Write the role of cambium in the secondary growth of dicot stem.

The increase in thickness or girth due to the activity of the cambium and the cork cambium is known as secondary growth. In a typical dicot stem, secondary growth starts in the interstellar region and extends towards the extra-stellar region. In the steller region, it starts growing to the activity of cambium and in the extra-stellar region due to cork cambium. 

Secondary growth in dicot stem: On the basis of the activity of cambium and cork cambium, secondary growth in the stem can be explained in the following heading.

1. The activity of the cambium:
   
   
   1. The Formation of the cambium Ring: The vascular bundles of the dicot stem having strips of cambium in between the xylem and phloem which are known as intrafascicular cambium. During secondary growth, the cells of medullary rays in a line with the interfascicular cambium develop meristematic activity and form strips of cambium called interfascicular cambium. The intra and interfascicular cambium unite to form a complete ring called the cambial ring. The activity of the cambial ring gives rise to secondary growth. The cambium has two types of cells (i) the fusiform initials which are elongated and form fibers, sieve tubes, tracheids. (ii) ray initials which produce parenchymatous cells of the rays in wood and phloem.
      
      
   2. Formation of Secondary Tissues: The cambium ring becomes active as a whole and starts cutting off new cells. The cells cut off on the outer side get differentiated into the phloem and are called secondary phloem. The cells cut off on the inner side are modified into the elements of the xylem which constitute secondary xylem. The secondary phloem consists of sieve tubes, companion cells, phloem fibers, and phloem parenchyma. The secondary xylem consists of pitted vessels, tracheids, xylem fibers, and xylem parenchyma. The activity of the cambium ring is more on the inner side than on the outer with the result that the xylem increases more rapidly in bulk than the phloem. Due to increasing pressure, the primary phloem is completely crushed and is replaced by small patches. The xylem is pushed towards the pith and remains more or less intact. The bulk of the trunk of a tree consists of secondary xylem.
      
      
   3. Formation of Secondary Medullary Rays: Ray initials of the cambium ring from some narrow bands of parenchymatous cells. These cells extend radially from the pith to the phloem. These are called secondary medullary rays or vascular rays. The rays present in the xylem are called xylem rays and rays present in phloem are called phloem rays.
      
      
   4. Formation of Annual Rings: The activity of vascular cambium is greatly affected by the variations in the climate. It is more pronounced in temperature regions. The cambium stops dividing in winter. In the spring season or early summer, the cambium becomes more active and produces a large number of vessels with the wider lumen. These are called springwood or earlywood. During the autumn or winter season, the cambium becomes less active and produces vessels with narrow lumens. Tracheids and wood fibers are formed in large numbers. These woods are called autumn wood or latewood. Hence the annual rings are formed after year. In the oldest part of the tree, annual rings can be used in determining the age of the tree. In tropical regions, the climate is more or less uniform. Therefore the annual rings are not well developed and do not correlate with the age of the tree.
      
      
      
      
   5. Sapwood and Heartwood: In older stems, the woody trunk is differentiated into two regions. The outer light-colored region is called sapwood or laburnum and the central dark-colored region is called heartwood or duranum. The cells of sapwood are living and functional. They take part in the conduction of water and the storage of food. The heartwood consists of dead cells. During the growth process, the ring of sapwood gradually processes rings of sapwood gradually convert into heartwood. The living cells of sapwood lose their protoplast and water content. The lumen of the xylem vessels gets blocked by the ingrowth of the parenchymatous cells. The adjacent parenchyma tissue enters through the pits of vessels and gradually enlarges to form a ballon-like structure, which is called tyloses. The heartwood is stronger and more durable than sapwood. The heartwood becomes resistant to attacks from bacteria and fungi due to the presence of antiseptic oils.
2. The activity of the cork cambium (secondary growth in the cortex): It is the result of the activity of a secondary meristem called cork cambium, which appears between the hypodermis and primary cortex. Some of the parenchyma cells in the peripheral layers of the cortex undergo dedifferentiation and become meristematic. These cells now represent the cork cambium or phellogen. The cork cambium starts exhibiting mitotic activity on both sides, just as the cambial ring in the stele.
   
   The mitotic activity on the inner surface of the cork cambium results in the formation of cells, which undergo differentiation into a living tissue, called secondary cortex or phelloderm, just above the primary cortex. The mitotic activity on the outer surface results in the formation of cells, which undergo differentiation into dead tissue, called cork or phellem, just below the epidermis. The cork covers and masks the hypodermis. The tissue resulting from secondary growth in the cortex, the cork cambium, and the secondary cortex together represent a region called the periderm.
   
   The periderm along with the primary cortex represents the bark. In several dicot plants, the bark peels off regularly. Due to the formation of the periderm, the epidermis is subjected to pressure and as a result, it breaks at several places to form an opening called lenticels. The lenticels, also known as aerating pores, enclose a group of living cells called complementary cells. Through these cells exchange of respiratory gases and to some extent transpiration takes place. Thus, secondary growth in the cortex results in the formation of the periderm. Due to the addition of this region, there is an increase in the girth of the girth.