1. 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

3. Phototropism Phototropism – growth toward (+) or away from (–) light
Plant shoots display positive phototropism
Plant roots display negative phototropism

4. 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!

5. 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

6. 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

7. Plant Growth
Meristems – a region of undifferentiated plant cells that are capable of dividing and developing into specialized plant tissue

8. 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

9. 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

10. Plant Growth What will this carving look like 50 years from now?
Discuss with your partner.