Chapter 7: Leaves
External Structure of Foliage Leaves
leaf blade 
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
blade of just one part
compound leaf
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
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
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
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
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
venus fly trap
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