stelar secondary growth.
(ii) Extra-Stelar secondary growth.
A brief description of these two is given below.
(i) Stelar Secondary Growth : The vascular bundles in the form of a ring shown by stele in the primary structure of dicot stem are called fascicular cambium. These cambia fuse to form a continuous cambium ring.
Cambium ring produces new cells. In the cambium two types of initiating cells are found. These are (i) Fusiform initials and (ii) Ray initials.
The fusiform initials give rise to secondary xylem and the secondary phloem.
The ray initials produce phloem to the inside. These rays are useful in lateral conduction and storage. These rays are called, ‘secondary medullary rays.’
In Dicot stem, the primary phloem and xylem are crushed and removed with the increase in stem thickness. They get replaced by secondary phloem and xylem. These remain active for a very long period of time.
Annual Rings : Seasonal variations in temperate regions effect the activity of the cambium. During spring season, the plant needs more water because of the presence of more flowers and leaves. In this season the wood formed shows more number of xylem vessels with wider lumens. This is called spring wood. The colour of this wood is light. In autumn, the plants do not need more water and mineral salts. In this season wood produced shows less number of xylem vessels. The wood formed in this season is dark in colour and called autum wood. In this way two types of wood are formed. It has dark and light colour. This forms dark and light coloured rings in the year. These are called annual rings. By counting the number of rings in the trunk one can tell the age of a tree.
In countries like India, annual rings do not appear clearly. This is because India is a tropical country and seasonal variations are not very sharp.
Some times Hormonal changes or heavy leaf fall or diseases also produce annual rings. These are called ‘pseudo annual rings’. In such cases the number of annual rings is not the correct estimate of age.
The thickness of these rings depends on the seasonal conditions. In active monsoons they are thinner.
Heart Wood and Sap Wood : As the age of the tree increases the wood undergoes a number of physical and chemical changes. The older wood is dark in colour which is shown by the centre of the trunk of the tree. This is called heart wood. This wood gives good mechanical strength to the tree.
Perpheral part of the stem of the tree is light in colour and is called sap wood. It can conduct water, minerals and storage of food materials. With the passage of time sap wood changes into heart wood.
(II) Extra-Stelar Secondary Growth : A pressure is exerted due to the formation of secondary xylem and phloem inside the stele. This makes epidermis to cause rupture. Meanwhile periderm is formed from the cortex. From the middle of cortex the secondary growth of corten begins. This is called cork cambrium.
The cork cells are rectangular or cubical in shape. They are arranged in radial rows. The cork tissue contains waxy substance. The cork does not allow water and gases. Cork can be compressed. A very good quality cork is obtained from the Oak tree. Bottle corks are made from this cork.
Lenticels : The exchange of gases takes place in the secondary cortex because cork is dead. The gaseous exchange in the cork tissue produces lens shaped structure. These are called ‘lenticels’ which promote transpiration.
Bark : The water supply to the outer side of the cork is cut off which makes outer layers dry. Strips peel off from outside the cork. This is called bark. In some plants bark is peeled off as thin sheets. In some plants the bark is thick. It is removed as irregular shreds. This is called scaly bark.
Shopping Cart
