Leaves: Photosynthesis, Adaptations, and Morphology

• Leaves partake in photosynthesis and gaseous exchange
• Leaves take in carbon dioxide and release oxygen during photosynthesis, and the reverse during respiration
• Leaves undergo transpiration, where water vapor is lost from the surface, reducing water loss by closing stomata

Photosynthesis

• Photosynthesis is a biological process where light energy is captured and converted into chemical energy

Plant Adaptations

• Desert plants (xerophytes) have thick, reduced leaves, stomata that open only at night, and a thick waxy layer
• Floating leaves feature stomata located only on the upper epidermis
• Submerged leaves lack stomata
• Conifers have waxy needle leaves, are evergreen, and possess thick, waxy cuticles with sunken stomata

Leaf Morphology

• Simple leaves feature a single blade per petiole, while compound leaves are divided into smaller leaflets originating from a single axillary leave

Compound Leaves

• Pinnately compound leaves have leaflets in pairs along an extension of the petiole
• Palmately compound leaves have all leaflets attached at one point at the end of the petiole
• Leaf attachment refers to how leaves are attached to the stem/node, including connate perfoliate, perfoliate and clasping types
• Sessile, petiolate, and sheathing describe different leaf attachments

Venation

• Venation is the arrangement of veins in the leaf
• Parallel venation features veins running parallel, common in monocots
• Netted venation includes pinnately veined leaves with one primary vein in an enlarged midrib, and palmately veined leaves where several primary veins fan out from the base

Phyllotaxy

• Phyllotaxy refers to leaf arrangement on a stem
• Alternate arrangement means leaves attach alternately in a spiral, with one leaf per node
• Opposite arrangement means two leaves arise from the opposite node
• Whorled arrangement means three or more leaves occur at the node

Leaf Shape/Outline

• Leaf shape can be linear (thin), lanceolate (spear-shaped), ovate (oval-shaped), or cordate (heart-shaped)
• The base of a leaf can be cuneate, rounded, truncate, or cordate
• Margin refers to the shape of the leaf's edge, which can be entire, undulate, finely serrate, coarsely serrate, double serrate, crenate, or lobed
• The apex or tip of the leaf can be acuminate, cuteapiculate, aristate, or caudate

Leaf Anatomy

• The tissues in a dicot leaf blade include the cuticle, upper epidermis, palisade mesophyll, vein/vascular bundle, spongy mesophyll, and lower epidermis
• Cuticle is a waxy covering over the epidermis of aerial parts, enabling plants to survive in dry conditions by minimizing water loss and reducing pathogen entry
• The cuticle is made up of lipid and hydrocarbon polymers impregnated with wax, making it hydrophobic and waterproof

Epidermis

• The transparent upper epidermis allows light to penetrate into the mesophyll where photosynthesis occurs
• Ordinary epidermal cells may vary in shape depending on the species and are usually covered by a cuticle
• Trichomes/hair cells reduce water loss, protect the leaf from predatory animals, and vary in shape and structure
• Guard cells close/open stomata in the epidermis

Mesophyll

• Palisade mesophyll is a type of cell in the leaf's internal structure
• Veins/vascular bundles consist of the bundle sheath, xylem, and phloem
• Spongy mesophyll contains air space needed for carbon dioxide flow during photosynthesis
• Stomata in the lower epidermis control water movement and serve as the passageway for atmospheric gases, averaging 100 per square mm

Guard Cells

• Guard cells are always next to stomata
• Factors affecting the opening/closing of stomata include light, water/moisture availability, and temperature changes
• Light makes stomata open, while darkness makes them close
• Stomata close to reduce water loss when water/moisture availability is low
• Higher temperatures will make stomata close

Carbon Dioxide and Stomatal Density

• High carbon dioxide levels lead to low stomatal density; low carbon dioxide levels lead to high stomatal density
• Monocots are usually narrow, wrap around the stem in a sheath, and have parallel venation
• Ligule is a collar extension of the sheath curving around the stem and an auricle is an appendage that surrounds the stem at the junction of the blade and sheath
• The mesophyll layer of monocots is not well differentiated into palisade and spongy layers
• Eudicots usually have a broad, flattened blade and netted venation
• Bulliform cells are large, thin-walled cells on the upper epidermis located on both sides of the midvein that may help the leaf roll or fold inward during drought, reducing water loss

Modified Leaves

• Cotyledons/seed leaves are considered first true leaves
• Colored bracts are petal-like leaves that attract pollinators
• Insect-trapping leaves help plants in nutrient-poor conditions digest insect bodies to obtain nitrogen and essential nutrients
• Tendrils allow plants to cling to other objects and have reduced leaves
• Spines are reduced leaves that discourage herbivory, reduce water loss, and serve as the primary organ of photosynthesis

Leaf Functions

• Storage leaves retain water in large vacuoles
• Absorptive leaves capture aquatic gases
• Reproductive leaves reproduce
• Window leaves are when the plant is buried in soil with the transparent part exposed to light, reducing water loss
• Flower pot leaves catch water and debris for nutrient collection

Leaf Abscission

• In temperate climates, most woody plants with broad leaves shed them in the fall, involving physiological and anatomical changes

Abscission Process

• As autumn approaches, plants reabsorb sugar and transport essential minerals out of the leaves
• Chlorophyll breaks down, and red water-soluble pigments are synthesized and stored in vacuoles, causing leaves to turn red/orange A protective layer of cork cells develops on the stem side of the abscission zone where a leaf petiole detaches, composed of thin-walled parenchyma cells
• Enzymes dissolve the middle lamella, the "cement" that holds primary cell walls together in the abscission zone
• After the leaf detaches, a protective layer of cork seals off the area, forming a leaf scar

Economic Importance of Leaves

• Leaves are used as food, spices, and drinks, such as cabbage, lettuce, spinach, and celery
• Leaves are used in dyes and fibers, such as abaca and pineapple, and can be used for fuel as flammable resin, and for drugs