3. 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
5. 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.
7. 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.
8. 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.
9. 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.
10. 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
11. 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.
12.
13. 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
14. 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.
15. Cont..
They are distinguished into sieve cells and sieve
tubes. Sieve cells occur in pteridophytes and
gymnosperms, while sieve tubes occur in angiosperms
16. 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.
17. 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.
18. 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.
.
19. 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.
20.
21. 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.
22. 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.
23. 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
24. 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).
25. USES OF BARK
Industrial fuel
Soil amendment
Ground cover
Potting medium
Tannic acid extraction
