Leaf Structure and Characteristics
– Leaves are the most important organs of most vascular plants.
– They are broad, flat, and thin, maximizing the surface area exposed to light for photosynthesis.
– Leaves have a complex internal organization to maximize exposure of chloroplasts to light and control water loss.
– The primary photosynthetic tissue in most leaves is the palisade mesophyll, located on the upper side of the leaf.
– Leaves have distinct upper and lower surfaces with different characteristics such as color, hairiness, and number of stomata.
– Leaves can have various shapes, sizes, textures, and colors.
– Broad, flat leaves with complex venation are known as megaphylls and are found in flowering plants.
– Some leaves are simple with only a single vein and are known as microphylls, found in lycopods.
– There are leaf-like structures in vascular plants that are not homologous with true leaves, such as phylloclades and phyllodes.
– Non-vascular plants also have leaf-like structures, such as the phyllids of mosses and liverworts.

Leaf Function
– Leaves are the sites of photosynthesis, where plants create their own food using sunlight, carbon dioxide, and water.
– They draw water from the ground through xylem and obtain carbon dioxide from the atmosphere through stomata.
– Leaves transport synthesized sugars to areas of active growth through the phloem.
– In addition to photosynthesis, leaves are involved in transpiration, the process of drawing water up from the roots.
– Leaves can store chemical energy, water, and serve specialized functions such as tendrils or insect traps.

Leaf Adaptations
– Some leaf forms are adapted to modulate light absorption to avoid excessive heat, ultraviolet damage, or desiccation.
– Xerophytes have leaf adaptations to cope with drought conditions.
– Conifers have needle-like or scale-like leaves that are advantageous in cold climates with snow and frost.
– Some plants have pendent leaves adapted to windy conditions.
– Leaf adaptations can also include sacrificing light-absorption efficiency for protection from herbivory.

Leaf Evolution and Importance
– Leaves are fundamental structural units for constructing cones in gymnosperms and flowers in flowering plants.
– The internal organization of leaves has evolved to maximize chloroplast exposure to light and carbon dioxide absorption.
– Leaves are waterproofed by the plant cuticle and gas exchange is controlled by stomata.
– Leaf shape and structure vary among species and can change within a species due to adaptation to climate and other factors.
– Leaves play a crucial role in the autotrophic nature of plants, providing energy through photosynthesis and supporting plant growth.

Leaf Arrangement and Morphology
– The arrangement of leaves on the stem is known as phyllotaxis.
– Paired leaf attachments at each node
– Opposite arrangement with each pair rotated 90° from the previous
– Whorled or verticillate arrangement with three or more leaves, branches, or flower parts attaching at each point on the stem
– Rosulate arrangement where leaves form a rosette
– Rows arrangement with leaves in two rows, also known as distichous or 2-ranked
– Mathematical models represent the stem apex as a circle, with each new node rotated by a constant angle from the previous node
– The number of leaves growing from a node depends on the plant species
– Divergence angle is often represented as a fraction of a full rotation around the stem
– Rotation fractions of 1/2, 1/3, 2/5, 3/8, and 5/13 are common in different plant species
– Many divergence angles are approximately 137.5°, which is the golden angle
– Simple leaf has an undivided blade, but may have lobes that do not reach the main vein
– Compound leaf has a fully subdivided blade, with leaflets separated along a main or secondary vein
– Palmately compound leaves have leaflets radiating from a common point of attachment
– Pinnately compound leaves have leaflets arranged on either side of the main axis
– Bifoliolate leaves consist of only two leaflets
– Petiolate leaves have a leaf stalk called a petiole
– Sessile leaves have no petiole and attach directly to the stem
– Subpetiolate leaves have a short petiole or appear to be sessile
– Clasping or decurrent leaves partially surround the stem
– Perfoliate leaves completely surround the stem
– Veins are the vascular tissue within a leaf
– Different patterns of venation exist, such as branching veins or parallel veins
– Veins play a crucial role in transporting water, nutrients, and sugars throughout the leaf.
– Veins are the vascular structure of leaves, providing transportation of water and nutrients between leaf and stem.
– Veins play a crucial role in maintaining leaf water status and photosynthetic capacity.
– Veins also contribute to the mechanical support of the leaf.
– Veins extend into the leaf via the petiole.
– Veins form cylindrical bundles between the two layers of epidermis.
– Venation patterns are specific to taxa.
– Angiosperms possess two main types of venation: parallel and reticulate.
– Monocots typically have parallel venation.
– Eudicots and magnoliids (dicots) generally exhibit reticulate venation.
– Exceptions to these patterns exist.
– Veins entering the leaf from the petiole are called primary or first-order veins.
– Branches of primary veins are secondary or second-order veins.
– Primary and secondary veins are considered major veins.
– Some authors include third-order veins as major veins.
– Higher order veins are sequentially numbered and associated with narrower vein diameter.
– In parallel veined leaves, primary veins run parallel and equidistant for most of the leaf length.
– Primary veins converge or fuse towards the apex.
– Many smaller minor veins interconnect the  Source:

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