Chapter 34 - Plant Form and Function

Autotroph vs Heterotroph: What is the difference?

• Autotrophs synthesize organic molecules from inorganic carbon sources (CO2) using light (photosynthesis) or chemical energy •

• Heterotrophs cannot fix carbon and must obtain organic molecules by consuming other organisms or organic matter

• Autotrophs function as primary producers, forming the base of food webs

• Key distinction: source of carbon and energy determines ecological role and energy flow

Surface Area to Vol Ratio Importance in Plants

• High surface area-to-volume ratio increases efficiency of resource exchange, including light capture and water/nutrient absorption

• Leaves are flattened structures that maximize surface area for photosynthesis and gas exchange

• Roots are highly branched and tubular, increasing contact with soil and enhancing absorption of water and dissolved minerals

• Increased surface area also increases water loss, requiring regulation through stomata and protective cuticle

• This principle explains why plant structures are thin, flat, or highly branched

Phenotypic Plasticity & Its Importance

• Ability of a single genotype to produce different phenotypes depending on environmental conditions, meaning plant form is flexible rather than fixed

• Observed in roots (depth changes based on oxygen/water availability) and leaves (sun vs shade differences in size and thickness)

• Studied using reciprocal transplant experiments, where genetically identical plants are grown in different environments

• Represented by a norm of reaction, showing how phenotype changes across environments

• Provides adaptive advantage by allowing plants to optimize resource acquisition under varying conditions

Root System Function & Characteristics

• Anchors the plant in soil, preventing displacement and providing structural stability

• Absorbs water and dissolved minerals from soil via root hairs and epidermal cells

• Can store carbohydrates in specialized storage roots for later energy use

• Often extends beyond the aboveground canopy and may represent the majority of plant biomass

• Exhibits phenotypic plasticity by growing toward resource-rich soil patches

Shoot System Structure and Function

• Composed of stems, leaves, and buds that grow above ground and capture light and CO2

• Stems contain nodes (sites of leaf attachment) and internodes (segments between nodes)

• Apical buds drive vertical growth, while axillary buds can develop into branches or flowers

• Primary function = photosynthesis, gas exchange, and reproduction

• Can adjust growth patterns based on environmental conditions (light availability, competition)

Modified Roots

• Prop roots (e.g., corn) provide additional support by anchoring the plant against wind

• Pneumatophores (e.g., mangroves) grow upward to facilitate oxygen diffusion in waterlogged, low-oxygen soils

• Storage roots (e.g., carrots, beets) accumulate carbohydrates for later use during reproduction

• Adventitious roots arise from stems or leaves instead of the primary root, aiding in support or climbing

• These modifications enhance survival in specialized environments

Leaf Structure: Key Component and Functions

• Leaves consist of a blade (broad, flattened surface) and petiole (stalk connecting to stem)

• Large surface area maximizes light capture for photosynthesis

• Axillary bud at base distinguishes a leaf from a leaflet in compound leaves

• Primary functions include photosynthesis, gas exchange, and transpiration

• Leaf anatomy is optimized for efficient energy capture and exchange

Leaf Adaptations & Response to Environment

• Sun leaves are thicker and have smaller surface area to reduce water loss in high-light environments

• Shade leaves are thinner and broader to maximize light capture under low-light conditions

• Needle-like leaves reduce transpiration in dry or cold environments

• These differences demonstrate phenotypic plasticity within the same plant species

• Leaf structure balances photosynthesis with water conservation

Main Types of Leaf Arrangements

• Alternate leaves are staggered along the stem, reducing self-shading and maximizing light exposure

• Opposite leaves occur in pairs at the same node, allowing symmetrical growth

• Whorled leaves form rings of multiple leaves at a node, increasing light capture from multiple angles

• Rosette arrangement keeps leaves close to the ground, reducing water loss and wind damage

• Arrangement influences efficiency of light interception and plant growth

Examples & Functions of Modified Leaves

• Bulbs (e.g., onion) are thickened leaf bases that store nutrients for later growth

