Plant Structure and Growth

Basic Plant Organs

Plants are composed of three primary organs, each with specialized structures and functions essential for survival and growth.

• Roots: Anchor the plant, absorb water and minerals, and store carbohydrates. Root hairs are extensions of epidermal cells that increase surface area for absorption.

• Stems: Support leaves and reproductive structures. The terminal bud at the tip directs upward growth and inhibits axillary buds, while axillary buds can form branches. Pinching/pruning removes the terminal bud to encourage lateral growth.

• Leaves: Main site of photosynthesis. Key structures include the petiole (stalk connecting leaf to stem), margin (leaf edge), and blade (lamina) (broad, flat area).

Plant Tissue Types

Plant tissues are organized into three main types, each with distinct roles:

• Dermal Tissue: Single layer of closely packed cells covering the plant, protecting against water loss and pathogens.

• Vascular Tissue: Continuous system for transporting water, minerals, and nutrients throughout the plant (includes xylem and phloem).

• Ground Tissue: Functions in storage, photosynthesis, and support; includes all tissues not part of dermal or vascular systems.

Plant Cell Types

• Parenchyma Cells: Thin-walled, living cells with large vacuoles; involved in photosynthesis, storage, and tissue repair. Found in leaves (mesophyll), stems, roots, and fruits.

• Xylem Cells: Dead, tubular cells that conduct water and minerals upward from roots. Located in vascular bundles, forming the central cylinder in stems and roots.

• Phloem Cells: Living cells that transport sugars and organic compounds from leaves to other plant parts. Includes:

  • Sieve-tube elements: Elongated, tubular cells without nuclei at maturity; responsible for translocation of sugars and amino acids.

  • Companion cells: Small, nucleated cells connected to sieve-tube elements via plasmodesmata; provide metabolic support and regulate loading/unloading of sugars.

Meristems and Plant Growth

Meristems are regions of perpetually embryonic tissue where cells divide for plant growth.

• Apical Meristems: Located at root and shoot tips; responsible for primary growth (lengthening).

• Lateral Meristems: Located in roots, stems, and branches; responsible for secondary growth (thickening).

Primary vs. Secondary Growth

• Primary Growth: Increases length of stems and roots, allowing plants to grow taller and roots to extend deeper.

• Secondary Growth: Increases thickness/girth, resulting in woody structures. Both rely on meristematic activity; primary growth establishes the framework, secondary growth builds upon it.

Plant Life Cycles

• Annuals: Complete their life cycle in one year.

• Biennials: Require two years to complete their life cycle.

• Perennials: Live for many years, reproducing multiple times.

Primary Growth of a Root

Root growth is organized into distinct zones, each with specialized functions:

• Zone of Cell Division: Includes the apical meristem; cells divide rapidly.

• Zone of Elongation: Cells elongate, pushing the root tip forward.

• Zone of Differentiation (Maturation): Cells differentiate into specialized types (dermal, ground, vascular).

• Root Cap: Protects the growing tip.

Tree Trunk Structure

From the center outward, the layers are: Pith, Medullary rays (wood rays), Heartwood, Sapwood, Cambium, Phloem (inner bark), Bark (outer bark).

Resource Acquisition, Nutrition, and Transport in Vascular Plants

Turgor Pressure in Plant Cells

Turgor pressure (hydrostatic pressure) is the force exerted by water inside the cell against the cell wall, maintaining cell rigidity and plant structure.

Flaccid vs. Turgid Cells

• Flaccid Cell: Limp, resulting in wilting due to water loss.

• Turgid Cell: Firm, healthy state due to water uptake.

Macronutrients vs. Micronutrients

• Macronutrients: Required in large amounts (C, H, N, O, P, S, K, Ca, Mg).

• Micronutrients: Needed in trace amounts (Fe, Mn, Zn, Cu, etc.).

Mutualistic Relationships in Plants

• Rhizobacteria: Fix atmospheric nitrogen at roots, making it available to plants.

• Mycorrhizae: Fungi that form mutualistic associations with roots, enhancing water and nutrient uptake.

Nitrogen Nutrition in Plants

Nitrogen is cycled through various forms and organisms in the soil, ultimately becoming available to plants for growth.

Unusual Plant Adaptations

• Epiphytes: Grow on other plants for support but do not parasitize them.

