8. Plant Tissues and Anatomy - part 04

Tissue Systems :

Plant tissues are derived from meristems and their structure and functions depend on the position.
Types of tissue systems [ On the basis of their structure and location] :

A. Epidermal tissue system :

It forms the outer covering of plant body and is derived from protoderm or dermatogen.
The two types of structures are seen in epidermal tissue system viz

Epidermis is the outermost protective cell layer made up of compactly arranged cells without intercellular spaces.
Cells show presence of central large vacuole, thin cyctoplasm and a nucleus.
The outer side of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water.
It may bear hairs. Root epidermis has root hairs.
These are unicellular elongated and involved in absorption of sap from the soil.
In stem, epidermal hairs are called trichomes.
These are generally multicellular, branched or unbranched, stiff or soft or even secretory.
These help in preventing water loss due to transpiration.
Small gateways in the epidermal cells are called as stoma. Such stoma are controlled or guarded by specially modified cells called guard cells.
These guard cells may be kidney shaped (dicot) or dumbbell shaped (monocot), collectively called as Stomata.
Stoma, guard cells and subsidiary cells form a unit called stomatal apparatus.
Stomata are further covered by subsidiary cells.
Guard cells have chloroplasts to carry out photosynthesis.
Guard cells change their turgor pressure causing its opening and closing, thus they play a vital role in exchange of gases and water vapour.

B. Ground tissue system :

All the plant tissues excluding epidermal and vascular tissue is ground tissue.
It is made up of simple permanent tissue e.g. paranchyma.
It is present in cortex, pericycle, pith and medullary rays in the primary stem and root.
Collenchyma and schlerenchyma in the hypodermis and chloroplasts containing mesophyll tissue in leaves is also ground tissue.

C. Vascular tissue system :

These are the distinct patches of the complex tissue viz. Xylem and phloem.
On the basis of their arrangement in the plant body these are radial when both the complex tissue are situated separately on separate radius as separate bundle. This is a common feature of roots.
In the stem, the complex tissue is collectively present as neighbours of each other on the same radius in the form of xylem inside and phloem outside hence called conointcollateral vascular bundles.
These bundles may be further of open type (secondary growth takes place) containing cambium in between them and closed type if cambium is not present (secondary growth absent).
When phloem is present in a vascular bundle on both the sides of xylem and intervening cambium tissue, it is called bicollateral vascular bundle.
It is a feature of family Cucurbitaceae.
When one vascular tissue is completely encircling the other, it is called as concentric vascular bundle, this may be leptocentric (phloem encircled by xylem) or hadrocentric (xylem encircled by phloem).
When one complex tissue is encircling on both the faces of the other it is amphicribral (xylem encircled by phloem on both faces) and amphivasal (phloem encircled by xylem on both faces).

Secondary growth in plants :

The vertical growth of the roots and stems in length with the help of apical meristem is called as primary growth.
Dicotyledonous plants and gymnosperms exhibit increase in girth of root and stem.
In dicot stem, secondary growth begins with the formation of a continuous cambium ring.
The cambium present between the primary xylem and primary phloem of a vascular bundle is called intraasicular cambium.
The cells of medullary rays adjoining these intrafascicular cambium strips become meristematic (regain the capacity to divide) and form the interascicular cambium.
Thus a complete and continous ring of vascular cambium is formed.
The cambium ring cuts off new cells, towards both the sides, inner and outer.
The cells that are cut-off towards pith (inner side) mature into secondary xylem and cells that are cut-off towards periphery mature into secondary phloem.
Generally, amount of secondary xylem is more than the secondary phloem.
In woody plants, secondary tissues constitute the bulk of the body. This provides support, conduction of water and minerals and protection.
Lateral meristems play a major role in development of secondary tissues.

Formation of cambial ring :

With the onset of favourable season, meristematic cells of intrafascicular cambium become active.
Simultaneously, the ray parenchyma cells, both fusiform initials and ray initials become meristematic. This is known as dedierentiation.
These form patch of cambial cells (meristematic cells) in between the adjacent bundles and produce interfascicular cambium.
Now both intrafascicular and interfascicular cambium join and form a complete ring. This is known as cambial ring.
This is possible because they lie in one plane.

Secondary growth in roots :

It is also observed in most of the dicot and gymnospermic roots by producing secondary vascular tissue and periderm.
Secondary growth is produced by vascular cambium and cork cambium respectively.
Conjuctive parenchyma cells present on the inner edges of primary phloem bundles become meristematic.
These cells add secondary xylem and secondary phloem on the inner and outer side respectively.
These events are similar to secondary growth in stems.

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