1. Leaves
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2. Introduction All leaves originate as primordia in buds.
At maturity, most leaves have:
Stalk = petiole
Leaves sessile if lacking petiole
Flattened blade = lamina
Network of veins = vascular bundles
Stipules at base of petiole
Leaves of flowering plants associated with leaf gaps and have axillary buds at base.

3. Introduction Leaves may be simple or compound.
Simple leaves - With a single blade
Compound leaves - Blade divided into leaflets
Pinnately compound leaves - Leaflets in pairs along rachis (petiole)
Bipinnately compound leaf - Leaflets subdivided
Palmately compound leaves - All leaflets attached at same point at end of petiole.
Bipinnately compound leaf
Palmately compound leaf

4. Introduction
Green leaves capture light energy by means of photosynthesis.
Photosynthesis - Trapping and storing of energy in sugar molecules that are constructed from water and carbon dioxide
Stomata - Tiny pores on lower surfaces of leaves
Allow carbon dioxide to enter and oxygen to diffuse out
Water vapor also escapes via stomata.
Guard Cells control water loss by opening or closing pore of stomatal apparatus.

5. Leaf Arrangements and Types
Leaves are attached to stems at nodes, with stem regions between known as internodes.
Phyllotaxy - Arrangement of leaves on stem
Alternate - One leaf per node
Opposite - Two leaves per node
Whorled - Three of more leaves at a node
Alternate
Whorled
Opposite

6. Leaf Arrangements and Types
Venation - Arrangement of veins in a leaf or leaflet blade
Pinnately veined leaves - Main midvein included within enlarged midrib.
Secondary veins branch from midvein.
Palmately veined leaves - Several primary veins fan out from base of blade.
Pinnate venation
Palmate venation

7. Leaf Arrangements and Types
Monocots - Primary veins parallel = Parallel venation
Dicots - Primary veins divergent in various ways = netted or reticulate venation.
Dichotomous venation - Veins fork evenly and progressively from base of blade.
Reticulate venation
Dichotomous venation
Parallel venation

8. Internal Structure of Leaves
Three regions: Epidermis, mesophyll, veins (vascular bundles)
Epidermis - Single layer of cells covering the entire surface of the leaf
Devoid of chloroplasts
Coated with cuticle (with cutin)
Functions to protect tissues inside leaves
Waste materials may accumulate in epidermal cells.
Different types of glands may also be present in the epidermis.

9. Stomata
Lower epidermis typically has thinner layer of cutin and is perforated by numerous stomata.
Stomata bordered by two guard cells.
Guard cells originate from the same parent cell, and contain chloroplasts.
Primary functions:
Regulate gas exchange between leaf interior and atmosphere
Regulate evaporation of water
Changes in amount of water in guard cells cause them to inflate or deflate.
Inflate - Stomata open
Deflate - Stomata close

11. Mesophyll and Veins Palisade Mesophyll Spongy Mesophyll
Most photosynthesis takes place in the mesophyll between the two epidermal layers.
Palisade Mesophyll
Compactly stacked, barrel- shaped parenchyma cells, commonly in two rows
Contains most of leaf’s chloroplasts
Spongy Mesophyll
Loosely arranged parenchyma cells with abundant air spaces

12. Dicot leaf cross section
Mesophyll and Veins
Veins (vascular bundles) are scattered throughout mesophyll.
Consist of xylem and phloem tissues surrounded by bundle sheath of thicker-walled parenchyma
Dicot leaf cross section

13. Monocot leaf cross section
Mesophyll and Veins
Monocots have some differences:
Usually do not have mesophyll differentiated into palisade and spongy layers
Often have bulliform cells on either side of main central vein
Bulliform cells partly collapse under dry conditions.
Causes leaf to fold or roll, reducing transpiration
Monocot leaf cross section

14. Specialized Leaves Shade Leaves
Receive less total light than sun leaves
Compared to sun leaves, shade leaves:
Tend to be larger
Tend to be thinner
Have fewer well-defined mesophyll layers and fewer chloroplasts
Have fewer hairs
Sun leaf
Shade leaf

