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Question
Explain the process of secondary growth in stems of wood angiosperms with the help of schematic diagrams. What is its significance?
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Solution
Secondary growth of a woody angiosperm stem occurs by two types of cambium, vascular cambium and cork cambium.
Vascular Cambium: It is formed partly by primary intrafassicular cambial strips and partly by secondary interfascicular strips from medullary rays. Cells of vascular cambium divide both on the outerside as well as on the inner side. Vascular cambial cells called fusiform initial form secondary pholem on the outer side and secondary xylem on the inner side. At places vascular cambium possesses ray initials. They form vascular rays, pholem rays in secondary pholem and wood rays in secondary xylem. Secondary pholem forms a narrow circle on the outside. As new secondary pholem becomes functional, the previous older pholem gets crushed. Secondary xylem persists. As a result wood grows with age. In order to accommodate it, the vascular cambium also grows in diameter by addition of new cells. The phenomenon is called dilation.
Cork cambium (Phellogen): It produces secondary growth tissues collectively called periderm. Cork cambium develops secondarily from a subepidermal layer of living cells. It produces phellem or cork on the outside and secondary cortex or phelloderm on the inner side. Cork consists of dead, suberised and impermeable cells. At places aerating pores called lenticels occur. A lenticel has loosely arranged suberised complementary cells. The interspaces help in gaseous exchange.
Significance: Secondary growth is necessary for the growth of girth of the stem. It also provides additional mechanical strength to the stem.
Vascular Cambium: It is formed partly by primary intrafassicular cambial strips and partly by secondary interfascicular strips from medullary rays. Cells of vascular cambium divide both on the outerside as well as on the inner side. Vascular cambial cells called fusiform initial form secondary pholem on the outer side and secondary xylem on the inner side. At places vascular cambium possesses ray initials. They form vascular rays, pholem rays in secondary pholem and wood rays in secondary xylem. Secondary pholem forms a narrow circle on the outside. As new secondary pholem becomes functional, the previous older pholem gets crushed. Secondary xylem persists. As a result wood grows with age. In order to accommodate it, the vascular cambium also grows in diameter by addition of new cells. The phenomenon is called dilation.
Cork cambium (Phellogen): It produces secondary growth tissues collectively called periderm. Cork cambium develops secondarily from a subepidermal layer of living cells. It produces phellem or cork on the outside and secondary cortex or phelloderm on the inner side. Cork consists of dead, suberised and impermeable cells. At places aerating pores called lenticels occur. A lenticel has loosely arranged suberised complementary cells. The interspaces help in gaseous exchange.
Significance: Secondary growth is necessary for the growth of girth of the stem. It also provides additional mechanical strength to the stem.
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