Plant Form and Function

Introduction

Plants exhibit a remarkable diversity in form and function, which is essential for their survival, reproduction, and adaptation to various environments. This study guide covers the major groups of plants, their structural organization, tissue systems, and growth processes, providing foundational knowledge for General Biology students.

Major Groups of Plants

Vascular vs. Nonvascular Plants

Plants are classified into two major groups based on the presence or absence of specialized conducting tissues.

• Nonvascular plants: Lack lignified xylem and phloem; rely on diffusion for water and nutrient transport; typically water-dependent. Examples include mosses and liverworts.

• Vascular plants: Possess specialized conducting tissues (xylem and phloem) that are lignified, allowing efficient transport of water, minerals, and nutrients. This group includes ferns, gymnosperms, and angiosperms.

Example: Gymnosperms (e.g., pine trees) and angiosperms (e.g., flowering plants) are both vascular plants.

Angiosperms: Monocots vs. Dicots (Eudicots)

Angiosperms, or flowering plants, are divided into two major groups based on seed structure and other diagnostic features.

• Monocots:

  • One cotyledon (seed leaf)

  • Parallel leaf venation

  • Scattered vascular bundles in the stem

  • Flower parts in threes or multiples of three

  • Examples: Wheat, corn, grasses, orchids, bamboo, palms, lilies

• Dicots (Eudicots):

  • Two cotyledons

  • Branched (netted) leaf venation

  • Vascular bundles are arranged in a ring

  • Flower parts in fours or fives (or multiples)

  • Examples: Most fruits, vegetables, ornamentals, shrubs, and trees

Plant Organs and Their Functions

Basic Plant Organs

The typical plant body consists of three basic organs: roots, stems, and leaves. Each organ is composed of several tissue types that work together to perform specific functions.

• Roots: Anchor the plant, absorb and transport water and minerals, store carbohydrates (e.g., carrots, turnips, sweet potatoes).

• Stems: Support leaves and reproductive structures, transport fluids between roots and leaves, sometimes store food or water.

• Leaves: Main photosynthetic organs, facilitate gas exchange.

Modified Plant Parts

Plants may have modified roots, stems, and leaves for specialized functions.

• Modified roots: Large taproots for food storage (e.g., carrots, beets).

• Modified stems:

  • Stolons: Horizontal stems for asexual reproduction (e.g., strawberry runners).

  • Rhizomes: Underground stems for storage and asexual reproduction (e.g., ginger).

  • Tubers: Swollen ends of rhizomes for storage (e.g., potato).

• Modified leaves:

  • Spines: Protection (e.g., cactus).

  • Tendrils: Climbing (e.g., pea plants).

  • Storage leaves: Store water or nutrients (e.g., onion bulbs).

Example: Potato is a stem tuber, while sweet potato is a storage root.

Plant Tissue Systems

Overview of Tissue Systems

Each plant organ contains three main tissue systems: dermal, vascular, and ground tissue.

• Dermal tissue: Protective outer covering; includes the epidermis and cuticle.

• Vascular tissue: Support and long-distance transport; composed of xylem and phloem.

• Ground tissue: Bulk of the plant body; involved in photosynthesis, storage, and support.

Dermal Tissue

The dermal tissue system forms the outer protective layer of the plant.

• Epidermis: Tightly packed cells; first line of defense.

• Cuticle: Waxy layer that reduces water loss.

• Stomata: Tiny pores in the epidermis (mainly on leaves) that regulate gas and water exchange; flanked by guard cells that control opening and closing.

Vascular Tissue

The vascular tissue system is responsible for transport and support.

• Xylem: Conducts water and dissolved minerals upward from roots; composed of tracheids and vessel elements (dead at maturity, lignified cell walls).

• Phloem: Transports sugars and other organic nutrients; composed of sieve-tube elements (alive at maturity, lack organelles) and companion cells (support sieve-tube elements).

Ground Tissue

Ground tissue makes up most of the plant body and is involved in photosynthesis, storage, and support.

• Parenchyma: Photosynthetic cells (mesophyll in leaves), storage cells.

• Collenchyma: Support cells with thickened cell walls.

• Sclerenchyma: Rigid support cells with lignified walls.

Plant Growth and Meristems

Primary vs. Secondary Growth

Plant growth occurs through specialized regions called meristems.

• Primary growth: Lengthens roots and shoots; occurs at apical meristems (tips of roots and shoots).

• Secondary growth: Increases diameter (thickness) of stems and roots; occurs at lateral meristems (vascular cambium and cork cambium).

Meristems

• Apical meristems: Found at the tips of roots and shoots; responsible for primary growth.

• Lateral meristems: Vascular cambium (produces secondary xylem and phloem) and cork cambium (produces cork for outer bark).

Zones of Growth in Roots

• Zone of cell division: Includes apical meristem and newly formed cells.

• Zone of elongation: Cells elongate, increasing root length.

• Zone of differentiation: Cells mature into specific tissue types (dermal, vascular, ground).

Cell Expansion

Plant cells expand by absorbing water into their vacuoles, generating turgor pressure against the cell wall. The orientation of cellulose microfibrils in the cell wall influences the direction of cell expansion.

• Turgor pressure: Internal pressure from water uptake.

• Cell-wall loosening: Allows expansion.

• Microfibril orientation: Determines direction of growth.

Specialized Structures and Functions

Buds

• Apical (terminal) bud: Located at the tip of a stem; responsible for vertical growth.

• Axillary bud: Found in the angle between leaf and stem; can give rise to branches or flowers.

Sieve-Tube Elements and Companion Cells

Sieve-tube elements are specialized phloem cells for transporting sugars; they lack most organelles and rely on companion cells for metabolic support.

• Sieve-tube elements: Long, tube-like cells with sieve plates at their ends; alive at maturity.

• Companion cells: Adjacent to sieve-tube elements; connected by plasmodesmata; provide metabolic support.

Cell Division in Vascular Cambium

• Periclinal division: Cell division parallel to the surface, increasing thickness (secondary growth).

• Anticlinal division: Cell division perpendicular to the surface, increasing circumference.

Summary Table: Plant Tissue Systems

Key Equations and Terms

• Turgor Pressure Equation: (Pressure equals force divided by area)

• Photosynthesis Equation:

• Transpiration: The process by which water vapor exits the plant through stomata.

Additional info:

• Some context and terminology have been expanded for clarity and completeness.

• Scientific names and terms (e.g., Arabidopsis) are italicized where relevant.