Plant Growth Mechanisms and Meristems

Plant Growth: Key Elements and Processes

• Growth Mechanisms: Plants grow through a combination of cell growth (increasing cell size) and cell division (mitosis, increasing cell number). Cell differentiation (specialisation) also occurs.

Meristems: The Key to Continued Growth

• Meristems: Undifferentiated plant cells that continually divide and differentiate are crucial for continued growth and repair. They are found in various locations, each with specific effects on plant development.
• Apical meristems: Located at root and shoot tips, initiate primary growth (length/height). These differentiate into protoderm (epidermis), ground meristem (ground tissue), and procambium (xylem and phloem).
• Secondary meristems(cambium): Enable secondary growth (diameter increase) in woody plants.
• Vascular cambium: Produces secondary xylem and phloem, creating wood and increasing thickness.
• Cork cambium: Replaces epidermis with bark (cork).

Primary Growth

• Primary growth: Apical meristems at stem and root tips are responsible for rapid cell division and elongation, enabling plants to reach towards light and absorb water.
• Apical dominance: The apical bud (top shoot) suppresses the growth of lateral buds, influencing plant architecture. Removal of the apical bud can promote lateral branching, as seen in pruning.

Secondary Growth

• Secondary growth: Increases the thickness or girth of woody plants.
• Lateral meristems (vascular and cork cambium): These generate secondary xylem (wood, inward) and phloem (outward), causing the trunk/stem to thicken.
• Cork cambium: Creates the bark and protects the plant against damage and water loss, replacing the epidermis in older parts of the plant.

Annual Rings

• Annual Rings: Variations in cell size and wall thickness in secondary xylem from spring (early wood) and autumn (late wood) create visible rings. Analysis of annual rings provides information on tree age and past environmental conditions.

Plant Hormones and Growth Responses

• Plant Hormones: Chemicals that regulate diverse aspects of plant growth, development, and responses to environment. They differ from animal hormones in their site of production and location of action. In plants, hormones can interact synergistically or antagonistically.
• Auxins: Primarily responsible for cell elongation and control of apical dominance, leaf development, flowering, ripening, and rooting of cuttings.
• Cytokinins: Promote cell division (cytokinesis), delay senescence (aging), and influence differentiation in various plant parts.
• Gibberellins (GAs): Stimulate stem elongation, seed germination, and fruit/flower development.
• Abscisic Acid (ABA): Inhibits growth and induces dormancy, primarily as a response to stresses such as drought, cold, or decreased light.
• Ethylene: A gaseous hormone that triggers fruit ripening, leaf and flower abscission, and other processes.
• Nontraditional hormones (jasmonates, oligosaccharins, strigolactones, brassinosteroids): Exhibit various roles in defense, seed germination, mycorrhizae, and other developmental processes.

Growth Patterns

• Indeterminate growth: Some parts of the plant (stems and roots) continually grow throughout their life.
• Determinate growth: Other parts (leaves, flowers) stop growing once reaching a specific size.