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Last updated date: 16th Jun 2024
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Answer
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Hint: Lenticels are formed at the sites of stomata by the break-down of the epidermis during periderm formation. They serve as passages for gas exchange. Hence they are analogous to the stomata of the primary epidermis.

Complete answer:
Lenticels are small, isolated, and raised openings on the surface of old stems. The phellogen of lenticel is continuous with that of the periderm. At the base of lenticel, there is a loose mass of parenchymatous cells, known as complementary cells or filling cells. These are thin-walled, colorless, and normally non-suberized. Complementary cells are derived from lenticel phellogen. The intracellular spaces between the complementary cells enable the ventilation of internal tissue. In temperate plants, in addition to the complementary cells, there are few layers of compactly arranged and suberized cells. They are also produced by lenticel phellogen. They collectively form the closing layer or closing Membrane. The extra-stelar secondary thickening takes place outside the stele. Lenticel formation and periderm formation is a part of it.

Additional Information:
- The closing layer is usually formed towards the end of the growing season.
- It closes the lenticel, blocks the air-flow through it and thereby, prevents lenticel from winter exposure.
- At the onset of the next growing season or rainy season, lenticel phellogen again produces complementary cells.

Note:
- The production of complementary cells causes the rupture of the closing Membrane above and re-establishes gas exchange. Towards the inner side, lenticel phellogen may produce phelloderm, which is in close contact with the Primary cortex.
- The extra-stelar secondary thickening takes place outside the stele. It involves the formation of Lenticels and periderm outside the cortex.
- Periderm is the protective covering that replaces the epidermis. Lenticels are small openings on the perimeter, for gas exchange.