• Parasitic Plants: Obtain nutrients from host plants via specialized structures (haustoria).

• Carnivorous Plants: Trap and digest animals to supplement nutrient intake in poor soils.

Casparian Strip and Root Structure

The Casparian strip is a band of cell wall material in the endodermis that blocks passive flow of substances into the vascular cylinder, ensuring selective uptake of minerals and water.

Long-Distance Transport of Water

• Transpiration: Water loss from leaves creates a pull that drives upward movement of water through xylem.

• Cohesion: Water molecules stick together, maintaining a continuous column in xylem.

• Adhesion: Water molecules adhere to xylem walls, aiding capillary action.

Guard Cells and Water Loss

Guard cells regulate the opening and closing of stomata by changing turgor pressure, thus controlling water loss and gas exchange.

Translocation of Phloem Sap

Translocation is the movement of sugars from sources (leaves) to sinks (roots, fruits) via phloem, driven by pressure flow generated by active loading and unloading of sugars.

Reproduction of Flowering Plants

Life Cycle of an Angiosperm

The angiosperm life cycle alternates between haploid (n) and diploid (2n) generations, involving pollination, fertilization, seed formation, and germination.

Key Reproductive Processes

• Pollination: Transfer of pollen from anther to stigma.

• Fertilization: Fusion of sperm and egg to form a zygote.

• Germination: Seed absorbs water, triggering growth and development.

Double Fertilization

Unique to angiosperms, double fertilization involves one sperm fertilizing the egg (zygote) and another fusing with two polar nuclei to form endosperm (nutritive tissue).

Origin of Plant Structures

• Embryo: Develops from fertilized egg (zygote).

• Seed: Composed of seed coat, endosperm, and embryo.

• Fruit: Develops from ovary wall (pericarp, exocarp, mesocarp).

Evolutionary Advantages of Seeds

• Protection for embryo

• Stored food supply

• Dormancy and survival through adverse conditions

• Water independence

• Enhanced dispersal and colonization

Breaking Seed Dormancy

• Temperature changes

• Freezing or intense heat

• Light exposure

• Moisture and humidity

• Seasonal cues

Seed Structure

Seed Germination

During germination, the seed absorbs water, metabolic changes begin, the root emerges, followed by the shoot, and leaves expand to begin photosynthesis.

Purpose of Fruit

Fruits protect seeds and aid in their dispersal by wind, water, or animals.

Methods of Fruit and Seed Dispersal

• Wind Dispersal: Lightweight seeds/fruits adapted for air currents (wings, parachutes).

• Water Dispersal: Buoyant fruits/seeds adapted to float (fibrous husks, air pockets).

• Animal Dispersal: Seeds consumed and excreted or attached to animal fur/feathers.

Asexual vs. Sexual Reproduction

• Sexual Reproduction: Produces genetic diversity; advantageous in changing environments but more complex and risky for seedlings.

• Asexual Reproduction: Produces clones; efficient in stable environments, does not require pollinators.

Plant Responses to Internal and External Signals

Plant Tropisms

• Phototropism: Growth toward light (e.g., sunflower bending toward sun).

• Gravitropism: Growth in response to gravity (e.g., roots growing downward).

• Thigmotropism: Growth in response to touch (e.g., tendrils coiling around support).

Plant Hormones

Plant Responses to Light

Plants detect and regulate responses to light using photoreceptors, which mediate processes such as phototropism and photoperiodism.

Photoperiodism

Photoperiodism is the physiological response to the relative lengths of night and day, often regulating flowering time.

Plant Responses to Environmental Stresses

• Drought: Close stomata, release abscisic acid, inhibit growth, roll leaves, grow deeper roots.

• Flooding: Release ethylene, root cell death, formation of air tubes for oxygen supply.

• Salt: Impede salt uptake, produce solutes to retain water.

• Heat: Evaporative cooling via transpiration, produce heat shock proteins.

• Cold: Alter membrane lipid composition for fluidity.

Plant Defenses Against Herbivores and Pathogens

• Herbivores: Physical defenses (thorns), chemical defenses (toxins), recruitment of predatory animals.

• Pathogens: Epidermis as first line of defense; pathogen recognition and host-specific responses as second line.