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• Plants can also detect stimuli and respond to them, however their responses do not involve the movement of the whole body, only certain parts of the plants are involved
• Plants respond to four main kinds of stimuli: light, gravity, water and touch
• Some plants also show responses to stimuli such as touch, temperature and chemicals
• Tropisms


  Directional growth responses of plants towards external stimuli
• Tropisms


  • Phototropism
  • Geotropism
  • Hydrotropism
  • Thigmotropism
• Positive tropism


  The part of a plant grows towards the stimulus
• Negative tropism


  The part of a plant grows away from the stimulus
• Phototropism


  • Shoots of plants show positive phototropism, growing towards the direction of light
  • Roots grow away from light, showing negative phototropism
• Geotropism


  • Roots of plants show positive geotropism, growing downwards towards gravity
  • Shoots of plants show negative geotropism, growing upwards away from gravity
• Hydrotropism


  • Roots of plants show positive hydrotropism, growing towards water
  • Shoots grow away from water, showing negative hydrotropism
• Thigmotropism


  • Tendrils or twining stems show positive thigmotropism, clinging onto objects
  • Roots show negative thigmotropism, avoiding objects that obstruct their search for water
• Nastic movements


  Non-directional movements of plant parts in response to external stimuli
• Nastic movements


  • Photonastic
  • Seismonastic
  • Thermonastic
• Photonastic


  • Response to light, e.g. morning glory blooming with sunlight, sunflower blooming towards sun
• Seismonastic


  • Response to touch, e.g. mimosa pudica folding leaves when touched, venus fly trap closing to trap insects
• Thermonastic


  • Response to temperature, e.g. tulip petals blooming in high surrounding temperature in spring
• Stimuli and responses in humans


  The ability of an organism to detect a stimulus through touching, smelling, tasting, hearing and seeing
• Sensory organs


  • Eyes
  • Ears
  • Nose
  • Tongue
  • Skin
• Certain sensory organs are sensitive to certain types of stimuli
• Eyes are only sensitive to light but not to sound
• Human sensory organs and the stimuli they detect


  • Eyes - light
  • Ears - sound and balance
  • Nose - chemicals in air
  • Tongue - chemicals in food
  • Skin - touch
• Eye


  Front view has sclera, iris and pupil
• Parts of the human eye and their functions


  • Conjunctiva - transparent membrane protecting front of sclera
  • Aqueous humor - transparent fluid maintaining eye shape and focusing light
  • Pupil - opening in center of iris controlling light entry
  • Iris - colored part controlling pupil size
  • Cornea - transparent layer refracting and focusing light onto retina
  • Suspensory ligaments - holding lens in position
  • Ciliary muscle - changing lens thickness
  • Lens - transparent, elastic, convex focusing light onto retina
  • Sclera - strong layer maintaining eye shape and protecting
  • Choroid - black layer preventing light reflection, supplying oxygen and nutrients
  • Retina - layer with photoreceptors detecting light and producing nerve impulses
  • Optic nerve - carrying nerve impulses from retina to brain
  • Blind spot - part of retina without photoreceptors
  • Vitreous humor - transparent gel maintaining eye shape and focusing light
• Photoreceptors in retina


  Rod cells - sensitive to light intensity

  Cone cells - sensitive to light color
• Blinking spreads tears to keep eyes moist and clean, and tears contain chemicals to kill bacteria
• Stereoscopic vision


  Vision that uses both eyes at the same time
• Monocular vision


  Vision that uses both eyes but separately
• Animals with stereoscopic vision


  • Humans
  • Cats
  • Owls
• Animals with monocular vision


  • Rats
  • Chickens
  • Rabbits
• Animals with stereoscopic vision


  Have a narrow field of vision but their fields of vision overlap, producing 3D vision
• Animals with monocular vision


  Have a wide field of vision but their fields of vision do not overlap or overlap only slightly, producing 2D vision
• 3D vision

  Allows animals to accurately estimate the distance, size and depth of objects
• 2D vision

  Prevents animals from accurately estimating the distance, size and depth of objects
• Predators have stereoscopic vision

  Helps them to accurately estimate distance to capture prey
• Prey have monocular vision

  Allows them to detect predators approaching from any direction
• Normal vision


  Thickness of the lenses changes so that images of distant and near objects are focused exactly on the retina, resulting in very sharp and clear images
• Defective vision


  Light rays are not focused exactly on the retina, resulting in blurred images
• Most common defects of vision


  • Short-sightedness
  • Long-sightedness
  • Astigmatism
• Short-sightedness (Myopia)


  • Eye lenses are too thick or eyeball is too long, causing light from distant objects to be focused in front of the retina
  • Person can see near objects clearly but distant objects are blurry
• Correction for short-sightedness


  Wearing glasses with concave lenses to diverge the light before it enters the eyes, allowing it to be focused exactly on the retina