Chapter C4 (1).pdf

Dr.Mohamed Elshebly 2005 BOTANY CHAPTER 4 © stem PREPARED BY : ‫محمد الشبلي‬.‫د‬ 1 DrMohamed Elshebly 0925121809  Dr.Mohame...

Dr.Mohamed Elshebly 2005 BOTANY CHAPTER 4 © stem PREPARED BY : ‫محمد الشبلي‬.‫د‬ 1 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly STEM Stem: is axial organ of shoot. , radial structure, no root hairs &grows continuously. Note: , plant sometimes appears to be stemless, rosette-like Morphology of the Stem: Shoot Tip: stem shoot apex produces leaf primordia that develop into leaves. Nodes : places where leaf or leaves, axillary buds are attached. Internodes :The elongated portion of a stem between two nodes. Two types Bud: Terminal bud, which develops at the apical end of the stem, Axillary buds are lateral buds that form at nodes & may develop into lateral vegetative shoots or flowers. Bud Scales: Small, modified leaves or stipules that cover & protect the bud. Naked buds Some buds lack scale coverings. Lenticel: Openings for gas & water exchange in the stem surface, are found on tree bark of some plants with secondary growth. 2 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly Permanant tissues of the stem : 1 – Ground tissue system : A – In conifers & Dicots stem : cortex between vascular & epidermis  It consist of parenchyma with intercellular spaces , but may include sclerenchyma & collenchyma tissues.  If the vascular tissue forms a ring between the cortex & the center ( in the center is the pith ) B – Monocot stem : With usually dispersed vascular bundles in the ground tissue that is not differentiated into cortex or pith. 2 – Vascular Tissue System : A -- In dicots & Conifers :  In secondary growth : The yearly additions of secondary xylem are called growth rings. ( which determine the ages the tree in base of the trunk ).  Rings can be counted visually or by use of increment borer.  Associated with the xylem & phloem rays are gum ducts in dicots which contain resins , oil , gum , mucilages.  Secondary xylem tissue wood may be composed of sapwood & heartwood. 3 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly Sapwood : consist of live xylem & active in transport of water & mineral. Heartwood : inner , composed of inactive xylem that store secondary metabolites. NOTE : Organic compounds such as oils , gum resins & tannins fill the cells that prevent wood rot caused by fungi & other organisms. B -- Monocot stem : stem have no vascular cambium or cork cambium & therefore no secondary growth.  ex.. herbaceous.  In large plants bodies such as palms , a thickening of the trunk occurs by multiple divisions of parenchyma. 3 -- secondary dermal tissue :  Are usually found in older plants.  NOTE : Only rarely , secondary tissues occur in bryophytes, ferns & monocots.  In woody dicots & conifers :  The periderm composed of : 1. : Phellum ( Cork cells ) they are generated centrifugally, are non- living & have suberized cell walls. 4 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly 2. Cork cambium (phellogen) 3. phelloderm.. The cork cover stem for : 1- protection , 2 - it’s compressible, 3 -- water & oil resistant 4 -- protect against insect & fungal attack & cold. Bark : The totality of all tissues produced by the phellogen, both primary & secondary ones & some other types, Functions of stem : 1. Photosynthesis. 2. Long distance Transport of hormones & organic in the phloem , also water & mineral in xylem. 3. Support the shoot system. 4. Site of response of roots to gravity ( gravitropism ) & shoots to unilateral light ( phototropism ). 5. Food storage.. 6. Sites of initiation of adventitious roots, new leaves and inflorescences (cluster of flowers). Stem Modifications: Stems may be of various forms to serve different functions, such as for food or water storage, for subterranean or aerial anchoring devices, , asexual reproduction, or for climbing. Rhizome: Found near or below the soil surface, underground stem that produces scale-like leaves & adventitious roots at the nodes. Rhizomes in a potato develop edible tuber. Corm : A solid, bulb-like, underground stem without fleshy scales forms a corm. It has greatly shortened internodes. Bulb: is a short, underground, food-storage extremely reduced internodes & surrounding fleshy scale leaves. Ex ,,, onion 5 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly Tuber: are swollen, underground food storage stems arising at the tips of rhizomes. They bear buds," eyes, at the nodes on the potato tuber. These (eyes) develop into potato sprouts (shoots) when the potato starts to grow. Pseudo bulb: Many orchids grow on branches or trunks of other plants (epiphytes). These plants develop a fleshy stem with water storage Other orchids, in contrast, are terrestrial (grow in the soil). Spur Shoot: Some trees have short, woody stems with shortened internodes, such as apple & Ginkgo. In apple, the spur shoots produce flowers & fruits. Stolon: A lateral stem, ―runner, from the base of a plant develops long internodes, & where the apex touches the soil, a new plant with shoots and adventitious roots forms at a node, Ex... is strawberry. Succulent Stem: Fleshy water storage are found in the spurge & cactus families. Vine: stems with long internodes & may have one of various types of climbing devices such as tendrils in grape. Stem - Water Transport: Transpirational “Pull” Q.. How then does water rise from the soil to the leaves of tall trees?  Root pressure : force generated by osmotic pressure that helps to drive fluids upward into the xylem , but alone insuffcient for the movement of sap against gravity. especially in tallest trees.  Instead , the lifting force generated by : 1 -- The evaporation provides a ―pull for water movement within the plant & Loss of water by evaporation is called transpiration. 2 – The cohesive & adhesive forces of molecules in the vessels. 6 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly  Water moves upward in the plant from a region of higher water concentration in the soil to a region of lower water concentration Pathway of Water Transport  Water in the soil is absorbed by the epidermal root hairs.  It then moves across the root through living cortical cells  Its filtered through endodermal cells ,  Then passes through the pericycle & enters the water-conducing cells (tracheids and vessel elements) of the xylem.  Then rises up from the root to the stem & into the leaves. Note : water can move through the roots by three separate pathways 7 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly 1 – Apoplast. (spaces between cells & in the wall ). 2. symplast (cytoplasm to cytoplasm by plasmodesmata ) 3. Transmembrane (transcellular) : cross the plasma membrane (may crossing tonoplast. Osmosis : if cells are surrounded by smaller concentration of salts than in the cytoplasm , water will flow into the cell. Turgor pressure : is the combined pressure of cell & vacuoles wall that support the shape of the cell. Stem - Food Transport:  Translocation is the transport of soluble organic materials (food) from one part of a plant to another.  These organic materials are mainly sugars (from photosynthesis) and amino acids and amides (products of nitrogen metabolism). FOOD TRANSOPORT STEPS :  Primarily, they come from leaves, which we call sources, & are transported to areas called sinks, TYPES OF SINKS : A -- Metabolic sinks include young growing developing parts. B -- Storage sinks include tubers, bulbs, corms, fruits, or parenchyma tissue.  The transport occurs in the phloem (At maturity, the sieve tube elements contain cytoplasm & organelles but usually lack nuclei.) 8 DrMohamed Elshebly 0925121809  Dr.Mohamed Elshebly  ‫ق‬Good Luck My Doctors ,,,,, Dr.Mohammed Elshebly ‫نهاية منهج الجزئي لكم كل التحيات واتمنى لكم‬ ‫التوفيق‬ ‫محمد الشبلي‬.‫د‬ 9 DrMohamed Elshebly 0925121809