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Tree bark

  1. 1. Alex K George BSF-10-002
  2. 2. BARK The term bark refers to all tissues of a woody stem or root occurring just outside of the vascular cambium. Bark is a very complex structure, consisting of cells that formed from lateral meristems
  3. 3. STRUCTURE 1. OUTER BARK 2. INNER BARK
  4. 4. OUTER BARK The outer, mostly dead tissues (outer bark) form a protective barrier between the plant axis and the abiotic and biotic environment. INNER BARK The inner tissue (secondary phloem), including living cells (inner bark), is where sugar transport for the plant occurs, and the inner bark also can have defences against herbivores, such as cells with tough cell walls (secondary phloem fibres or sclereids) or cells and tubes filled with bitter or toxic chemicals.
  5. 5. OUTER BARK 1. Cork (PHELLEM) 2. Cork cambium (PHELLOGEN) 3. Secondary cortex (PHELLODERM) PERIDERM
  6. 6. cork cells Cells pushed to the outside generally have layering of suberin in their walls, and thereby become cork cells (also called phellem). Suberin, often composed of suberic acid and phellonic acid, is a fatty and waxy substance, which makes the cells mostly impervious to water and unable to exchange gases and nutrients, hence these cells soon die and entrap air.
  7. 7. Cont.. Uniform size,arranged in radial rows without intracellular spaces It mostly consists of densely packed parenchymatous cells with walls that become suberized or lignified (or both) before the cells die and become air-filled.
  8. 8. Phelloderm Cells pushed from the cork cambium to the inside are called phelloderm, and these can remain alive because they have unthickened and unspecialized cell walls and, hence, can exchange gases and obtain nutrients.
  9. 9. Cont.. The cells of phelloderm are parenchymatous living,isodiametric and radially arranged with intercellular spaces Have chloroplast they carry out photosynthesis. Cell wall made of cellulose
  10. 10. Lenticels Lenticels are lens shaped opening or breaks in the cork tissue Formed due to rupture in the epidermis during secondary growth Lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem. These occur in most woody trees.
  11. 11. Inner bark The phloem elements which are produced by the vascular cambium are called secondary phloem and are larger in size called metaphloem. Phloem elements are 1. Sieve elements 2.Companion cells 3. Phloem parenchyma 4.Phloem fiber
  12. 12. Sieve elements Sieve elements are the conducting elements of the phloem. They have thick primary walls. Contain sieve plate. The sieve elements are arranged one above the other and form vertical sieve tubes . It contains a lining layer of cytoplasm. A special protein called slime body is seen in it. The conduction of food material takes place through cytoplasmic strands.
  13. 13. Cont.. They are distinguished into sieve cells and sieve tubes. Sieve cells occur in pteridophytes and gymnosperms, while sieve tubes occur in angiosperms
  14. 14. Cont.. Sieve cells have sieve areas on their lateral walls only and are not arranged one above the other in linear rows. They are not associated with companion cells. Sieve tubes are arranged one above the other in linear rows and have sieve plates on their end walls. They are associated with the companion cells.
  15. 15. companion cells. The thin-walled, elongated, specialized parenchyma cells, which are associated with the sieve elements, are called companion cells. They assist the sieve tubes in the conduction of food materials.
  16. 16. Phloem parenchyma The parenchyma cells associated with the phloem are called phloem parenchyma. These are living cells. They store starch and fats.  in some plants they also contain resins and tannins. .
  17. 17. Phloem fibres The fibres of sclerenchyma associated with phloem are called phloem fibres or bast fibres. They are narrow, vertically elongated cells with very thick walls and a small lumen. Among the four kinds of phloem elements, phloem fibres are the only dead tissue. These are the strengthening and supporting cells.
  18. 18. CHEMICAL COMPOSITION The lignin content of bark is much higher than that of wood, and the polysaccharide or sugar content is correspondingly lower.  The extractive-free cellulose portion of bark is only 20- 35%, compared to 40-45% for wood. The ash content of bark is usually higher than that of wood. Extractive content of bark is high compared to wood, commonly amounting to 15-26% of unextracted bark weight compared to 2-9% for wood. 
  19. 19. Bark extractives include various starches, resins, and waxes. A major portion of the extractable chemicals, one- fourth to one-half by weight, is tannic acid, Bark extract is usually more highly acidic than extract from wood of the same species.
  20. 20. BARK FUNCTIONS 1. Transport -Photosynthates as well as some proteins and RNA are principally transported around trees from sources to sinks through sieve elements in the phloem. 2. Growth -The girth increments in tree stems that make height growth are mostly due to expansion of cells derived from the vascular cambium. 3. Biomechanical support -provide rigidity and flexibility is to upright tree stem
  21. 21. 4. Defence –against both biotic and abiotic stress 5. Storage -store carbohydrates, fats, oils, latex, and resins 6. Carbon Fixation -In some species the photosynthetic tissue is a persistent epidermis with abundant lenticels (e.g., Betula) whereas in others photosynthesis occurs in cortical tissues rejuvenated by continued cell division (e.g. Populus).
  22. 22. USES OF BARK Industrial fuel Soil amendment Ground cover Potting medium Tannic acid extraction
  23. 23. Reference Http://www.Ecology.Info/article.Aspx?Cid=10&id=45 Forest products and wood science John G. Haygreen and jim l. Bowyer
  24. 24. THANK U!

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