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Leaves (Morphology and Anatomy of Other Leaf Types (Insect Traps…
Leaves
Morphology and Anatomy of Other Leaf Types
Bud Scales
Perennial plants shoot apical meristems shielded from low temps and wind by bud scales
Role is primary protection, not photosynthesis
Common modification of leaves
Produce a thin layer of corky bark and wax
Spines
Protection from animals
Have no blade and are needle shaped, so inhibit lamina formation
Cactus spines are modified leaves of axillary buds
No mesophyll parenchyma or vascular tissue
Mesophyll has fibers that deposit lignin in their walls- makes them hard and resistant to decay
Leaves of Conifers
Contain unpalatable chemicals
Perennial remaining on stem for many years = evergreens
Sclerophylls with thick epidermis and hypodermis and always simple
Needles – pines, firs, spruces
Stems can range in length and can have small or large scales
Tendrils
Contain cells that are capable of sensing contact with an object
When tendril touches something, that side stops growing but the other side continues elongation
Elongation forms a coil that grows around objects FIGURE 6-44
Peas, cucumbers, and squash
Sclerophyllous Foliage Leaves – perennial leaves existing on a plant for 2 or more years
Sclerenchyma present as a layer below epidermis and in the bundle sheaths
Cuticle is very thick, and waxes are abundant on these types of leaves
Feasible, hardiness makes them resistant to animals, fungi, freezing temps, and UV light
Leaves with Kranz Anatomy
Lack palisade parenchyma and spongy mesophyll
Have large bundle sheaths with a ring of mesophyll cells
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C4 plants
Succulent leaves – thick and fleshy which helps reduce surface-to-volume ratio and favor water conservation
Leaves are spherical or cylindrical which helps with surface-to-volume ratio, but it can reduce the capacity or carbon dioxide uptake
Mesophyll has few air spaces which reduces water loss through stomata
Makes mesophyll transparent allowing light to penetrate further into the leaf
Insect Traps
Lamina is tubular and secretes a watery digestive fluids
Parenchymous, photosynthetic, lots of stomata, vascular bundles, aerenchyma and chlorenchyma
Epidermis is absorptive not impermeable
Passive traps – incapable of movement
Pitcher contains trichomes that help insects walk towards their death
Active traps – move during capture
Good example: Venus fly traps FIGURE 6-45 C
Environments with poor nitrates and ammonia, plants eat insects to get nitrogen
Nitrogen helps build plants amino acids and nucleotides
External Structure of Foliage Leaves
Blade’s lower side – dorsal surface
Blade’s upper side – ventral surface
Flat, light harvesting portion of leaf – leaf blade or lamina
Leaf shape is valuable tool for identification
Can be simple or compound leaf
Functions for turbulence, barriers, and abscising pathogenic leafs
Pinnately compound – leaflets attached individually (A)
Leaflets always arranged in two rows
Figure 6-7
Palmately compound – leaflets attached at the same point (B)
Compound leaf has leaflets – small blades
Huge range of shapes, yet none are more advantageous than the other
Attached by a petiolule – extension of petiole to the rachis
Stalk that hold blade out into light – petiole
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Can be many different widths and lengths
Long, flexible allow for fluttering in the wind allowing fresh air to surface
Leaves with petioles – petiolate
Leaves without petioles – sessile leaves
Results in leaf base wrapping around the stem = sheathing leaf base
Has abscission zones – enzymes that cleave at the petiole when leaf life is over
Adjacent undamaged cells for a corky protective scar – leaf scar
Internal Structure of Foliage Leaves
Epidermis
Waterproof, translucent, allow entry of carbon dioxide
Have coating of cutin hat helps retain water and make waterproof
Water loss through epidermis – transpiration
Leaf and stem epidermis – many flat, tabular, ordinary epidermis cells
Guard cells and trichomes can be abundant
Upper epidermis – little to no stomata due to inefficiency of keeping water
Not all bad because it can prevent fungi from entering leaf
Lower epidermis – almost all stomata here because efficient at keeping water, less direct sunlight
Can fall into epidermal cavities called crypts – helps trap water to re-enter stomata instead of blowing away
Trichomes – leaf hairs
Consequences – deflect excessive sunlight on upper epidermis and prevent rapid air movement on lower epidermis
Advantages – Make walking or chewing difficult for insects, prevent water coverage of stomata
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Glandular trichomes secrete stinging compounds to prevent animals form eating the leaf
Mesophyll – ground tissue internal to epidermis
Spongy mesophyll – permits carbon dioxide to diffuse rapidly away from stomata, beneath mesophyll on lower portion
Palisade parenchyma – main photosynthetic tissue, beneath mesophyll in upper portion
1-4 thick layers with intercellular spaces surrounding them
Vascular Tissue – between palisade parenchyma and spongy mesophyll
Eudicots leaf has one large midrib – lateral veins branch into minor veins
Minor veins - release water from xylem and loading sugar onto phloem
Sites of material exchange, have large surface area, do not contain fiber cells, have both phloem and xylem
Midrib and lateral veins – function is conduction
Can be arranged as bundle sheaths around vascular tissue
Always contain primary xylem on upper side and primary phloem on lower side
Larger veins can produce mucilage, tannin, or starch storage cells
Have bundle sheath extension – mass of fibers above, below, or both
Fibers give rigidity to blade and moves water out to mesophyll
Petiole – transition between stem and lamina
Petiole mesophyll – like the cortex, compact, and not aerenchymatous
Can have lots of collenchyma
Contains fewer stomata and trichomes
Leaf traces – bundles of vascular tissue that reach toward petioles
Can be arranged into ring, plate, or many other patterns
Stipules – two small flaps of tissue at base of petiole
Function – protect shoot apical meristem and contribute to photosynthesis
Initiation and Development of Leaves
Monocots
Leaf primordium -expansion of shoot apical meristem
More apical meristem cells involve with the primordium until it’s a cylinder that encircles the shoot apical meristem
This cylinder becomes the lamina with inner adaxial epidermis and outer abaxial epidermis
Basal expansion – protoxylem and protophylem are always being stretched and disrupted in basal meristem
Apical meristem adjacent cells grow with primordium forming a hood-like shape
Vessel elements – sieve tube members differentiate rapidly so conduction is never interrupted
Basal Angiosperms and Eudicots
Leaf primordium – cells interior to the protoderm growing outward and forming protrusion
As grow upward increase in thickness establishes bulk of midrib
Row of cells growing outward of primordium becomes the lamina
Now consist of two wings and midrib
Cells in wings are meristematic and division expands lamina
Stomata, trichomes, and vascular bundles differentiate, and petiole becomes distinct
Consists of leaf protoderms and leaf ground meristem, young xylem and phloem
