Chapter 7: Leaves

External Structure of Foliage Leaves

leaf blade image

light-harvesting portion

also called lamina

lower side is it's dorsal surface (abaxial)

large veins protrude like backbones

upper side is the ventral surface (adaxial)

smooth

petiole (stalk)

holds blade out into light

prevents shading by above leaves

long

thin

flexible

allow blade to flutter in wind

cools leaf

brings cool air to surface

petiolate

plant with petiole

sessile leaf

plant without petiole

monocots

grasses

irises

lilies

agaves

yuccas

sheathing leaf base

leaf base wraps around the stem

simple image

blade of just one part

compound leaf image

blade divided into several individual parts (many small blades)

leaflets

attached by petiolule

rachis

extension of petiolule

veins (bundles of vascular tissue)

distribute water from stem to leaf

simultaneously collect sugars produced by photosynthesis

reticulate venation

basal angiospersm

eudicots

netted pattern

parallel venation

monocots

long strap-shaped leaves

larger veins run side by side with few obvious interconnections

abscission zone image

oriented perpendicular to petiole at base of leaf

cells are involved in cutting off the leaf when its useful life is over

release enzyme that weakens cell walls

leaf twists in wind and falls off

provides protection for plant when leafs fall off

undamaged cells swell and become corky

forms protective scar tissue

leaf scar image

Internal Structure of Foliage Leaves

epidermis

transpiration

water loss

serious problem if soil is so dry that roots cannot replace lost water

consists of

flat, tubular, ordinary epidermal cells

guard cells

trichomes

glandular

non glandular

mesophyll

ground tissues interior to the leaf epidermis

palisade parenchyma image

main photosynthetic tissue of most plants

along the upper surface of most leaves

large surface gives maximum area for dissolution

spongy mesophyll

lower portion of leaf

open, loose aerenchyma

permits CO2 to diffuse rapidly away from stomata into all parts of leaf interior

vascular tissues

between palisade parenchyma and spongy mesophyll

edicot leaf usually has

one large midrib (midvein)

lateral veins emerge and branch into

minor veins

most important for releasing water from xylem and loading sugar into phloem

involved mainly in conduction

primary phloem on lower side

bundle sheath

helps support leaf blade

makes it difficult for insects to chew through vascular tissues

do not contain fibers

primary xylem on upper side

bundle sheath extension image

larger veins have mass of fibers above or below

fibers

give rigidity to blade

provide additional means by which water moves from the bundle out to the mesophyll

petiole

massive in plants like

palms

rhubarb

celery

water lilies

transition between stem and lamina

petiole mesophyll

like cortex

somewhat compact

not especially aerenchymatous

leaf traces

one, three, five, or more vascular bundles

branch from stem vascular bundles and diverge toward petiole

stipules

two small flaps of tissue at the base

protect shoot apical meristem

contribute significant amount of photosynthesis

Initiation and Development of Leaves

basal angiosperms and edicots

leaves produced only through activity of shoot apical meristem

cells just interior to the protoderm grow outward at base of meristem

form protrusion called leaf primodium

becomes taller than shoot apical meristem

consists of

leaf protoderm

leaf ground meristem

all cells meristematic

dense cytoplasm

small vacuoles

monocots

initiated by expansion of some shoot apical meristem cells

form leaf primordium

hoodlike shape

cylinder

encircles shoot apical meristem

gives rise to lamina

Morphology and Anatomy of Other Leaf Types

succulent leaves

adapted to desert habitats

thick

fleshy

their shape reduces surface-to-volume ration

favors water conservation

reduces capacity for CO2 uptake

mesophyll contains very few air spaces

reduces water loss through reducing internal evaporative surface area.

photosynthesis occurs more deeply due to transparent mesophyll

sclerophyllous foliage leaves

barberry

holly

Agave

Yucca

hardness makes them more resistant to

animals

fungi

freezing temperatures

ultraviolet light

sclerenchymatous leaves

sclerophyllous

sclerophylls (leaves)

sclerenchyma just below epidermis in bundle sheaths

leaves of conifers

leaves are sclerophylls

thick cuticle

epidermis & hypodermis have thick walls

always simple

never compound

needles

occur in

pines

firs

spruces

small, flat, scale-like leaves

junipers

cypresses

arborvitae

mostly perennial

evergreens

bud scales

smalll

rarely compound

petiole is short or absent

frequently produce thin layer of corky bark

spines

cacti

two types of leaves

green cactus body

microscopic green leaves

clusters of spines are axillary buds

have no blade

no mesophyll parenchyma or vascular tissue is present

tendrils

peas

cucumbers

squash

grow indefinitely

have sense of touch

leaves with Kranz anatomy

occurs in C4 plants

lack palisade mesophyll

lack spongy mesophyll

have prominent bundle sheaths

composed of large chlorophyllous cells

ring of mesophyll cells surround each sheath to radiate from vascular bundle

Kranz anatomy

adapts C4 plants to arid environments

insect traps

plants gain nitrogen they need by eating insects

active traps

passive traps

pitcher leaves image

venus fly trap image

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