BISC WEEK 10

Root System Structures

• Taproot: central, dominant root

• Lateral roots: branch off the taproot

• Root hairs: (site of absorption)

• Fibrous roots: dense, branching root system with no dominant taproot

Root System Functions

• Anchors the plant in soil

• Absorbs water and nutrients from soil

• Conducts water and selected ions up to the shoot

• Obtains sugar (energy) from the shoot

• Stores material produced in the shoot for later use

Root System: How structure supports function

Tube-shaped = high surface area/volume (SA/V) → faster absorption (Fick's Law: more surface area = faster diffusion) 

phenotypic plasticity (grow toward resource-rich soil, shallower in waterlogged/anoxic soil) and morphological diversity (taproot vs. fibrous) that reduce competition between species

Shoot System Structures

• Stems: vertical aboveground structures

• Node: point where a leaf attaches

• Internode: segment of stem between nodes

• Axillary (lateral) bud: found at nodes; may grow into a branch

• Apical bud: at the tip of stem/branch; site of growth that extends stem length

• Branch: lateral extension of the shoot from an axillary bud

Shoot System Functions

• Obtains light and CO₂ from atmosphere to produce sugars (photosynthesis) 

• Apical/axillary buds develop into flowers

Root System: How structure supports function

• Branching pattern + phototropism maximize light capture, minimize shading 

Leaf Structure

• Blade: expanded, flattened portion

• Petiole: stalk attaching blade to stem

• Simple leaf: one blade + petiole

• Compound leaf: blade divided into leaflets

• Doubly compound leaf: leaflets themselves divided again

Leaf Function

Main site of photosynthesis

Leaf: How does structure support function

• Flattened shape = highest SA:V of all three systems 

• maximizes light/CO₂ absorption (Fick's Law)

• trade-off = more water loss (transpiration)

Dermal Tissue System

outer-most layer of cells and represents the interface between the organism and the external environment.

Function:

• protection & gas exchange in shoots.

• protection & water/nutrient absorption in roots.

Epidermal cells

Secrete waxy cuticle; protect from water loss, pathogens, herbivores

Located in outer layer of shoots and leaves

Guard Cells

Surround stomata; open/close pores to regulate gas exchange (CO₂ in, O₂ out) and water loss

located mostly in leaves (also stems)

Trichomes

Hair-like; reflect sunlight, reduce water loss, deter herbivores, trap/digest insects

located in shoot system (leaves/stems)

Root hairs

Increase surface area for absorption

located in roots (zone of maturation)

Ground Tissue System

Parenchyma → thin wall only; photosynthesis (leaves) + storage (roots); totipotent

Collenchyma → uneven thick wall, flexible; support for growing stems 

Sclerenchyma → thick rigid wall + lignin; dead at maturity; support/protection

• Fibres = long (rope, paper)

• Sclereids = short (seed coats, gritty pear texture)

Totipotent

can regenerate whole plant — basis of cuttings/cloning

Vascular Tissue System

Xylem:

• One-way transport of water & dissolved nutrients, root → shoot; also structural support.

Phloem:

• Two-way transport of sugars, amino acids, hormones (roots ↔ shoots)

Primary Growth

Produced by apical meristem (tip of root/shoot)

Divides: increases length of root and shoot system

3 types of meristems derived from apical meristem

• Protoderm → epidermis (dermal)

• Ground meristem → parenchyma/collenchyma/sclerenchyma (ground)

• Procambium → xylem/phloem (vascular)

Meristem

undifferentiated, dividing cells

Cortex

ground tissue between vascular tissue & epidermis

Cambium

a special type of meristem (also called a lateral meristem) that differs from an apical meristem in two ways.

Secondary Growth

Widens Shoots and Roots

cambium → width/girth (only in woody plants)

2 types of cambium

• Vascular cambium → secondary xylem (to the inside) + secondary phloem (to outside)

• Cork cambium → cork cells (out only)

Wood

Accumulated secondary xylem

bark

Everything outside vascular cambium (secondary phloem + cork cambium + cork)

Heartwood vs sapwood

Heartwood = old, no longer conducts water, darker; Sapwood = young, still conducts, lighter

Growth rings

Early wood (big, thin-walled, spring) vs late wood (small, thick-walled, dry/cold season

Rays

Parenchyma rows for lateral transport

Lenticles

Gaps in bark for gas exchange