Study of gross internal structures of plant organs by the method of section cutting is called plant anatomy.
Tissue is formed by a group of cells having
- Same origin
- Same structure
- Same function
Gross classification of tissue
- Permanent tissue
- Meristematic tissue
- Secretory tissue (concerned with secretion of chemicals)
- The cells have no power of cell division.
- Two types:
- Living or thin walled.
- Non-living or thick walled (lignin present).
- Growth is stopped either completely or for the time being.
Tissues with partially hindered growth may turn into meristematic again (e.g. Patabahar = Croton = Codiaeum variegatum (Euphorbiaceae family)).
Good to know
- Lignin plays a crucial part in conducting water in plant stems. The polysaccharide components of plant cell walls are highly hydrophilic and thus permeable to water, whereas lignin is more hydrophobic.
- Lignin is present in all vascular plants, but not in bryophytes.
- Thick walled tissues may be living or dead.
Classification of permanent tissue
1. Simple tissue
- Composed of one kind of cells.
- Forms a homogeneous or uniform mass.
- Parenchyma (living)
- Sclerenchyma (Non-living or dead)
2. Complex tissue
- Composed of more than one kinds of cells.
- Forms a heterogeneous mass.
- Works together as a unit.
- Made up of parenchyma and sclerenchyma cells, though parenchyma may not be present sometimes.
- Collenchyma cells are always absent in complex tissue.
Parenchyma tissue (meristematic though permanent)
- Protoplasm present.
- Cell walls generally thin. Sometimes thick.
- Round, oval, polygonal, isodiametric etc.
- Equally expanded.
- Intercellular space present.
- Vegetative propagation (e.g. cutting) via plant parts is possible due to meristematic potential of parenchyma cells.
- Major parts of plant body.
- Spongy parenchyma + palisade parenchyma (together form mesophyll tissue).
- Xylem, phloem.
- Fruit pulp (juicy vesicle of a citrus fruit), endosperm of seed.
- Edible flesh of fruit.
Special types of parenchyma
- With large intercellular space.
- In cortex of aquatic plants.
- Contain well developed air space.
- Have chloroplast.
- They carry on photosynthesis.
- g. Spongy parenchyma, palisade parenchyma.
Function of parenchyma cells
- Main function is to store food in the form of starch, grain, fat etc.
- In case of xerophytic or succulent plants, they store water.
- Aerenchyma helps to float.
- Helps in exchange of gas.
- Chlorenchyma helps in photosynthesis.
- Living tissue.
- Cell wall thick but not always.
- Thickness by cellulose and pectin.
- Uneven thickness.
- Elongated, rectangular, oblique cells with tapering or flat end.
- Water content is huge in cell wall. So the cells are extensible or elastic.
- Colouring agent: tannin, resin.
- Main source of mechanical support in the young stage of growing organs as they lack secondary tissue (Later on, secondary growth takes place. So, collenchyma cells are not required further).
- Normally after epidermis.
- In growing organs.
- Mainly in supporting tissue of dicot leaf and stem.
- In periphery of stem and leaf.
- May be present in root cortex when roots are exposed to sunlight.
- Not found in root, stem and leaf of monocot.
- Mainly found in dicot.
Good to know
- The collenchyma cells have dynamic cell walls.
- Cell lacks living protoplasm.
- Cell walls are thick because of lignin deposition. So, called lignified cell.
- Chloroplast absent.
- Also called mechanical tissue.
- Provides mechanical support i.e. strength to the cell.
- Pit is a characteristic of dead cell.
- It is caused by unequal accumulation of lignin.
Classification of Sclerenchyma Tissue
- Fiber cell
- Sclereid cell
- Elongated cells.
- With pointed tip***.
- Cell wall is always thick. Thickness is caused by lignin.
- Sometimes, the cell walls are so thickened that their cavities are reduced in size.
- In young stage, protoplasm is present. On maturation, protoplasm disappears. Cells become empty and dead. In rare cases, protoplasm may remain even upon maturity.
- Small size rounded pits are present.
- Cell shape is hexagonal in T.S.
- Cavities take upon pink colour when safranin is applied.
Occurrence of fibre
- Cortex, pericycle of stem (not in epidermis).
- In vascular tissue (in xylem and phloem).
- Can also be present as a single cell, continuous band.
- 1 to 3 mm in angiospermic plants.
- Exceptionally lengthy fibres: 20-550 mm.
- (Have economic importance)
Linum usitatissimum– Flax- Tisi- Linaceae.
Cannabis sativa– Hemp- Cannabaceae.
Corchorus capsularis– Jute- Paat- Malvaceae.
Boehmeria nivea– Ramie- Urticaceae.
On the basis of location of fibre
- Xylary fibre or wood fibre
- Extraxylary fibre or bast fiber
- Cortical fibre
- Phloem fibre
- Perivascular fibre
1. Xylary fibre or wood fibre
- Found in xylem tissue.
- Form the integrated part in xylem.
- Single celled, group or scattered arrangement.
- Develop from the same meristematic tissue like other xylem elements.
2. Extraxylary fiber or bast fiber
- Outside the xylem.
- Pits are present.
- End wall may be blunt.
- Thickness may be caused by hemicellulose.
a. Cortical fibre
- Present in cortex.
b. Phloem fibre
- Originate from primary or secondary phloem.
c. Perivascular fibre
- Found in the peripheral region of the vascular cylinder.
- Position: Inside the cortex tissue but not in the phloem tissue.
(inside the cortex, outside the phloem)
- More or less can be elongated.
- Isodiametric too but not so much.
- Found in: Xerophytic and hydrophytic plants.
- Causes thickness and hardness.
- Brachy sclereid or stone cell.
- Macro sclereid.
- Ostea sclereid.
- Astro sclereid.
1. Brachysclereid or stone cell
- Looks like pedicel present in epidermis layer of seed and fruit. Also found in leaf and stem and the cortex of xerophyte.
- Shape stone like.
- Short cell.
- Found in pith and cortex of stem.
- Found in fruit pulp.
- Rod shaped.
- Found just beneath the epidermis.
- Widely distributed in leaf cell.
- In Sundarban.
- Also occur in seed coat.
- Commonly found in fruit, cortex and pith.
- May be present as a single cell or group.
- Found in cortex of leaf and stem of xerophytic plant.
- Bone shaped.
- Enlarged at both ends.
- Found in hypodermal layer after epidermis.
- Found in leaf, fruits and seeds.
- Star shaped with projection.
- Found in intercellular space of hydrophytic plant.
e.g. leaf cortex and stem cortex.