Lecture 4c: Plant Tissues

Introduction to Plant Tissues

  • This is the conclusion of lecture four in horticulture 300, focusing on plant tissues.

  • The lecture follows previous discussions on an introduction to plants, plant cells, and basic genetics.

Overview of Plant Tissues

  • After the micro-level exploration of plant cells, this lecture expands to understand how those cells mature and specialize into tissues.

  • Growth cells, known as meristematic cells, are undifferentiated, dividing rapidly to produce specialized cells for the plant.

    • Meristem is defined as actively dividing cells that contribute to growth.

  • As cells mature, they develop into specialized structures necessary for plant function:

    • Structural cells: Providing shape and rigidity to the plant.

    • Vascular tissues: Responsible for conducting water and nutrients from roots to leaves and distributing manufactured food (sugars) back to roots.

Nodes are growth points in plants where leaves, branches, or buds arise. They play a critical role in the plant's architecture and overall growth strategy. The spaces between nodes are called internodes, and they can vary in length depending on the species and environmental conditions.

Function of Nodes

  • Nodes are integral for plant regeneration, as they can give rise to new growth and branching.

  • In some plants, such as vines, nodes may form adventitious roots that help support the plant as it grows.

  • Each node has the potential to produce a lateral bud, leading to the development of side branches which can increase the plant's density and coverage.

Types of Nodes

  • Vegetative Nodes: These are responsible for producing leaves and branches, participating in the plant's photosynthetic capabilities.

  • Flowering Nodes: Specialized nodes that develop flowers, crucial for reproduction.

  • Adventitious Nodes: Occur at non-traditional locations on the plant, often in response to stress or injury, allowing the plant to adapt and grow in challenging conditions.

Hormonal Influence on Nodes

  • The growth and development of nodes are influenced by plant hormones, primarily auxins and cytokinins, which help regulate the balance between vegetative and reproductive growth.

Categories of Plant Tissues

  • Plant tissues categorize primarily into:

    • Meristematic tissues: Young, undifferentiated cells that eventually develop into specialized tissues.

    • Differentiated tissues: Mature cells organized into functional structures.

Types of Meristematic Tissues

  • Apical meristem: Present in all plants, typically located at the tips of roots and shoots.

  • Lateral meristem: Found in dicotyledons (dicots) for thickening growth, additionally termed vascular cambium in woody species.

  • Intercalary meristem: Found in grasses (monocots); aids in growth at nodes and internodes.

Function of Meristematic Tissues

  • Apical meristem drives upward growth in shoots and roots.

    • Shoot tips: Apical meristem at the top directs vertical growth.

    • Root tips: Serves a similar purpose but encounters resistance from soil.

    • Lateral meristem enables diameter growth in woody plants (e.g., trees).

    • Both lateral and apical meristem allow for the production of various plant tissues over time.

Basic Plant Tissues

Protoderm

  • Develops into the epidermis, the outer layer of herbaceous and green plant tissues.

Ground Meristem

  • Responsible for structural cells, producing three primary cell types:

    • Parenchyma:

    • Thin-walled, flexible living cells; involved in photosynthesis, storage, and healing.

    • Collenchyma:

    • Provides flexible support in herbaceous tissues; living at maturity.

    • Sclerenchyma:

    • Thick-walled cells, often dead at maturity; provides rigid support and structural integrity due to lignin presence.

Vascular Tissues

  • Essential for nutrient and water transport:

    • Xylem: Conducts water and dissolved minerals from roots to leaves.

    • Phloem: Transports photosynthates (sugars) from leaves to the rest of the plant.

Structure of Vascular Tissues

  • Xylem consists of:

    • Tracheids: Long and thin, present in all vascular plants.

    • Vessel elements: Found in angiosperms, larger than tracheids; specialized for efficient water transport.

  • Phloem includes:

    • Sieve tube elements: Conduct nutrients.

    • Companion cells: Provide metabolic support for sieve tubes, crucial for nutrient distribution.

Growth Patterns and Mechanisms

  • Plants exhibit growth through both vertical expansion and increasing girth:

    • Apical growth elongates upward, while lateral growth expands outward.

  • Nodes are growth points where buds arise; spaces between nodes are called internodes.

Adventitious and Latent Buds

  • Adventitious buds: Form from non-nodal areas, often in response to damage; may grow quickly and ineffectively.

  • Latent buds: Hidden under the bark; can activate under certain conditions, serving as backup growth points.

Hormonal Regulation

  • Auxin: A plant hormone influencing growth; affects the relationship and growth between apical and lateral buds.

Anatomy of Woody vs. Herbaceous Plants

  • Woody Plants: Have distinct growth rings and patterns due to lateral meristem activity.

  • Herbaceous Plants: Typically lack substantial secondary growth; mostly retain primary growth.

Vascular Arrangement

  • Monocots vs. Dicots: Xylem and phloem arrangement differs; monocots typically have scattered vascular bundles while dicots exhibit ring organization.

  • Compartmentalization of decay: An important mechanism for trees to prevent decay spread by sealing off damaged areas.

Properties of Water in Plants

  • Cohesion and adhesion facilitate water transport through xylem:

    • Cohesion: Water molecules stick together, essential for capillary action.

    • Adhesion: Water molecules stick to surfaces, aiding in upward movement in xylem vessels.

Transpiration Pull and Water Movement

  • Water is pushed from the roots and pulled upwards through leaves.

    • Transpiration pull generates significant negative pressure that contributes to ascension of water.

    • Matric forces work together with root pressure.