Chapter 34 - Plant Form and Function

Surface Area to Vol Ratio Importance in Plants

High SA:V ratio increases efficiency of resource exchange

• Leaves: flattened structures to maximize photosynthesis and gas exchange

• Roots: highly branched/tubular, better contact with soil → better water/nutrient absorption

• Challenge: increased water loss which requires regulation through stomata and protective cuticle

Phenotypic Plasticity & Its Importance

• Ability of a single genotype to produce different phenotypes depending on environmental conditions

  • Plant form = flexible, not fixed

  • Optimize resource acquisition under varying conditions

• Observed in roots (depth changes based on resource availability)

• Observed in leaves (sun vs shade differences in size and thickness)

Phenotypic Plasticity: Research

• Investigated how much a plant’s growth depends on its environment

• Propagated genetically identical plants from plant cuttings; planted these new offspring in three different environments along the elevation gradient → reciprocal transplant experiment

• Results represented a norm of reaction

  • Each norm of reaction line was different for the high, low, and medium plants

  • Where the lines crossed each other indicated local adaptations

• Conclusion: environmental conditions have a profound influence on body size and shape: genetically identical plants looked different at each location

Root System Function & Characteristics

• Anchors the plant (structural stability)

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

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

• Often represents 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 capture light and CO2

  • Photosynthesis, gas exchange, and reproduction

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

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

• Can adjust growth patterns based on environmental conditions

Modified Roots

• Prop roots (corn): provide additional support by anchoring the plant against wind

• Pneumatophores (mangroves): grow upward to facilitate oxygen diffusion in waterlogged/low-oxygen soils

• Storage roots (carrots, beets): accumulate CHO for later use during reproduction

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

Modified Stems

• Water-storage (cactus): stems store water, spines are modified leaves

• Stolons (strawberry): produce new individuals at nodes above ground

• Rhizomes: produce new individuals at nodes belowground, store CHO

• Tubers (potatoes): store CHO like starch

• Thorns: protection from herbivores

Leaf Structure: Key Component and Functions

• Blade (broad, flattened surface) and petiole (stalk connecting to stem)

• Large SA to maximizes light capture

  • Photosynthesis, gas exchange, transpiration

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

Leaf Adaptations & Response to Environment

Balances photosynthesis with water conservation; differences in leaves demonstrate phenotypic plasticity within the same species

• Sun leaves: thicker w/ smaller SA to reduce water loss in high-light environments

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

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

Main Types of Leaf Arrangements

• Alternate leaves: 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

Examples & Functions of Modified Leaves

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

• Succulent leaves: store water (allowing survival in arid environments)

• Tendrils: wrap around structures to support climbing plants (i.e. ivy)

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

Three Major Types of Plant Tissue Systems

• Dermal: forms the outer covering, regulates interaction with the environment

• Ground: photosynthesis, storage, and support functions

• Vascular: transports water, minerals, organic compounds throughout the plant

• Grouped based on structure, location, and function; form an integrated system supporting plant survival and growth

Dermal Tissue Specializations/Adaptations

Help balance gas exchange with protection and water conservation

• Guard cells regulate opening/closing of stomata (pores that allow for gas exchange but can lead to water loss via transpiration)

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

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

Ground Tissue Cell Types

• Parenchyma cells: living; involved in photosynthesis, storage, and regeneration

• Collenchyma cells: unevenly thickened walls; provide flexible support in growing regions

• Sclerenchyma cells: thick, lignified secondary walls to provide rigid support (usually dead at maturity)

• Ground tissue makes up the bulk of the plant body

Xylem: Structure & Function

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

• Composed of tracheids and vessel elements

  • Dead at maturity and lack cytoplasm

• Thick, lignin-reinforced walls prevent collapse under tension

• Pits and perforation that allow water movement between cells

• Also contributes to structural support of the plant

Phloem: Structure and Function

• Transports sugars, amino acids, and hormones throughout the plant via a pressure-flow (positive pressure)

• Sieve-tube elements and companion cells

  • Living specialized conducting cells

  • Support sieve tubes by maintaining metabolism and regulating transport

• Bidirectional movement depending on source (sugar production) and sink (sugar use/storage)

Plant Cell Structures: Key Components

• Cell wall = made of cellulose; provides rigidity, defines cell shape

• Plasmodesmata = cytoplasmic channels that connect adjacent cells for transport/communication

• Chloroplasts = perform photosynthesis, converting light energy into chemical energy

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

Apical Meristems

Key drivers of primary growth (increase in length)

• Regions of actively dividing, undifferentiated cells 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

Primary Growth

Essential for increasing plant length (i.e. height and root depth)

• Involves 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

Root Growth Zones

Stages of cell development in primary growth

• Division zone: apical meristem where cells actively divide

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

• Maturation zone: has fully differentiated cells specialized for absorption and transport

  • Root hairs form here → increase SA for water/nutrient uptake

Primary Meristems

Originate from apical meristems, est. basic structure of the plant body, give rise to the major tissue systems

• Protoderm → dermal tissue system (epidermis)

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

• Procambium → vascular tissue system (xylem and phloem)

Vascular Arrangement: Monocots vs Dicots

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

  • Facilitates formation of vascular cambium

• Monocots: scattered vascular bundles (no secondary growth)

Secondary Growth

Increases plant diameter; enhances long-distance transport capacity and provides mechanical strength for large plant size

• Occurs via lateral meristems (cambia)

  • Cylindrical regions of undifferentiated/actively dividing cells that extend along length of roots and stems

• Vascular cambium produces secondary vascular tissues through repeated cell division

  • Secondary xylem forms wood (water transport)

  • Secondary phloem transports sugars, amino acids, and signaling molecules

• Cork cambium produces cork cells that form bark, reduce water loss, protect against physical damage

Vascular Cambium

Responsible for most of the increase in girth in woody plants

• Located b/w primary xylem and phloem in stems and roots

• Produces secondary xylem → inside and secondary phloem → outside

• Asymmetrical cell division leads to accumulation of wood over time

• Also produces supporting cells such as fibers and parenchyma

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/protects against pathogens + damage

• Gas exchange occurs through lenticles in the bark

Wood Structure: Heartwood vs Sapwood

• Sapwood = younger outer xylem that actively transports water

• Heartwood = older inner xylem that no longer transports water

  • 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

• Includes cork, cork cambium, and secondary phloem

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

• Cork cells are dead/impermeable (forms 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: large, thin-walled cells formed during favorable conditions

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

• Alternating layers create visible growth rings; ring width reflects environmental conditions (i.e. water availability)

Totipotency

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

• Requires specific hormones to trigger development

• Common in parenchyma cells, which can divide and differentiate

• Allows cloning and asexual reproduction in plants

• Not a characteristic present in animals

Auxin: Role in Plant Growth

• Hormone that regulates cell elongation in shoots

• Uneven distribution causes phototropism (growth toward light)

• Also regulates gravitropism (response to gravity)

  • Differential growth and directional growth responses

Shoot Growth: Response to Environment

• Shoots grow toward light sources to maximize photosynthesis

  • Ex: increase branching in areas w/ more light

  • Demonstrates phenotypic plasticity

  • Controlled by hormones (auxin)

Parenchyma Totipotency and Function

Most versatile/abundant plant cell type

• Can divide + 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