Leaves Lecture Notes (PDF)

Leaves Dr. Mohammad Yusuf  Outline  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 ...

Leaves Dr. Mohammad Yusuf  Outline  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  Introduction  All leaves originate from primordia in shoot apical meristem.  At maturity, most leaves have: Stalk = petiole – Leaves sessile if lacking petiole (monocots) Flattened blade = lamina Network of veins = vascular bundles Stipules (Leaf bud) at base of petiole Leaves of flowering plants associated with leaf gaps and have axillary buds at base.  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) Palmately compound leaf o Bipinnately compound leaf - Leaflets subdivided – Palmately compound leaves - All leaflets attached at same point at end of petiole. Bipinnately compound leaf  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.  Introduction  Other functions of leaves: Wastes from metabolic processes accumulate in leaves and are disposed of when leaves are shed. Play major role in movement of water absorbed by roots – Transpiration occurs when water evaporates from leaf surface. – Guttation - Root pressure forces water out hydathodes at tips of leaf veins in some plants.  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 Opposite Whorled  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 Palmate venation venation  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. Parallel Dichotomous venation Reticulate venation venation  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.  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. o Primary functions: « Regulate gas exchange between leaf interior and atmosphere « Regulate evaporation of water o Changes in amount of water in guard cells cause them to inflate or deflate. « Inflate - Stomata open « Deflate - Stomata close  Mesophyll and Veins  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  Mesophyll and Veins  Veins (vascular bundles) are scattered throughout mesophyll. Consist of xylem and phloem tissues surrounded by bundle sheath of thicker-walled parenchyma  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. o Causes leaf to fold or roll, reducing transpiration Monocot leaf cross section  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  Specialized Leaves  Leaves of Arid Regions 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  Specialized Leaves  Tendrils Modified leaves that curl around more rigid objects, helping the plant to climb or to support weak stems Tendrils – Garden peas  Spines Modified leaves that reduce leaf surface and water loss and protect from herbivory. – Cacti o Leaf tissue replaced with sclerenchyma. o Photosynthesis occurs in stems. Spine  Specialized Leaves  Thorns - Modified stems arising in the axils of leaves of woody plants Thorn  Prickles - Outgrowths from epidermis or cortex  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  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  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  Specialized Leaves  Reproductive Leaves Walking fern - New plants at leaf tips Air plant - Tiny plantlets along leaf margins Air Plant  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. Poinsettia Clary’s sage  Specialized Leaves  Insect-Trapping Leaves Grow in swampy areas and bogs – Nitrogen and other elements are deficient in soil. o Specialized leaves trap and digest insects. Pitcher Plants – Insects trapped and Pitcher plant digested inside cone- shaped leaves.  Specialized Leaves  Insect-Trapping Leaves Sundews – 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.  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  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.  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 MAPLE OAK BEECH – Protective layer o Cells coated and impregnated with suberin. – Separation layer o Pectins in middle lamella of cells are broken down by enzymes. Shape Margin