Alex K GeorgeBSF-10-002

BARKThe 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

STRUCTURE1. OUTER BARK2. INNER BARK

OUTER BARKThe outer, mostly dead tissues (outer bark) form a

protective barrier between the plant axis and the abiotic and biotic environment.

INNER BARKThe 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.

OUTER BARK1. Cork (PHELLEM)

2. Cork cambium (PHELLOGEN)

3. Secondary cortex (PHELLODERM)

PERIDERM

cork cellsCells 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.

Cont..Uniform size,arranged in radial rows without

intracellular spacesIt mostly consists of densely packed parenchymatous

cells with walls that become suberized or lignified (or both) before the cells die and become air-filled.

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.

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

LenticelsLenticels are lens shaped opening or breaks in the

cork tissueFormed due to rupture in the epidermis during

secondary growthLenticels permit the exchange of gases between the

outer atmosphere and the internal tissue of the stem. These occur in most woody trees.

Inner barkThe phloem elements which are produced by the

vascular cambium are called secondary phloem and are larger in size called metaphloem.

Phloem elements are1. Sieve elements 2. Companion cells 3. Phloem parenchyma 4.Phloem fiber

Sieve elementsSieve 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.

Cont..They are distinguished into sieve cells and sieve

tubes. Sieve cells occur in pteridophytes and gymnosperms, while sieve tubes occur in angiosperms

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.

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.

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..

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.

CHEMICAL COMPOSITIONThe 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.

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.

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

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).

USES OF BARKIndustrial fuelSoil amendmentGround coverPotting mediumTannic acid extraction

Reference Http://www.Ecology.Info/article.Aspx?Cid=10&id=45Forest products and wood science John G. Haygreen and jim l. Bowyer

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