Transcript File
Control Mechanisms Section 3.5 Unit C Objectives • define phototropism and gravitropism • relate concepts of phototropism and gravitropism as control systems to natural phenomena • list other control mechanisms found in plants, giving examples Introduction • Stimuli change in the environment that causes a change in the organism • Way that react depends on type and direction of stimulus • Examples? • Plants respond to stimuli as well as to get what they need • What do plants need to survive? Stimuli in Plants • Plants may respond by turning or bending toward or away from an external stimulus tropism Stimuli in plants • Look at these two pictures: What stimuli might be affecting the direction of plant growth? gravity light Phototropism • Phototropism • Growth movement in response to light stimulus • Positive phototropism stems; toward light • Negative phototropism roots (weak); away from light Time Lapse Videos: • http://www.youtube.com/watch?v=Ze8NV7cvW8k (Phototropism in Tomatoes -Time Lapse) • Also: http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropi sm/tropisms.html Phototropism • http://www.youtube.com/watch?v=g8mr0R3ibPU&feature=related (how time flies and sunflowers catch the sun HD) Plant Growth • The shoot of a plant grows out of the lengthening outer sheath of seed • Sheath will stop growing, allowing the stem and leaves to break through and keep growing • The stem and leaves will show phototropism if the light comes from one direction only Gravitropism •Gravitropism • Growth movement response to gravitational force (also known as geotropism) • Positive gravitropism roots; with gravitational force • Negative gravitropism stems; against gravitational force Gravitropism • The bean seed can be oriented any way and the root still always eventually grows down (positive gravitropism) – note effect of root tip removal Mechanism of Gravitropism • Can orient plant any way and roots will still grow down • If plant tips, starch grains shift and settle in new location • Movement detected and growth response results Analogy If I turn this jar of jellybeans on its side, the jellybeans will shift and settle on the new bottom Summary Plant Part Stimulus Tropism Stem Light root Light Positive phototropism Weak negative phototropism stem gravity root gravity Negative gravitropism Positive gravitropism Plant Growth: Darwin’s Experiment • Read the different treatments below carried out on oat plant seedling • What do you think would be the effect on direction of plant growth? Why? • • • • • 1) Control (no treatment) 2) Tip of plant removed 3) Cover tip with opaque cap 4) Cover tip with transparent cap 5) Cover base of plant with opaque shield Results • What did you hypothesize as the result of this experiment? • Results: p. 327 Fig C3.25 • What does this tell us? • The tip is somehow involved in detection of light stimulus (communicating with cells in area of bending area of elongation) Boysen-Jensen’s Experiment • Snipped off the tip and covered with 1) gelatin and 2) mineral mica • Saw phototropism in treatment with gelatin but not with mineral mica • Substance that was communicating info could diffuse through gelatin • From tip to area of elongation Auxins • Group of plant hormones • Isolated by F.W. Went • In shoot tips – auxins stimulate elongation • In root tips – auxins retard (slow down) elongation • Moves from tip to area of elongation to accomplish this • Uneven distribution of auxins results in tropisms Auxins and phototropism • Sunlight eradicates auxin, meaning that the part of the shoot tip of the plant which is receiving direct sunlight will have the least amount of auxin. • More auxin is present on the shaded side promotes more cell division and elongation, causing the plant to bend towards the sunlight after this lop-sided growth. Auxins and phototropism Auxins and hydrotropism • Note that in roots, auxins move towards moist side, slowing down elongation on that side, resulting in root bending to water More hormones • Plants have several other groups of hormones, including gibberillins (which control germination and seed development) and cytokinins • For more info see: http://www.planthormones.info/index.htm Other Control Mechanisms (“Tropisms”) • Plants can also respond to: • • • • Temperature how? Chemicals Water (hydrotropism) Light/dark periods ex. need so much darkness or light to flower • All these responses to stimuli are controlled by chemical messengers • How do you think climate change will affect blooming? Homework • p. 328 # 1-6 • Review p. 330 # 1-24 • Unit Review p. 334-336 (Knowledge and Application Section)