Introduction
Plant growth is a fundamental aspect of plant biology, encompassing the processes by which plants increase in size and develop their structures. This section of the International Baccalaureate (IB) Biology syllabus delves into the mechanisms and factors involved in plant growth, focusing on the role of meristems, plant hormones, and specific growth techniques such as micropropagation.
9.3.1 Plant Growth
Meristems
Meristems are regions of undifferentiated cells in plants that are capable of continuous cell division and growth. These cells are known as meristem cells, and they play a crucial role in the growth and development of plants.
- Types of Meristems:
- Apical Meristems: Located at the tips of roots and shoots, responsible for primary growth, which increases the length of the plant.
- Lateral Meristems: Found parallel to the sides of stems and roots, responsible for secondary growth, which increases the thickness of the plant. This includes the vascular cambium and cork cambium.
Apical Dominance
Apical dominance is the phenomenon where the main, central stem of the plant is dominant over other side stems. This is regulated by the plant hormone auxin, which is produced in the shoot apical meristem and inhibits the growth of axillary buds (buds that form at the junctions of the stem and leaves).
NoteApical dominance ensures that the plant grows vertically towards light, maximizing photosynthesis.
Differentiation of Meristem Cells
Meristem cells can differentiate into various specialized cells that form different tissues in the plant:
- Ground Meristem: Gives rise to ground tissues like parenchyma, collenchyma, and sclerenchyma.
- Protoderm: Develops into the epidermis, the outer protective layer of the plant.
- Procambium: Forms the vascular tissues, xylem, and phloem, which are responsible for transport within the plant.
Tropisms
Tropisms are directional growth responses that occur in response to environmental stimuli such as light, gravity, and water.
- Phototropism: Growth towards light, primarily regulated by the hormone auxin.
- Gravitropism (Geotropism): Growth in response to gravity; roots exhibit positive gravitropism (grow downwards), while shoots exhibit negative gravitropism (grow upwards).
- Hydrotropism: Growth towards moisture.
A classic example of phototropism is the bending of a plant stem towards a light source. This is due to the redistribution of auxin to the shaded side of the stem, causing cells on that side to elongate more than those on the light-exposed side.
9.3.2 Plant Hormones
Auxins
Auxins are a group of plant hormones that play a pivotal role in the regulation of plant growth and development.
- Indole-3-Acetic Acid (IAA): The most common naturally occurring auxin.
- Functions:
- Promotes cell elongation in shoots.
- Inhibits growth in roots at high concentrations.
- Regulates apical dominance by inhibiting axillary bud growth.
In practical applications, gardeners often prune the tips of plants to reduce auxin levels and encourage bushier growth by promoting axillary bud development.
Gibberellins
Gibberellins are another group of plant hormones that are involved in various growth processes, including stem elongation, seed germination, and flowering.
- Functions:
