1.2 - Meristematic Tissues and Growth

Meristematic tissues are undifferentiated living tissues involved in plants growth. Meristematic tissues can be categorized into two types.

• Apical meristems which can be found at the tips of plant shoots and roots.
• Lateral meristems are made up of vascular cambium and cork cambium.

Growth and development refer to the changes that happen in an organism from zygote stage to an adult.

Figure 3 - Apical and lateral meristems of plants.

Zone of Cell Growth

There are 3 zones of cell growth at the tips of the shoots and roots of plants – zone of cell division, zone of cell elongation and zone of cell differentiation. The zones of cell growth are important for the growth and development of plants.

Figure 4 - Zones of cell growth at the shoot tip (left) and root tip (right).

Zone of cell growth, Structure and functions

Zone of cell division

• Take place at the apical meristems where meristem cells are actively dividing by mitosis
• Cell number increases and elongates the plant stem
• New cells formed causing the cells formed previously to be pushed to the zone of cell elongation

Zone of cell elongation

• Made up of cells that are increasing in size
• Increase in size happens through absorption of water by osmosis and nutrients into the cells and vacuoles
• Smaller vacuoles combine together to form a larger vacuole through a process called vacuolation
• Water moves into the cell to create pressure against the cell wall which then pushes, elongates and widens the cells

Figure 5 - Cell elongation.

Zone of cell differentiation

• Consist of differentiated cells
• Cells reach their maximum size during differentiation
• Differentiation forms permanent tissues such as epidermis, cortex, xylem and phloem
• Specialized cells carry out specific functions. For example, epidermal cells differentiate and form
  • Guard cells which control the opening of stoma for photosynthesis
  • Root hair cells which increase the surface area for water and minerals absorption

Types of Growth

Plants undergo two types growth which are primary growth and secondary growth.

Primary growth

• Undergone by all plants.
• Happens after germination process.
• Occurs to increase the length of stem and roots.
• Takes place at the apical meristems at the shoot tips and root tips.
• Starts with active cell division at the apical meristems, followed by cell elongation and differentiation.
• At the shoot tips, leaf primordium and bud primordium will grow to form new leaves and shoots.
• This helps the plant to grow taller.
• At the root tips, the root cap becomes exhausted when they penetrate the soil. Cells of the root cap will be replaced by meristem cells.

Figure 6 - Primary growth.

Secondary growth

• Occurs in mostly in eudicots and some monocots (shrubs).
• Helps the plant to increase the circumference or diameter of the stem and root.
• Non-woody plants such as herbaceous plants do not undergo secondary growth.
• Most of the monocots do not undergo secondary growth as well except Draceana sp., Aloe sp., and Agave sp.
• Results from the division of lateral meristem cells located at the stem and root.
• Lateral meristems consist of vascular cambium and cork cambium.
  • Vascular cambium is situated between phloem tissues and xylem tissues
  • Cork cambium can be found underneath the epidermal layer

Figure 7 - Cross section of eudicot stem which shows secondary growth.

Figure 8 - Cell division at the vascular cambium and cork cambium during secondary growth at the stems. Note that the diameter of the plant stem is increasing.

Figure 9 - Secondary growth at the stem. 1. Secondary growth begins with cell division via mitosis at the vascular cambium. 2. The cells in the cambium ring divide inwards to form a new xylem and outwards to form a new phloem. The new xylem tissues will become secondary xylem whereas the new phloem tissues will become secondary phloem. 3. During secondary growth, the primary xylem will be pushed towards the pith. The secondary phloem will be pushed towards the epidermis. As a result, all the xylem tissues are compressed to form a stronger wood because their walls are thickened with lignin. 4. When the secondary xylem layers are compressed, this increases the circumference of the stem and causes the epidermis to stretch and crack. Cork cambium will divide actively to form cork cells at the outer side and cortex on the inner side. The cork layer protects the stem from insects and pathogens when the epidermis is cracked.

Figure 10 - Secondary growth at the roots helps to increase the circumference of the roots. This process is similar to the secondary growth that happens at the stem of eudicots. 1. Cells of the vascular cambium divides by mitosis to form a complete ring. 2. Cell division occurs inwards to form secondary xylem and outwards to form secondary phloem. 3. Active vascular cambium activity causes the roots to grow thicker. Cork cambium underneath the epidermis divides actively to form cork cells. The cork cells give protection to the root tissues.

Figure 11 - The different formation of secondary xylem can be used to determine the age of a tree. In spring, enough water supply and sunlight promote bigger formation of secondary xylem with thinner wall. In summer, the secondary xylem formed becomes smaller with thicker walls because the condition is unfavourable.

Necessity of primary growth

• Enables the elongation of plants in order to obtain maximum sunlight for photosynthesis.
• Primary phloem transports product of photosynthesis from the leaves to other parts of the plant.
• Primary xylem transports water and minerals from the roots to the leaves.
• Primary xylem also provides mechanical support to herbaceous plants or young plants.

Necessity of secondary growth

• Stabilize the plants by increasing the diameter of the stem and roots to suit the height of plants.
• Provides mechanical support to the plants.
• Produce more xylem and phloem tissues.
• To replace older and damaged xylem and phloem tissues.
• Produces stronger and thicker bark to protect the plants from excessive water loss, entry of pathogens and physical injuries.
• Produces more seeds so that the species can live longer.

Comparison between Primary Growth and Secondary Growth in Eudicots

• Similarities

  • Both types of growth increase the size of plants permenantly.
  • Both occur in woody plants.
  • Both involve cell division via mitosis.

• Differences

  • Primary growth

    • Meristem tissue involved: Apical meristems
    • Parts of the plant that undergo growth: Stems dan roots of young plants
    • Direction of growth: Longitudinally
    • Growth effects: Stems and roots grow longer
    • Tissues and structures formed: Epidermis, cortex and primary vascular tissues (primary xylem and phloem)
    • Presence of woody tissues: No
    • Thickness of bark: Thin
    • Presence of annual growth rings: No

  • Secondary growth

    • Meristem tissue involved: Lateral meristems (vascular cambium and cork cambium)
    • Parts of the plant that undergo growth: Matured stems and roots where primary growth has stopped
    • Direction of growth: Radially
    • Growth effects: Circumference of stems and roots increase
    • Tissues and structures formed: Bark, periderm (cork cambium and cork tissues), lenticels and secondary vascular tissues (secondary xylem and phloem)
    • Presence of woody tissues: Yes
    • Thickness of bark: Thick
    • Presence of annual growth rings: Yes

The Economic Importance of Plants that Have Undergone Secondary Growth

• Produce timber such as Shorea sp. (meranti) and Balanocarpus sp. (cengal)
• Strong and hard woods can be used to make structures for houseboats, furniture, fences, doors and others.
• The presence of annual rings makes the furniture looks more attractive and can be made as decorative items.
• The woods and barks of plants like Hopea sp. (merawan) and meranti can produce resin and oil. These can be commercialized as varnish, adhesive substances, perfume and medicine.
• Flowering plants can be used as decorative plants.
• Fruits produced can be sold to generate income and enhance economic growth of the country.