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Shoots – Chapter 25 Shoot Includes stem and leaves Two principal functions of stems are support and conduction The principle function of the leaf is photosynthesis Primary structure of stem Lots of variation in structure – Vascular system continuous cylinder – Vascular tissues a cylinder of bundles – Vascular tissues bundles scattered throughout ground tissue Continuous vascular cylinder Vascular bundles actually separated by narrow parenchyma – These areas are termed interfasicular regions Primary xylem forms from inner procambium Primary phloem forms from outer procambium Circular bundle arrangement Pith rays (interfasicular regions) are wide Vascular cambium partly procambial and partly interfascular in origin Scattered bundle arrangement Vascular bundles closed – Do not give rise to a cambium Leaf arrangement Arrangement of leaves on stem is called phyllotaxy – Helical – Distichous Two opposite ranks – Opposite – Decussate At right angles – Whorled Leaf structure Leaf consists of blade and petiole in eudicots Sessile leaves lack petioles Stipules can form at the base of the leaf Leaf structure In monocots, base of leaf is expanded into a sheath Simple leaves Blades are not divided into distinct parts Can be deeply lobed Compound leaves Pinnately compound – Leaves arise from rachis Palmately compound Compound leaves Buds are formed in axils of leaves and not of leaflets Leaflets all arise from same plane, while leaves arise from stem in various planes Leaf epidermis Compactly arranged and covered with cuticle (as are stems) Stomata are openings that facilitate gas exchange – Can occur on both sides of leaf or only one side – If on one side, usually on lower side – Trichomes help retard water loss Stomata arrangement Aquatic plant Xerophyte Mesophyll A ground tissue Site of photosynthesis – Lots of extracellular space – Lots of chloroplasts In eudicots, mesopyll is differentiated into palisade parenchyma and spongy parenchyma – Most photosynthesis occurs in palisade parenchyma Xerophyte Grasses Vascular bundles Veins are scattered throughout leaves – Netted or reticulate venation in eudicots – Parallel or striate venation in monocots Minor vs. major veins Veins Usually xylem on top, phloem on bottom Veins not exposed to intercellular spaces of mesophyll Surrounded by bundle sheath Bundle sheath extensions Mechanical support Conduct water to epidermis C4 vs. C3 leaves Mesopyll cells and bundle sheath cells form two concentric layers around vascular bundles (Kranz anatomy) Lots of big chloroplasts in BSC Veins close together (no more than 4 mesophyll cells apart) Mesophyll and bundle sheath cells not concentracally arranged Small chloroplasts in BSC Veins farther apart C4 vs. C3 plants C4 plants export photosynthates (products of photosynthesis) faster C4 plants have bulliform cells Environmental factors Phenotypic plasticity Light Abcission Leaf scar remains Transition region Flower Development of the flower ends the meristematic activity of the shoot apex Stem and leaf modifications Tendrils – Arise from stem or leaf Cladophylls Modified stems Perform photosynthesis Spine Thorn Tuber Tuber inside periderm is storage parenchyma Eyes contain group of buds Scale like leaf Bulb Thickened leaf tissue Corm Thickened stem tissue Thickened petioles Water storage