Plants in Space

• The shaded area has more auxin than the unshaded area because it receives less light.
• Phototropism is the growth of plants towards light, which occurs due to the presence of auxin on one side of the plant.
• • By growth of certain cells - this is called a TROPISM response. 
• By change in TURGOR of certain cells. - called a NASTIC response.
• Tropisms: long term, slow growth of the plant towards/Away from a stimulus - results in permanent changes to size of some cells
• Nastic response: occur more rapidly that tropisms and reversible. Rely changes in turgor pressure (exerted by water on cell walls) with certain plant cells
• Growth at the meristem in three stages:

  Zone of cell division

  Zone of cell elongation

  Zone of differentiation
• Zone of cell division: cells multiply thorugh mitosis
• Zone of cell elongation: cells growth in both length and width as water fills the vacuole and the elastic cell stretches
• Zone of differentiation: elongated cells become specialised.

  May become leaves in upper meristem - at roots they may become root hairs
• Chane in seasons can affect plants in different ways

  Dormancy

  Vernalisation

  Abscission
• Dormancy: A state of dormant growth in which a plant is not actively growing but is still able to photosynthesise
• Vernalisation: the need for a period of cold to trigger flowering in plants
• Abscission: The process by which a plant removes its leaves from the stem. - response to cold season
• Different Phytohormones:

  auxins

  cytokinins

  gibberellins

  Abscisic Acid

  Ethylene
• Cytokinins: produced in roots, stimulate cell division and delay senescence (ageing)
• Auxins: produced in shoot tips, promote cell division and elongation (tallest part of plant) - inhibit abscission
• Abscisic acid: A plant hormone that inhibits the growth of plants.
• Gibberellin: A plant hormone that stimulates the growth of shoots and roots. - break dormancy and initiate germination
• Ethylene: A hormone that stimulates the ripening of fruits and vegetables
• TROPISMS are plant growth responses to external STIMULI in which the stimulus direction determines the direction of the growth response. 

  Tropisms are named depending on the stimulus involved, and are identified as POSITIVE (towards the stimulus) or NEGATIVE (away from the stimulus). 

  Tropisms act to position the plant in the most favourable growth environment.