PBL 3: Leaves and Roots Flashcards
Main functions of leaves
-Photosynthesis
-Transpiration
-Control water loss
-Storage
-Prevent organisms (animals, insects, bacteria, fungi) from digesting the plant
General leaf structure
Leaf blade: light-harvesting part
-thin and flat, upper and lower side of leaf
Petiole: prevents self-shading, holds leaf out into the light
Monocots VS Eudicots/Dicots
MONOCOTS:
-vascular bundles (veins on leaf) arranged in parallel
-lack bark and secondary growth (no cambium)
-have a leaf sheath
-lack abscission zones
DICOTS/EUDICOTS:
-vascular bundles have branched veins
-has bark, secondary growth & abscission zones
-lacks leaf sheath
Simple VS Compound leaves
SIMPLE:
-only one leaf blade
Advantages: higher % of photosynthetic tissue
COMPOUND:
-leaf blade divided into several individual parts
-small leaf blades attached to a petiolule which is attached to the rachis.
Advantages: reduced tearing by wind, restricted access for insects (blade edge=barrier), greater access to CO2.
Ways of attachment in compound leaf
1.) Palmately: leaflets attached at the same point
2.) Pinnately: leaflets attached at different points along rachis
Internal structure and function of epidermis
Overall function: prevents high water loss, protective barrier, absorption of CO2
Structure:
-Stomata with two guard cells present on lower side of leaf: diffusion of materials and transportation process
–Sunken stomata (crypts) surrounded by trichomes prevent water loss and allow reabsorption of water
-Trichomes: small protrusions on leaf surface which provide shade, traps water, slows air flow
Internal structure and function of mesophyll layer
Interior ground tissue with two cell layers:
- palisade parenchyma” main photosynthetic tissue, one layer thick
- spongy mesophyll: allows for rapid CO2 diffusion away from stomata, located on the lower side of leaf, loose aerenchyma cells.
*C4 plants have no distinction between cell layers- has a bundle sheath around vascular tissue to concentrate CO2 around xylem+phloem.
Internal structure and function of vascular tissues
Located between palisade parenchyma and spongy mesophyll
- Midrib: large vascular bundle, contains primary xylem on upper side and primary phloem on lower side.
-Diverges into lateral veins which goes into minor veins - Minor veins: site of material exchange- loads sugar into phloem, release water from xylem
- Bundle sheath: fibres arranged around vascular tissue
- Conducts water and can have bundle sheath extension (mass of fibres around large veins, gives rigidity and protection)
Petiole
two flaps of tissue at its base called stipules (which die in mature leaves)
-Stipules protect apical meristem of young and small leaves
Specialized leaves: Succulent leaves
-Thick and fleshy
-Reduced CO2 intake which also leads to reduced water loss
-Mesophyll contains less airspaces: light can penetrate further into leaf, less SA for water to evaporate
-Survives in desert habitats
Specialized leaves: tendrils
-Modified leaf that has the ability to coil around objects and use it as support
-Continuous cell division: side facing object stays dormant whereas the other side elongates
Specialized leaves: sclerophyllous foliage
-Increased number of sclerenchyma cells below the epidermis and the bundle sheaths compared to parenchyma: more resistant to external attack and freezing temperatures (perennial-long lifespans)
Specialized leaves: flytrap/insect trap leaves
-Survive in habitats with poor nitrate and ammonia concentration
-Two ways of trapping that rely on receptors like trichomes: active/passive traps
Specialized leaves: bougainvillea
-Modification to appear colourful and attract pollinators
Specialized leaves: onion
Functions as storage
Specialized leaves: Conifers
-Contain sclerenchyma, thick cuticle + epidermis
-Lack abscission zones
-Evergreen/perennial
Specialized leaves: bud scales
-Protect dormant shoot apical meristems: cold temperature, wind, do NOT photosynthesise
Specialized leaves: spines
-Leaves of cacti: modified axilliary buds that do NOT photosynthesise (only stem cortex does)
Specialized leaves: Kranz anatomy
-Leaves of C4 plants
-No palisade parenchyma/spongy mesophyll
-Have a prominent bundle sheath composed of chlorophyll cells
Yellowing leaves explanation
Before leaves are cut off, nutrients are extracted: chlorophyll is still active and causes oxidative damage
-Chlorophyll is being broken down, exposing the underlying fat-soluble yellow pigments
-Red leaves: pigment comes from antioxidants
-General colourful leaves can have more prominent water-soluble pigments that mask chlorophyll
Main functions of roots
-Anchorage
-Absorption
-Production of hormones (cytokinin, gibberellin)
-Fleshy taproots: carbohydrate storage during winter.
External Root Structure: Basic Organization
Single tap root that branches into smaller lateral roots
-Tap root develops from embryonic root (radicle)
External Root Structure: Fibrous root system
-Present in monocots
-Roots don’t grow from radicle (it dies and new adventitious roots form from the root primordia)
-No secondary growth
External Root Structure: Adventitious Roots
-Don’t grow from pre-existing roots (e.g. radicle)
-Can form in eudicots (only under propagation/cutting)
-Growth comes from stem
Individual Root Structure: Root Cap
-Protects root apical meristem
-Constantly undergoes cell division
-Secretes mucigel
-Provides food for microbes (bacteria) which release nutrients and free ions within soil matrix
Mucigel
Polysaccharide substance that lubricates passage of root into the soil.
Individual Root Structure: Region/Zone of elongation
-Region of cell division and expansion
-Differentiation begins
Individual Root Structure: Root hair zones/Region of Maturation
-Root hairs increase root SA
-CO2 from respiration combines with soil to create carbonate, which increases ion availability in the soil.
-Absorption of water and minerals form a water pressure that creates upwards force (no elongation takes place)
-Microenvironment is called the rhizosphere (altered pH around the root hair)
Internal Root Structure Layout
1.) Epidermis: Outermost layer
2.) Cortex: layer between epidermis and endodermis
3.) Endodermis
4.) Pericycle
5.) Vascular tissue w/ Cambium between xylem+phloem
Internal Root Structure: Endodermis
Thin primary walls that are in contact with vascular tissue
-Neighboring endodermis cells are encrusted with lignin+suberin, making it waterproof
-Casparian strips: band of altered walls involved in controlling minerals that enter xylem and act as a filter.
–Apoplastic flow (through intercellular space) needs the Casparian strip to filter ions.
Internal Root Structure: Pericycle
Dormant parenchyma cells arranged in an irregular region.
-Hormone auxin can revert the parenchyma back into meristematic tissue for lateral growth.
Internal Root Structure: Vascular bundle
One vascular bundle consists of a central xylem, surrounded by phloem.
-Cambium present between xylem+phloem so the plant root can undergo secondary growth.
Specialized Roots: Storage roots
long-term storage roots for carbohydrates
-carrot, beetroot
Specialized Roots: Aerial roots
Free in the air, root epidermis consists of layers of dead cells
-Orchids
Specialized Roots: Prop roots
roots that grow in anaerobic environments (acts as a stabiliser, secondary growth- woody)
-Buttress roots, mangroves
Specialized Roots: Contractile roots
attached to bulb-digs into soil and pulls bulb down to soil.
Specialized Roots: Mycorrhizae
Symbiotic relationship between roots and soil fungi
-Fungus feeds on sugar produced by plant and plant absorbs phosphorus from fungus
-Plant communication through chemical signals by the fungi
