1.2 - Meristematic Tissues and Growth

Types and Positions of Meristematic Tissues

• Meristematic tissues are undifferentiated living tissues responsible for plant growth.
• Apical meristems are located at shoot tips and root tips.
• Lateral meristems consist of:
  • Vascular cambium.
  • Cork cambium.

Zones of Cell Growth

• Shoot tips and root tips have three zones of cell growth.
• Their activity produces primary growth.
• At a root tip, the order from the root cap is: zone of cell division → zone of cell elongation → zone of cell differentiation → matured tissue.

1. Zone of Cell Division

• Located at the apical meristem.
• Meristem cells divide actively through mitosis.
• The increase in cell number elongates the plant stem.
• Newly formed cells push earlier cells into the zone of cell elongation.

2. Zone of Cell Elongation

• Contains cells that are increasing in size.
• Water enters by osmosis; nutrients enter the cells and are stored in vacuoles.
• Small vacuoles fuse into a large vacuole; this is vacuolation.
• Water exerts pressure against the cell walls, pushing, elongating and widening the cells.

3. Zone of Cell Differentiation

• Contains cells that differentiate after reaching their maximum size.
• Cells change shape and structure into specialised cells with specific functions.
• Forms permanent tissues such as epidermis, cortex, xylem and phloem.
• Examples:
  • Leaf epidermal cells differentiate into guard cells that control stoma opening.
  • Root epidermal cells differentiate into root hair cells.

Types of Growth in Plants

Primary Growth

• Occurs after germination in all plants.
• Occurs at apical meristems at shoot tips and root tips.
• Active cell division is followed by cell elongation and differentiation.
• Elongates stems and roots and increases plant height.
• At shoot tips, leaf primordia and bud primordia form new leaves and shoots.
• At root tips, worn root-cap cells are replaced by meristem cells.

Secondary Growth

• Occurs mainly in eudicots and a small number of monocots (shrubs).
• Increases the circumference or diameter of stems and roots.
• Does not occur in non-woody plants such as herbaceous plants.
• Results from cell division in lateral meristems of stems and roots:
  • Vascular cambium lies between phloem and xylem tissues in vascular bundles.
  • Cork cambium lies under the epidermal layer.
• Most monocots do not undergo secondary growth; exceptions include Draceana sp., Aloe sp. and Agave sp.

Mechanism of Secondary Growth

Secondary Growth at the Stem

1. Vascular cambium divides actively through mitosis.
2. Cells in the cambium ring divide:
   • Inwards to form new xylem, which becomes secondary xylem.
   • Outwards to form new phloem, which becomes secondary phloem.
3. Primary xylem is pushed towards the pith; primary phloem is pushed towards the epidermis.
4. Compressed primary xylem forms a strong wood layer; lignin-thickened xylem walls provide mechanical support.
5. The thickening secondary xylem layer increases stem circumference, causing the epidermis to stretch and crack.
6. Cork cambium divides to form cork cells outside and cortex inside.
7. Cork protects the cracked stem from insect and pathogen attack.

Secondary Growth at the Root

1. Vascular cambium cells divide actively and join to form a complete ring.
2. Cambium-ring cells divide inwards to form secondary xylem and outwards to form secondary phloem.
3. Vascular cambium activity thickens the root.
4. Cork cambium beneath the epidermis forms cork cells that protect root tissues.

Annual Growth Rings

• Secondary growth in temperate climates occurs at different rates between seasons.
• Spring: sufficient water and sunlight produce larger, thin-walled secondary xylem; the ring is bright.
• Summer: unsuitable growth conditions produce smaller, thick-walled secondary xylem; the ring is dark.
• Annual growth rings can be used to determine the age of a plant.

Comparison between Primary Growth and Secondary Growth in Eudicots

Similarities

• Both permanently increase plant size.
• Both occur in woody plants.
• Both involve cell division by mitosis.

Differences

Aspect	Primary Growth	Secondary Growth
Meristem tissue involved	Apical meristem	Lateral meristem: vascular cambium and cork cambium
Part involved	Younger regions of stems and roots	Matured stems and roots after primary growth has ceased
Direction	Longitudinal	Radial
Effect	Increases stem and root length	Increases stem and root thickness or circumference
Structures formed	Epidermis, cortex, primary xylem and primary phloem	Bark, periderm, lenticels, secondary xylem and secondary phloem
Woody tissue	Absent	Present
Bark	Thin	Thick
Annual growth rings	Absent	Present on the stem

Necessity of Primary and Secondary Growth

Necessity of Primary Growth

• Maximises plant elongation to absorb sunlight for photosynthesis.
• Primary phloem transports products of photosynthesis from leaves to other plant parts.
• Primary xylem transports water and mineral salts from the soil through roots to leaves.
• Primary xylem supports herbaceous or young plants.

Necessity of Secondary Growth

• Increases stem and root diameters to provide stability suited to plant height.
• Provides mechanical support.
• Produces more xylem and phloem tissues.
• Continuously replaces old or damaged xylem and phloem tissues.
• Produces stronger, thicker bark that protects against excessive water loss, physical injuries and pathogen infections.
• Extends lifespan and increases opportunities for seed production and reproduction.

Economic Importance of Plants that Undergo Secondary Growth

• Strong, hard timber such as Shorea sp. (meranti) and Balanocarpus sp. (cengal) is used for houseboats, furniture, fences and doors.
• Annual rings make furniture attractive and suitable as decorative items.
• Wood and bark of plants such as Hopea sp. (merawan) and meranti produce resin and oil used for varnish, adhesives, perfume and medicine.
• Flowering plants are used as decorative plants.
• Commercial fruits such as mangoes and mangosteens generate income and boost economic growth.