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Plant Growth AP Biology 2007-2008 Growth in Animals Animals grow throughout the whole organism AP Biology many regions & tissues at different rates Growth in Plants Specific regions of growth: meristems stem cells: perpetually embryonic tissue regenerate new cells apical shoot meristem growth in length primary growth apical root meristem growth in length primary growth lateral meristem growth in girth secondary growth AP Biology Apical meristems AP Biology shoot root Root structure & growth AP Biology Root cap - protects the meristem Shoot growth Apical bud & primary growth of shoot region of stem growth axillary buds “waiting in the wings” protecting the meristem Young leaf primordium Apical meristem Older leaf primordium Lateral bud primordium AP Biology Vascular tissue Primary xylem Growth in woody plants Primary phloem Woody plants grow in Epidermis height from tip Lateral meristems primary growth apical meristem Woody plants grow in diameter from sides secondary growth lateral meristems Primary phloem Secondary phloem Secondary Primary xylem xylem vascular cambium makes 2° phloem & 2° xylem cork cambium makes bark Annual AP Biology growth layers Bark Secondary growth Secondary growth growth in diameter thickens & strengthens older part of tree Occurs in stems and roots, but rarely in leaves cork cambium makes bark growing ring around tree vascular cambium makes xylem & phloem growing ring around tree AP Biology Vascular cambium Why are early & late growth different? 1 cell layer of undifferentiated parenchyma cells Phloem produced to the outside Xylem produced to the inside bark cork cambium phloem xylem vascular cambium AP Biology late early last year’s xylem Early versus late growth – tree rings! Early (spring) wood consists of secondary xylem cells with very large diameters and thin cell walls Maximizes delivery of water to new leaves Late (summer) wood – thick-walled cells that transport less water but are great for support AP Biology cork cambium Woody stem How old is this tree? vascular cambium late early 3 2 1 xylem phloem AP Biology bark Tree trunk anatomy tree girdling What does girdling do to a tree? AP Biology Aaaargh! Murderer! Arborcide! Wood Anatomy Heartwood – old layers of Xylem No longer transport water and minerals Darker due to resins and compounds Sapwood – new layers of xylem Still transport xylem sap Large circumference every year – transports more sap each year Why can a tree survive when hollow? AP Biology AP Biology Plant hormones (Plant Growth Regulators) auxin gibberellins abscisic acid ethylene and more… AP Biology Auxin (IAA) Effects Many effects – root and fruit formation, secondary growth, and cell elongation controls cell division & differentiation (stimulates elongation) phototropism growth towards light asymmetrical distribution of auxin cells on darker side elongate faster than cells on brighter side – causes curve apical dominance Polar transport – unidirectional transport from tip to base AP Biology Cytokinins Stimulate cytokinesis or cell division Produced in actively growing tissues Move upward in xylem sap Signal axillary buds to grow Don’t act alone – act with Auxin to regulate differentiation Slow the progress of apoptosis AP Biology Gibberellins Family of hormones over 100 different gibberellins identified Effects stem elongation Enhance cell elongation and division fruit growth seed germination AP Biology plump grapes in grocery stores have been treated with gibberellin hormones while on the vine to make them larger and elongate the internodes for space Abscisic acid (ABA) Effects slows growth seed dormancy high concentrations of abscisic acid germination only after ABA is inactivated or leeched out survival value: seed will germinate only under optimal conditions light, temperature, moisture AP Biology Drought tolerance – high ABA causes stomatoa to close rapidly Ethylene Hormone gas released by plant cells Effects response to mechanical stress - curvature fruit ripening leaf drop like in Autumn apoptosis One bad apple spoils the whole bunch… AP Biology Fruit ripening Michigan Peaches… Apples… Adaptation hard, tart fruit protects developing seed from herbivores ripe, sweet, soft fruit attracts animals to disperse seed Mechanism triggers ripening process breakdown of cell wall softening conversion of starch to sugar sweetening positive feedback system ethylene triggers ripening ripening stimulates more ethylene production AP Biology Apoptosis in plants Many events in plants involve apoptosis response to hormones ethylene auxin death of annual plant after flowering senescence differentiation of xylem vessels loss of cytoplasm AP Biology shedding of autumn leaves What is the evolutionary advantage of loss of leaves in autumn? Don’t take this lying down… Ask Questions!! AP Biology 2007-2008