15. Specialized Leaves Leaves of Arid Regions Leaves of Aquatic Areas
Arid regions have limited availability of water, wide temperature ranges, and high light intensities.
Leaves reduce loss of water by:
Thick, leathery leaves
Fewer stomata or sunken stomata
Succulent, water-retaining leaves, or no leaves
Dense, hairy coverings
Leaves of Aquatic Areas
Less xylem and phloem
Mesophyll not differentiated into palisade and spongy layers.
Large air spaces

16. Specialized Leaves Tendrils Spines
Modified leaves that curl around more rigid objects, helping the plant to climb or to support weak stems
Garden peas
Tendrils
Spines
Modified leaves that reduce leaf surface and water loss, and protect from herbivory.
Spine
Cacti
Leaf tissue replaced with sclerenchyma.
Photosynthesis occurs in stems.

17. Specialized Leaves Prickles - Outgrowths from epidermis or cortex
Thorns - Modified stems arising in the axils of leaves of woody plants
Thorn
Prickle
Prickles - Outgrowths from epidermis or cortex

18. Specialized Leaves Storage leaves
Succulent leaves are modified for water storage.
Have parenchyma cells with large vacuoles
Many desert plants
Fleshy leaves store carbohydrates.
Onions, lily

19. Flower-pot leaf sliced lengthwise
Specialized Leaves
Flower-Pot Leaves
Leaves develop into urn-like pouches that become home of ant colonies.
Ants carry in soil and add nitrogenous wastes that provide good growing medium for the plant’s own roots.
Dischidia, an epiphyte of Australia
Flower-pot leaf sliced lengthwise

20. Specialized Leaves Window leaves In succulent desert plants of Africa
Leaves buried in ground, except for exposed end.
End has transparent, thick epidermis and transparent water storage cells underneath.
Allows light into leaf, while buried leaves keep plant from drying out

21. Specialized Leaves Reproductive Leaves
Walking fern - New plants at leaf tips
Air plant - Tiny plantlets along leaf margins
Air Plant

22. Specialized Leaves Floral Leaves (bracts)
At bases of flowers or flower stalks
Poinsettia - Flowers do not have petals, instead brightly colored bracts surround flowers.
Clary’s sage - Colorful bracts are at top of flowering stalks above flowers.
Clary’s sage
Poinsettia

23. Specialized Leaves Insect-Trapping Leaves
Grow in swampy areas and bogs
Nitrogen and other elements are deficient in soil.
Specialized leaves trap and digest insects.
Pitcher Plants
Insects trapped and digested inside cone-shaped leaves.
Pitcher plant

24. Specialized Leaves Insect-Trapping Leaves Sundews Venus’s Flytraps
Have round to oval leaves covered with glandular hairs that have a sticky fluid of digestive enzymes at tip
Venus’s Flytraps
Only in North Carolina and South Carolina
Blade halves trap insects.
Sundew
Venus’s Flytraps

25. Bladder of bladderwort
Specialized Leaves
Insect-Trapping Leaves
Bladderworts
Submerged or floating in shallow water
Tiny bladders on leaves have trap doors that trap insects inside bladders.
Bladder of bladderwort

26. Autumnal Changes in Leaf Color
Chloroplasts of mature leaves contain several groups of pigments:
Chlorophylls - Green
Carotenoids - Yellows
In fall, chlorophylls break down and other colors are revealed.
Water soluble anthocyanins (red or blue) and betacyanins (red) may also be present in the vacuole.

27. Abscission Deciduous plants drop leaves seasonally.
Abscission - Process by which leaves are shed
Occurs as a result of changes in abscission zone near base of petiole
Protective layer
Cells coated and impregnated with suberin.
Separation layer
Pectins in middle lamella of cells are broken down by enzymes.

28. Human and Ecological Relevance of Leaves
Landscaping - Shade trees
Food - Cabbage, lettuce, celery petioles, spices
Dyes
Perfumes - Oils of orange tree, lavender
Ropes and Twine - Agave, hemp fibers
Drugs - Narcotics, tobacco, marijuana
Beverages - Tea, tequila (agave leaves)
Insecticides - Rotenone
Waxes - Carnauba and caussu waxes
Aesthetics - Floral arrangements, gardens

29. Review Introduction Leaf Arrangements and Types
Internal Structure of Leaves
Stomata
Mesophyll and Veins
Specialized Leaves
Autumnal Changes in Leaf Color
Abscission
Human and Ecological Relevance of Leaves