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Chapter 6: Leaves (Morphology and Anatomy of Other Leaf Types (Succulent…
Chapter 6: Leaves
Morphology and Anatomy of Other Leaf Types
Leaves of Conifers
Bud Scales
Sclerophyllous Foliage Leaves
A plant having hard leaves stiffened by woody tissue
Spines
Succulent Leaves
Any plant with fleshy, thick tissues adapted to water storage
Tendrils
Leaves with Kranz Anatomy
Insect Traps
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Initiation and Development of Leaves
Monocots
Initiated by the expansion of some shoot apical meristem cells to form a leaf primordium
Outer surface of the tube is abaxial epidermis
Monocot: Any of a class of angiosperm plants having a single cotyledon in the seed.
Basal expansion - protoxylem and protophloem are constantly being stretched
After the first lead has grown to its full size, the next leaf continues elongating and emerges through the opening at the top of the previous leaf's tubular sheathing base
Inner surface of the tube is adaxial epidermis
Higher above the basal meristem, tissues are mature
Basal Angiosperms and Eudicots
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The primordium extends upward as a narrow cone, growing so rapidly that it becomes taller than the shoot apical meristem
Primordium grows upward - it increases in thickness, establishing the bulk of the midrib
Leaf primordium: at the base of the meristem, cells just interior to the protoderm grow outward, forming a protrusion (primordium)
All cells in the wings are meristematic, and their division and expansion enlarge the lamina rapidly
During lamina expansion - stomata, trichomes, and vascular bundles differentiate
Basal Angiosperms: broad group of the most primitive flowering plants.
Perennial Plants: Leaves are initiated in the summer or autumn
Eudicots: An angiosperm having two cotyledons in the seed
Spring - buds open and the primordial leaves expand rapidly
Epidermis becomes vulnerable after the leaves expand
Annual Plants - initiate leaves before the seed becomes dormant and dry
Internal Structure of Foliage Leaves
Mesophyll
Palisade Parenchyma
Often only one layer thick
At times it can be three to four layers thick
The main photosynthetic tissue of most plants
Spongy Mesophyll
Open, loose aerenchyma that permits carbon dioxide to diffuse rapidly away from stomata into all parts of the leaf's interior
Most common arrangement
Located in the lower portion of the leaf
May have a layer of palisade parenchyma along both surfaces
Leaves are held horizontally or vertically (Iris, Gladiolus, and Eucalyptus)
Mesophyll: Ground tissues interior to the leaf epidermis are collectively
Vascular Tissues
Eudicot leaves
Lateral veins merge into minor veins
Contains both primary xylem and primary phloem
One large midrib (midvein)
Bundle Sheath
Makes it difficult for insects to chew into vascular tissues
Minor veins are the sites of material exchange with the rest of the mesophyll
A layer or region of compactly arranged cells surrounding a vascular bundle in a plant.
Can contain either xylem or phloem or sometimes both
Bundle Sheath Extension
Help give rigidity to the blade
Additional means by which water can move
Veins often have a mass of fibers above, below, or both
Located between the palisade parenchyma and spongy mesophyll
Epidermis
The dorsiventral nature of leaves, however, causes their upper and lower epidermis to exist in significantly different microclimates
Water is swept away - if stomata in the upper epidermis are open and losing water
Must be reasonably waterproof but simultaneously translucent
Leaf stomata are frequently sunken into epidermal cavities
Water loss through the epidermis is called transpiration
Leaf epidermises are often remarkably hairy
Hairs provide excellent footholds against leaf flutter for insects of the appropriate size
External Structure of Foliage Leaves
Petiole (Stalk)
Prevents self-shading of the leaf blades
Long, thin, flexible petioles allow the blade to flutter in wind, cooling the leaf and brining fresh air to its surface
Holds the blade out into the light
Leaf flutter makes it difficult for insects to land on leaves
Considered a petiolate leaf
Sessile Leaf
Self-shading is not a problem
Close packing leaves - traps water molecules
If leaves are small or very long or narrow
Ex) Aeonium
Common - Arid, sunny regions
Monocots - grasses, irises, lilies, agaves, and yuccas
Typically lacks a petiole
Leaf Blade (Lamina)
Dorsal Surface (Abaxial Side) : Blade's lower side and larger veins protrude like backbones
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Flat, light-harvesting portion of the leaf
Ventral Surface (Adaxial Side) : Upper side of the leaf and is usually rather smooth
Different types of leaf blades
Pinnate
Reticulate
Palmate
3 main veins
Parallel
Sheathing Leaf Base
Blade can flex and flutter without a petiole
Monocots with petioles and laminas - palms, aroids, and bird-of-paradise plants
Leaf wraps around the stem to form the sheathing leaf base
2 Types of Leaves
Compound Leaf
Has many small blades (leaflets)
Leaves may be palmately compound, with all leaflets attached at the same point
Leaves may be pinnately compound, with all leaflets attached individually along the ranchis
Each leaflet is attached by a petiolule
Has a blade divided into several individual parts
The petiolule has an extension to the petiole, the rachis
Plants that grow immersed in streams usually have these types of leaves
Simple Leaf
Ex) Poplar, oak, and petiolar bundles
Always attached to a twig by a petiole
Has a blade of just one part
Different Leaf Shapes
Peltiform
Sagittiform
Hastiform
Cordiform
Leaf Veins
Reticulate venation - In basal angiosperms and eudicots, they occur in a nettled pattern
Parallel venation - In monocots with long, strap-shaped leaves, the larger veins run side by side with few obvious interconnections
Leaf Veins - bundles of vascular tissue
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