• Succulent leaves store water, allowing survival in arid environments

• Tendrils are modified leaves that wrap around structures to support climbing plants

• Pitcher plant leaves are modified into traps that capture and digest insects for nutrients

• Leaf modifications support survival under diverse environmental conditions

Three Major Types of Plant Tissue Systems

• Dermal tissue system forms the outer covering and regulates interaction with the environment

• Ground tissue system carries out photosynthesis, storage, and support functions

• Vascular tissue system transports water, minerals, and organic compounds throughout the plant

• Tissues are grouped based on structure, location, and function

• Together they form an integrated system supporting plant survival and growth

Dermal Tissue Specializations

• Stomata are pores that allow CO2 to enter and O2 to exit; guard cells regulate opening and closing

• Opening of stomata allows gas exchange but also leads to water loss via transpiration

• Cuticle is a waxy, hydrophobic layer that reduces evaporation and protects against pathogens

• Trichomes are hair-like structures that reduce water loss, reflect light, and deter herbivores

• These adaptations balance gas exchange with protection and water conservation

Ground Tissue Cell Types

• Parenchyma cells are living, versatile cells involved in photosynthesis, storage, and regeneration

• Collenchyma cells have unevenly thickened walls and provide flexible support in growing regions

• Sclerenchyma cells have thick, lignified secondary walls and provide rigid support; usually dead at maturity

• Different cell structures allow tissues to perform mechanical and metabolic functions

• Ground tissue makes up the bulk of the plant body

Xylem: Structure & Function

• Xylem transports water and dissolved minerals from roots to shoots via transpiration pull (negative pressure)

• Composed of tracheids and vessel elements, which are dead at maturity and lack cytoplasm

• Thick, lignin-reinforced walls prevent collapse under tension during water transport

• Pits and perforation plates allow water to move between cells

• Also contributes to structural support of the plant

Phloem: Structure and Function

• Phloem transports sugars, amino acids, and hormones throughout the plant

• Transport occurs via pressure-flow mechanism driven by osmotic gradients (positive pressure)

• Sieve-tube elements are specialized conducting cells that are alive but lack many organelles

• Companion cells support sieve tubes by maintaining metabolism and regulating transport

• Movement is bidirectional depending on source (sugar production) and sink (sugar use/storage)

Plant Cell Structures: Key Components

• Cell wall made of cellulose provides rigidity and defines cell shape

• Plasmodesmata are cytoplasmic channels that connect adjacent cells for transport and communication

• Chloroplasts perform photosynthesis, converting light energy into chemical energy

• Vacuole stores water, ions, pigments, and toxins and maintains turgor pressure

Apical Meristems

• Regions of actively dividing, undifferentiated cells located at root and shoot tips

• Responsible for producing all cells of the primary plant body

• Allow continuous growth throughout the plant’s life

• Give rise to primary meristems that differentiate into tissue systems

• Key drivers of primary growth (increase in length)

Primary Growth

• Involves three stages: cell division (mitosis), cell elongation (increase in size), and differentiation (specialization)

• Occurs at apical meristems in roots and shoots

• Produces primary tissues that form the primary plant body

• Extends plant into new areas for resource acquisition

• Essential for increasing plant height and root depth

Root Growth Zones

• Division zone contains apical meristem where cells actively divide

• Elongation zone contains cells that increase in length, pushing root tip forward

• Maturation zone contains fully differentiated cells specialized for absorption and transport

• Root hairs form in maturation zone, increasing surface area for water/nutrient uptake

• Zones reflect stages of cell development in primary growth

Primary Meristems

• Protoderm gives rise to dermal tissue system (epidermis)

• Ground meristem produces ground tissue system (parenchyma, collenchyma, sclerenchyma)

• Procambium develops into vascular tissue system (xylem and phloem)

