plant responces

• a stimulus is anything that causes a response in an organism
• a response is the activity of the cell, organism or part of an organism as a result of a stimulus
• a tropism is the growth response of a plant to a stimulus
• a positive tropism occurs when a plant grows towards the stimulus
• a negative tropism occurs when a plant grows away from the stimulus
• phototropism in the growth response of a plant to light
• stems are positively phototrophic - roots are negatively phototrophic
• geotropism is the grow response of a plant to gravity
• roots positively geotrophic -stems negatively geotrophic
• thigmotropism is the growth response of a plant to touch
• tendrils are positively thigmotrophic
• hydrotropism is a plants growth response to water
• roots and pollen tubes grow towards water
• chemotropism is a plants response to chemicals
• most roots are negatively chemotrophic to acids or heavy metals
• a growth regulator is a chemical that controls the growth of a plant - produced in meristematic tissue and transported to another part of the plant where they cause an effect
• growth promoters increase the rate of growth - example - auxins and giberellins
• functions of auxin - cause stem elongation, inhibits side branching in stems and causes phototrophism
• apical dominance means that auxins produced in the apical meristem pass down the stem and inhibits side branches from growing
• if the stem is exposed to light from one side IAA diffuses down the shady side - the higher concentration of IAA in the shaded cells causes elongation on the shady side without elongation on the bright side
• growth inhibitors decrease the rate of growth or stop it completely
• growth inhibitors are made in stem nodes, ripe fruit and decaying leaves
• ethene is a growth inhibitors that causes fruits to ripen, stimulates leaves to fall in autumn and causes ageing in plants - commonly used commercially to ripen bananas
• commercial use of plant growth regulators - weed killers eg. roundup
• anatomical protective features - thorns stop animals from eating plants, some plants have flowers that open during the day when pollinators like bees are more active
• chemical protective features - excessive heat - help to maintain their shape, some plants to produce stress proteins - spread of mirco-organism