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Roots, Stems, & Leaves Roots 2 Main Types: 1. Tap Roots • • long & thick primary root with small secondary roots Found in dicots like oaks, carrots, beets, dandelions 2. Fibrous roots • • • Branching with no single larger root Mainly found in monocots Extensive root systems help prevent topsoil erosion Root Structure & Function Structures: • Epidermis - both protects and absorbs • Root hairs - penetrate soil & provide larger surface area for absorption • Cortex - Ground tissue between epidermis & vascular cylinder • Endodermis - layer that surrounds vascular cylinder Function • Roots anchor the plant in the ground and absorb water & dissolved minerals from soil Leaves Characteristics: • Main organs of photosynthesis • Blade – thin, flattened sections for collecting sunlight • Petiole – stalk which attaches blade to stem Guard cells One of the paired epidermal cells that control the opening and closing of a stoma in plant tissue. Stems Functions: • Produce leaves, branches, & flowers • Hold leaves up to sunlight • Transport substances between roots & leaves Stem Structure • Stems are composed of dermal, vascular, & ground tissue • Nodes - where leaves are attached • Internode - regions in between nodes • Buds - undeveloped tissue that can produce new stems & leaves; found where leaves attach to nodes Specialized Stems Stems that store food and can remain dormant for a time • Tuber – usually grows underground, stores food (potato) • Bulb – central stem surrounded by short, thick leaves (onion) • Corm – similar to bulb but stem is thickened to store food (gladiolus flower) • Rhizome – horizontal underground stem that can form new shoots (ginger) Plant Adaptations & Plant Tropisms Plant Adaptations • Dormancy – period during which growth & activity decrease or stop Plant Adaptations Aquatic plants must tolerate mud nearly devoid of oxygen • Tissues with air-filled spaces for O2 to diffuse • Seeds often float and grow quickly Plant Adaptations Xerophytes - desert plants • Root systems spread out for long distances • Leaves may be reduced to spines; photosynthesis occurs in stems, which also store water Plant Adaptations Carnivorous plants – live in bogs with little nitrogen • Use specialized leaves to trap & digest insects to meet nutritional needs • Pitcher plants - drown insects in specialized leaves • Sundews - trap insects with sticky secretions • Venus’ flytrap - has leaves that snap shut Plant Adaptations Parasites & Epiphytes – extract water & nutrients from a host plant • Parasites harm the host • Epiphytes grow on bodies of other plants but gather their own water & produce their own food – Ex: Spanish moss, bromeliads, many orchids Plant Adaptations Chemical defenses – manufacture poisons, insect hormones, etc. that effect animals/insects Plant Tropisms Plant Tropisms - responses of plants to external stimuli • Gravitropism – stems grow up & roots grow down • Phototropism – plants grow toward light • Thigmotropism – response to touch – vines & climbing plants wrap around support structures Plant Hormones 1. Auxins – cause of phototropism • Stimulate cell elongation • Higher concentration on shaded side of the plant causing stem to elongate and bend -- Cause of gravitropism • higher concentration on lower part of root stem inhibit cell growth, causing root to grow downward. • Help move root through the soil and around objects in the soil Plant Hormones 2. Cytokinins – produced in growing roots and in developing fruits and seeds. • Cause dormant seeds to sprout • Cells grow thicker 3. Gibberellins – growth promoting • Increase size of stems and fruits 4. Ethylene – stimulates fruit to ripen