Meristem Tissues and Growth

Meristematic Tissues and Growth

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Meristematic Tissues and Growth

Zone of cell divison

The zone of cell division takes place at the apical meristems
The increase of the number of cells causes the elongation of the plant stem
When the new cells are forming, the cells formed previously are pushed to the zone of cell elongation.

Zone of cell elongation

The zone of cell elongation consists of cells that are increasing in size.
Happens through water diffusion by osmosis and the absorption of nutirents into the cells and stored in the vacoule.
Small vacoules fuse to form a large vacoule through vacuolation.
The diffused water exerts pressure against the cell wall which pushes, elongates and widens the cells.

Zone of cell differentiation

The zone of cell differentiation consists of differentiating cells that differentiate once they have reached their maximum size.
Cells differentiate to form permanent tissues such as epidermis,cortex,xylem and phloem.
The cells change their shapes and structures to become specialised cells with specific functions.

Primary growth:

Primary growth is the growth that occurs after germination and it takes place in all plants to elongate their stems and roots.
Takes place in the apical meristems at the shoot tips and root tips.
At the shoot tips, leaf primordia and shoot primodia will grow to form new leaves and shoots.
Enables plants to increase in height.
At the root rips, the root cap will become exhausted when they penetrate the soil.
Root cap to be replaced by the meristem cells.

Secondary growth:

Secondary growth occurs mainly in eudicots and a small number of monocots (shrub) to increase the circumference or diameter of plant stem and root
Lateral meristems consists of vascular cambium (located between phloem and xylem tissues in vascular bundles) and cork cambium (located under the epidermal layer).

Secondary growth starts when the vascular cambium divides actively through mitosis.
The cells in the cambium ring divide inwards to form new xylem and outwards to form new phloem. The new xylem tissues become secondary xylem and the new phloem tissues become secondary phloem.
When undergoing secondary growth, primary xylem is pushed towards the pith whereas primary phloem is pushed towards the epidermis.
As a result, the primary xylem tissues are compressed to form a stronger wood layer.
When the secondary xylem layer is compressed,there is an addition to the circumference of the stem which causes the epidermis of the stem to strecth and crack.
Cork cambium actively divides to form cork cells on the outer side and cortex on the inner side.

Vascular cambium cells divide actively and combine to form a complete ring.
The cells in the cambium ring divide inwards to form secondary xylem and outward to form secondary phloem.
Due to vascular cambium activity, the root becomes thicker.
The cork cambium located under the epidermis divides actively to form cork cells. The cork cells provide protection to the root tissues.

The necessity of primary growth

Allow the maximum elongation of plants to absorb sunlight for photosynthesis.
Primary phloem can transport the products of photosynthesis from the leaves to other parts of the plants.
Primary xylem can transport water and mineral salts form the soil via the roots to the leaves.
Primary xylem provides support to herbaceous or young plants.

The necessity of secondary growth

Provides stability to plants by increasing the stem and root diameters to suit the height of plants.
Provides mechanical support to plants.
Produces more xylem and phloem tissues.
Produces xylem and phloem tissues continuosly to replace old and damaged xylem and phloem tissues.
Produces stronger and thicker bark to provide protection to the plants.
Able to live longer by increasing the chances of seed production and reproduction.

Similarities between primary growth and secondary growth in eudicots:

Differences between primary growth and secondary growth in eudicots

Aspect	Primary growth
Tissue Involved	Apical meristem
Parts of plant	Occurs on stems and roots in younger region of plants
Direction of growth	Growth occurs longitudinally
Growth effect	Increases the length of stems and roots of plants
Tissue formed	Epidermis,cortex,primary vascular tissues (primary xylem and primary phloem)
Woody tissue	Absence
Bark thickness	Thin
Annual growth ring	Absence

Aspect	Secondary growth
Tissue Involved	Lateral meristem (vascular cambium and cork cambium)
Parts of plant	Occurs when primary growth has ceased on matured stems and roots.
Direction of growth	Growth occurs radially
Growth effect	Increases the thickness or circumference of stems and roots of plant
Tissue formed	Bark,periderm (cork cambium and cork tissues),lenticels and secondary vascular tissues (secondary xylem and secondary phloem).
Woody tissue	Presence
Bark thickness	Thick
Annual growth ring	Presence

Plants that undergone secondary growth are important in terms of economy due to the woods:

Timber has great economic value for wood industry
Types of tree which have great economic value for wood industry are merbau,jati,cengal,akasia,rubber and meranti trees
The trees that are hard and tough can be used in construction and furniture sectors
Akasia and batai can be used to produce paper
Merawan and meranti can be used to produce oils
Hibiscus and bougainvillea plants are used as long-lived plants