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Chapter 6: Leaves (Morphology and Anatomy of Other Leaf Types (Leaves of…
Chapter 6: Leaves
Morphology and Anatomy of Other Leaf Types
Succulent Leaves
Desert habitats
Crassulaceae
Portulacaceae
Leaves are thick and fleshy
Reduces surface/volume ratio
Water conservation
Inside the leaf
Mesophyll contains few air spaces
Reducing internal evaporative surface area
Reduction of water loss thru stomata
Sclerophyllous Foliage Leaves
Necessary positive marginal profit from photosynthetic sugar investment
Leaves generally tend to be flexible, soft, and edible
Sclerophyllous leaves are usually perennial
Life cycle of longer than a year
Justifies cost of elevated [sclerenchyma] tissue
Resistant to UV light, freezing, fungi, etc.
Usually thick cuticle and epidermis
Leaves of Conifers
Sclerophylls
thick cell walls of epidermis and hypodermis
Thick cuticle
Always simple, never compound leaves
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Mostly perennial leaves
Bud Scales
In perennial plants
Protection of dormant shoot apical meristems
Little to no photosynthetic action
Small and mostly simple modified leaves
Production of a thin layer of corky bark for protection
Spines
Modified leaves of axillary buds
spines have no blade, and are needle shaped
Mutations that inhibit lamina formation
Absence of mesophyll parenchyma and vascular tissue
Replaced with closely packed fibers, which die after maturation and are resistant to natural decomposition processes
Stem cortex=site of cacti photosynthetsis, so loss of palisade mesophyll is not bad
Tendrils
Contain sensory organs
Grow until contact, that side stops growing and the other continues to grow
Highly modified leaves
Leaves with Kranz Anatomy
C4 photosynthesis
Leaves lack palisade parenchyma and spongy mesophyll cell layers
Prominent bundle sheaths
Composed of large chlorophyllous cells
Rang of mesophyll cells radiates from vascular bundle and surrounds each bundle sheath
Kranz anatomy
Adaptation to arid environments
Insect Traps
Insectivory has evolved in plants
Habitats poor in Nitrates
Habitats poor in Ammonias
Digestion of insects provides necessary AAs and nucleotides
Active traps (move during capture)
Venus flytraps
Passive traps (incapable of movement
Pitcher leaves of
Nepenthes
External Structure of Foliage Leaves
Functions
Photosynthesis
Prevent excess loss of water
Protect plant from fungi, bacteria, epifoliar foliage
Flat,Wide, and Usually thin
Lamina=Light Harvesting portion
Dorsal surface
Ventral surface
Petiole (stalk)
Allows Blade to flutter in the wind
Prevents overshadowing of leaves
No petiole=Sessile leaf
Sheathing leaf Base
Leaf Blade
Simple (blade of just one part
Greater percentage of simple leaf plants require less support and more photosynthetic action
Compound (blade divided into multiple parts)
Contains many leaflets
Leaflets attached by petiolule
Petiolule attached to petiole by Rachis
Pinnately vs. palmately compound
Compound leaves are better than simple
Veins
Distribute water from the stem to the leaf
Reticulate venation
Parallel venation
Abscision zone
Usually in the petiole
Oriented perpendicular to the petiole
Cutting off leaf when its useful life is over
Leaf scar
Necessary to prevent infection
Similar to the function of a harmonic scalpel in surgery vs, normal scalpel
Internal Structure of Foliage Leaves
Epidermis
Water loss via Transpiration
Waterproof but Translant
Allow entrance of CO2
Unilateral distribution of stomata
Contain stomata
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Trichromes
Mesophyll
Ground tissues interior to epidermis
Palisade parenchyma
Main photosynthetic tissue of most plants
Targeted separation of palisade cells for light exposure
Generally only 1 layers thick, intense sunlight may merit 3-4 layers
Spongy mesophyll
lower portion of leaf
Open, loose aerenchyma
Permits rapid CO2 diffusion away from stomata into leaf interior
Vascular Tissues
Between the palisade and spongy mesophyll
Midrib (midvein)
Lateral veins---> Minor veins
Minor veins release water from xylem and load sugar into phloem
Lateral veins and midrib mostly involved in conudction
Primary xylem on upper side
Primary phloem on lower side
Bundle sheath
Fibers arranged around vascular tissues
Support and protection
Bundle sheath extension
Petiole
transition between the stem and the lamina
Epidermis contains fewer stomata and trichomes, but otherwise similar to the lamina
Petiole mesophyll is cortex-like
Vascular tissues are the most variable
Leaf traces
May remain distinct or may fuse into a single trace
Stipules
Two small flaps of tissue at petiole base
Protection of apical meristem
Photosynthetic contribution occasionally
Initiation and Development of Leaves
Basal Angiosperms and Eudicots
Leaves are produced through the activity of a shoot apical meristem
Leaf primordium
Extends upwards as a narrrow cone taller than the shoot apical meristem
primordium consists of leaf protoderm and leaf ground meristem
All cells are meristematic
Provascular tissue
Primary xylem and phloem
Laminar expansion
Stomata, trichomes, and vascular bundles differentiation
Petiole becomes distinct from the midrib
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Entire pattern is completed while the leaf is very small
Monocots
Initiated by the expansion of shoot apical meristem cells
Formation of a cylinder tubular shoot
One side=lamina
Abaxial epidermis
Adaxial epidermis
Perpetual cycle of tubular growth, with one side differentiating into functional tissue types.
Basal expansion pattern
Protoxylem and protophloem stretched and disrupted in basal meristem
Necessary rapid differentiation of vessel elements and sieve tube members
