Tropisms and Growth

Accelerated Biology

2

Tropisms

• Stimulus – an environmental condition that causes a plant response

• Tropism – growth of plant in a certain way in response to a stimulus

Phototropism• Phototropism – growth toward (+) or away from (–) lightPhototropism – growth toward (+) or away from (–) light

• Plant shoots display positive phototropismPlant shoots display positive phototropism• Plant roots display negative phototropismPlant roots display negative phototropism

Gravitropism• Gravitropism – growth toward (+) or away from (–) gravity

– plant shoots display negative gravitropism– Plant roots display positive gravitropism– seedling shoot grows upwards and roots grow downwards, no

matter how seed landed!

Thigmotropism• Thigmotropism – response to physical contact

– plant shoots can coil or grasp and use objects for support while growing toward light• Ex) morning glories, clematis

– this is also how the Venus fly trap closes

Plant Hormones• Auxin – a plant hormone that causes cell walls to become more

flexible which allows cells to elongate as they grow– Auxin accumulates on the dark side of the plant– cells on the dark side elongate more than cells on the light side– causes stem to grow towards the light

Plant Growth• Meristems – a region of

undifferentiated plant cells that are capable of dividing and developing into specialized plant tissue

Plant Growth• Primary Growth

– Apical meristems – located at the tips of the stems and roots

– stems and roots become longer, without becoming wider

– like stacking plates one on top of another, the pile will only get taller, but never wider

Plant Growth• Secondary Growth

– most dramatic in woody plants

– occurs in two meristems• cork cambium –

produces cork cells• vascular cambium –

produces vascular tissue– stem grows in diameter– result = annual rings

Plant Growth• What will this carving look like

50 years from now?• Discuss with your partner.