• All originate from apical meristems

• These meristems establish the basic structure of the plant body

Vascular Arrangement: Monocots vs Dicots

• Dicots have vascular bundles arranged in a ring, allowing for secondary growth

• Monocots have scattered vascular bundles, limiting secondary growth

• Arrangement affects plant structure, support, and growth potential

• In dicots, ring arrangement facilitates formation of vascular cambium

• Important for distinguishing plant groups

Secondary Growth

• Occurs via lateral meristems (cambia) → cylindrical regions of undifferentiated, actively dividing cells that extend along the length of roots and stems

• Vascular cambium (located between primary xylem and phloem) produces secondary vascular tissues through repeated cell division

• Secondary xylem (produced to the inside) forms wood → responsible for water and mineral transport and provides structural support due to lignified cell walls

• Secondary phloem (produced to the outside) transports sugars, amino acids, and signaling molecules throughout the plant

• Cork cambium (located near the outer surface of stems/roots) produces cork cells → dead, waxy, suberin-containing cells that form bark and reduce water loss while protecting against physical damage

• Overall increases plant diameter, enhances long-distance transport capacity, and provides mechanical strength necessary for large plant size-

Vascular Cambium

• Located between primary xylem and phloem in stems and roots

• Produces secondary xylem toward the inside and secondary phloem toward the outside

• Cell division is asymmetrical, leading to accumulation of wood over time

• Also produces supporting cells such as fibers and parenchyma

• Responsible for most of the increase in girth in woody plants

Cork Cambium

• Located near the outer surface of stems and roots

• Produces cork cells that are dead and coated with suberin, making them impermeable

• Forms protective bark that reduces water loss and protects against pathogens and damage

• Replaces epidermis in woody plants as they grow thicker

• Gas exchange occurs through lenticles in the bark

Wood Structure: Heartwood vs Sapwood

• Sapwood is younger outer xylem that actively transports water

• Heartwood is older inner xylem that no longer transports water

• Heartwood accumulates resins and other compounds for protection

• Secondary xylem provides structural support for the plant

• Wood forms through continuous activity of vascular cambium

Bark Composition

• Bark includes cork, cork cambium, and secondary phloem

• Provides protection against physical damage, pathogens, and water loss

• Cork cells are dead and impermeable, forming an effective barrier

• Secondary phloem transports nutrients within bark layers

• Lenticels allow gas exchange through bark

Annual Growth Rings

• Form due to seasonal changes in vascular cambium activity

• Early wood consists of large, thin-walled cells formed during favorable conditions

• Late wood consists of small, thick-walled cells formed during stressful conditions

• Alternating layers create visible growth rings

• Ring width reflects environmental conditions such as water availability

Totipotency

• Ability of a single plant cell to regenerate into a complete plant

• Requires specific hormones such as auxin and cytokinins to trigger development

• Common in parenchyma cells, which can divide and differentiate

• Allows cloning and asexual reproduction in plants

• Demonstrates high developmental flexibility compared to animals

Auxin: Role in Plant Growth

• Auxin is a plant hormone that regulates cell elongation in shoots

• Uneven distribution causes phototropism (growth toward light)

• Also regulates gravitropism (response to gravity)

• Promotes differential growth by stimulating elongation on one side of stem

• Critical for directional growth responses

Shoot Growth: Response to Environment

• Shoots grow toward light sources to maximize photosynthesis

• Plants increase branching in areas with higher light availability

• Growth direction is controlled by hormones like auxin

• Environmental signals influence growth patterns

• Demonstrates phenotypic plasticity in shoot system

Plants: Optimizing Growth

• Roots grow toward areas with high nutrient and water availability

• Shoots grow toward regions with optimal light exposure

• Growth is dynamic and responsive to environmental cues

• Hormones coordinate responses between root and shoot systems

• Maximizes efficiency of resource capture

Parenchyma Totipotency and Function

• Parenchyma cells retain ability to divide and differentiate into other cell types

• Can form callus tissue during wound repair

• Important for plant regeneration and cloning

• Serve roles in storage, photosynthesis, and metabolism

• Most versatile and abundant plant cell type