PBL 3: Leaves and Roots

Question 1

Main functions of leaves

Answer 1

-Photosynthesis
-Transpiration
-Control water loss
-Storage
-Prevent organisms (animals, insects, bacteria, fungi) from digesting the plant

Question 2

General leaf structure

Answer 2

Leaf blade: light-harvesting part
-thin and flat, upper and lower side of leaf

Petiole: prevents self-shading, holds leaf out into the light

Question 3

Monocots VS Eudicots/Dicots

Answer 3

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

Question 4

Simple VS Compound leaves

Answer 4

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.

Question 5

Ways of attachment in compound leaf

Answer 5

1.) Palmately: leaflets attached at the same point

2.) Pinnately: leaflets attached at different points along rachis

Question 6

Internal structure and function of epidermis

Answer 6

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

Question 7

Internal structure and function of mesophyll layer

Answer 7

Interior ground tissue with two cell layers:

1. palisade parenchyma” main photosynthetic tissue, one layer thick
2. 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.

Question 8

Internal structure and function of vascular tissues

Answer 8

Located between palisade parenchyma and spongy mesophyll

1. 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
2. Minor veins: site of material exchange- loads sugar into phloem, release water from xylem
3. 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)

Question 9

Petiole

Answer 9

two flaps of tissue at its base called stipules (which die in mature leaves)
-Stipules protect apical meristem of young and small leaves

Question 10

Specialized leaves: Succulent leaves

Answer 10

-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

Question 11

Specialized leaves: tendrils

Answer 11

-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

Question 12

Specialized leaves: sclerophyllous foliage

Answer 12

-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)

Question 13

Specialized leaves: flytrap/insect trap leaves

Answer 13

-Survive in habitats with poor nitrate and ammonia concentration
-Two ways of trapping that rely on receptors like trichomes: active/passive traps

Question 14

Specialized leaves: bougainvillea

Answer 14

-Modification to appear colourful and attract pollinators

Question 15

Specialized leaves: onion

Answer 15

Functions as storage

Question 16

Specialized leaves: Conifers

Answer 16

-Contain sclerenchyma, thick cuticle + epidermis
-Lack abscission zones
-Evergreen/perennial

Question 17

Specialized leaves: bud scales

Answer 17

-Protect dormant shoot apical meristems: cold temperature, wind, do NOT photosynthesise

Question 18

Specialized leaves: spines

Answer 18

-Leaves of cacti: modified axilliary buds that do NOT photosynthesise (only stem cortex does)

Question 19

Specialized leaves: Kranz anatomy

Answer 19

-Leaves of C4 plants
-No palisade parenchyma/spongy mesophyll
-Have a prominent bundle sheath composed of chlorophyll cells

Question 20

Yellowing leaves explanation

Answer 20

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

Question 21

Main functions of roots

Answer 21

-Anchorage
-Absorption
-Production of hormones (cytokinin, gibberellin)
-Fleshy taproots: carbohydrate storage during winter.

Question 22

External Root Structure: Basic Organization

Answer 22

Single tap root that branches into smaller lateral roots
-Tap root develops from embryonic root (radicle)

Question 23

External Root Structure: Fibrous root system

Answer 23

-Present in monocots
-Roots don’t grow from radicle (it dies and new adventitious roots form from the root primordia)
-No secondary growth

Question 24

External Root Structure: Adventitious Roots

Answer 24

-Don’t grow from pre-existing roots (e.g. radicle)
-Can form in eudicots (only under propagation/cutting)
-Growth comes from stem

Question 25

Individual Root Structure: Root Cap

Answer 25

-Protects root apical meristem
-Constantly undergoes cell division
-Secretes mucigel
-Provides food for microbes (bacteria) which release nutrients and free ions within soil matrix

Question 26

Mucigel

Answer 26

Polysaccharide substance that lubricates passage of root into the soil.

Question 27

Individual Root Structure: Region/Zone of elongation

Answer 27

-Region of cell division and expansion
-Differentiation begins

Question 28

Individual Root Structure: Root hair zones/Region of Maturation

Answer 28

-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)

Question 29

Internal Root Structure Layout

Answer 29

1.) Epidermis: Outermost layer
2.) Cortex: layer between epidermis and endodermis

3.) Endodermis
4.) Pericycle
5.) Vascular tissue w/ Cambium between xylem+phloem

Question 30

Internal Root Structure: Endodermis

Answer 30

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.

Question 31

Internal Root Structure: Pericycle

Answer 31

Dormant parenchyma cells arranged in an irregular region.
-Hormone auxin can revert the parenchyma back into meristematic tissue for lateral growth.

Question 32

Internal Root Structure: Vascular bundle

Answer 32

One vascular bundle consists of a central xylem, surrounded by phloem.
-Cambium present between xylem+phloem so the plant root can undergo secondary growth.

Question 33

Specialized Roots: Storage roots

Answer 33

long-term storage roots for carbohydrates
-carrot, beetroot

Question 34

Specialized Roots: Aerial roots

Answer 34

Free in the air, root epidermis consists of layers of dead cells
-Orchids

Question 35

Specialized Roots: Prop roots

Answer 35

roots that grow in anaerobic environments (acts as a stabiliser, secondary growth- woody)

-Buttress roots, mangroves

Question 36

Specialized Roots: Contractile roots

Answer 36

attached to bulb-digs into soil and pulls bulb down to soil.

Question 37

Specialized Roots: Mycorrhizae

Answer 37

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