1. Leaves

2. Outline Overview Leaf Arrangements and Types Internal Structures of Leaves  Stomata  Mesophyll and Veins Specialized Leaves Autumnal Changes in Color Abscission Relevance of Leaves

3. Overview All leaves originate as primordia in the buds. At maturity, most leaves have a stalk (petiole) and a flattened blade (lamina) with a network of veins (vascular bundles). Leaves of flowering plants are associated with leaf gaps and have an axillary bud at the base.  May be simple (single blade) or compound (divided into leaflets).

4. Overview Pinnately compound leaves have leaflets in pairs along the rachis (extension of the petiole), while palmately compound leaves have all the leaflets attached at the same point at the end of the petiole.  Pinnately compound leaves may be further subdivided an thus be referred to as bipinnately compound.

5. Overview Green leaves capture sunlight and thus go through photosynthesis.  Lower surfaces of leaves are dotted with stomata which allow carbon dioxide to enter and oxygen and water to diffuse out. - Guard Cells control stomatal opening.  Transpiration occurs when water evaporates from the leaf surface.  Guttation - Root pressure forces water out of hydathodes (openings at tip of leaf veins). Leaves are sealed off at the base of their petiole and drop during fall (abscission)

6. Leaf Arrangements and Types Leaves vary in: - in size: duckweeds (1 mm wide) to palms (6 meters long) - shapes: flattened, tubular, feathery, cup-shaped, spine-like, needle-like - texture: smooth, hairy, slippery, sticky, waxy, glossy Leaves are attached to stems at nodes, with stem regions between nodes known as internodes. Leaf arrangement on stem (Phylotaxy) generally occurs in one of three ways: - Alternate (spiral): one leaf per node - Opposite: two leaves per node - Whorled: three or more leaves per node

7. Leaf Arrangements and Types Arrangement of veins in a leaf or leaflet blade may also be pinnate or palmate.  Pinnately veined leaves have one primary vein called midvein within a midrib. - Secondary veins branch from midvein.  Palmately veined leaves have several primary veins that fan out from the base of the blade. - Parallel in monocots - Divergent in dicots (netted or reticulate venation)  Dichotomous venation: veins fork evenly and progressively from base of blade (Gingko leaf)

8. Fig. 7.4

9. Internal Structure of Leaves Epidermis is a single layer of cells covering the entire surface of the leaf.  Upper epidermal cells are devoid of chloroplasts.  Waxy cutin called the cuticle is often present  Different glands (occur in the form of depressions, protuberances, or appendages on the leaf surface) may also be present in the epidermis. Glands secrete stick substances.

11. Stomata Lower epidermis of most plans is perforated by numerous stomata (opening formed between two guard cells).  Guard cells originate from the same parent cell as the rest of the epidermis but differ in that they contain chloroplasts. - Primary function includes regulating gas exchange between leaf interior and the atmosphere, and regulating the evaporation of water entering the plant at the roots.  Guard cells are surrounded by thickened but flexible cell wall. Water pressure inside each pair of guard cells regulates their expansion or deflate resulting in the formation of an opening (stoma) between guard cells when they expand or the collapse of the opening when they deflate.

13. Mesophyll and Veins Most photosynthesis takes place in the mesophyll between the two epidermal layers.  Palisade Mesophyll - Uppermost layer Contain most of leaf’s chloroplasts.  Spongy Mesophyll - Lower layer  Monocot leaves usually do not have mesophyll differentiated into palisade and spongy. Veins (Vascular bundles) are scattered throughout the mesophyll.  Consist of xylem and phloem tissues surrounded by the bundle sheath.  Some monocot leaves (grasses) have large, thin-walled bulliform cells on either side of the midrib toward the upper surface. Under dry conditions bulliform cells partly collapse causing the leaf blade to fold or roll which in turn reduces transpiration.

15. Fig. 7.10

16. Specialized Leaves Shade Leaves  Since they receive less total light, they tend to be thinner, have fewer mesophyll layers and chloroplasts, and fewer hairs than leaves on the same tree exposed to direct light (sun leaves). Leaves of Arid Regions  Many have thick cuticle, leathery leaves and few and sunken stomata.  Some have succulent, water-retaining leaves, or dense, hairy coverings.  Pine needles have a thick cuticle and a layer of thick-walled cells (hypodermis) beneath the epidermis.

17. Fig. 7.11b

18. Fig. 7.12

19. Specialized Leaves Tendrils  Modified leaves that curl around more rigid objects helping the plant to climb or support weak stems. - Become coiled like a spring as they develop.  When contact is made, the tip curls around the object, and the direction of the coil reverses.

20. Fig. 7.14

21. Specialized Leaves Spines, Thorns, and Prickles  Spines - Modified leaves designed to reduce water loss and protect from herbivory.  Thorns - Modified stems arising in the axils of leaves of woody plants (e.g. honey locust).  Prickles - Outgrowths from the epidermis or cortex (e.g. roses, raspberries).

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23. Specialized Leaves Storage Leaves – Succulent leaves, retain water in large, thin-walled parenchyma with no chloroplasts. Fleshy leaves (e.g. onions, lily bulbs) store carbohydrates. Flower-Pot Leaves - Urn-Like Pouches which become home to ant colonies. Ants carry in soil and waste products and the flower pot then produces adventitious roots. Window Leaves – In some plants of Kalahari desert (South Africa) leaves are shaped like ice cream cones and are buried in ground. Reproductive Leaves - New plants at tips. Floral Leaves – Bracts (e.g. poinsettia).

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25. Specialized Leaves Insect-Trapping Leaves  Pitcher Plants  Sundews Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

26. Specialized Leaves Insect-Trapping Leaves  Venus’s Flytraps  Bladderworts Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display

27. Autumnal Changes in Leaf Color Cholorplasts of mature leaves contain several groups of pigments.  Chlorophylls - Green  Carotenoids – carotenes (yellow) and xanthophylls (pale yellow). Water soluble anthocyanins (red or blue) and betacyanins (red) may also be present in the vacuole.

28. Mechanism of fall color development  Water soluble anthocyanins accumulate in the vacuoles. If the cell sap is acidic it turns red, if its alkaline it turns blue.  A reduction in daylight causes the green chlorophyll to break and the yellow color of carotenoids and pale yellow color of xanthophylls to predominate.

29. Leaf Abscission Deciduous plants drop their leaves seasonally. At the abscission zone near the base of the petiole two layers of cells are differentiated due to hormonal changes associated with aging. One layer (several cells thick) at the stem side forms the protective layer whose cells are coated with suberin. On the leaf side, a separation layer is differentiated. The cells swell and become gelatinous. Due to seasonal changes (low temperature, shorter days, less light intensity) the pectins of the middle lamella break down by enzymes making the attachement of leaf to stem very loose. Wind or rain can then break the leaf off the stem.

30. Leaf Abscission Zone

31. Human and Ecological Relevance of Leaves Landscaping Food Dyes Ropes and Twine Drugs  Tobacco  Marijuana Insecticides Waxes

32. Review Overview Leaf Arrangements and Types Internal Structures of Leaves  Stomata  Mesophyll and Veins Specialized Leaves Autumnal Changes in Color Abscission Relevance of Leaves

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