Biology Today - February 2018

Thermoperiodicity

Regular change in body

temperature that occurs at

specific time interval.

Diurnal thermoperiodicity

Temperature is high during the day and low during night. High day temperature favours photosynthesis while lower

night temperature stimulates growth and storage in plants. It promotes seed germination in many plants and determines

the period of animal acitivity. For example, desert animals live in burrows, during the hot daytime.

Seasonal thermoperiodicity

Different seasons of year have different temperatures. They favour different aspects of plant and animal life or phenology.

Seasonal thermoperiodicity, therefore, controls flowering, fruiting, fruit dispersal, leaf shedding, leaf bearing, seed

germination, etc., of plants. In animals, it determines growth, reproduction, development, colouration and morphology.

Homeostasis

• The phenomenon of maintaining constant internal environment despite
  changes in external temperature is called homeostasis.


• Endothermal animals show homeostasis by:
  
  
  • Retaining heat produced by metabolic reactions.
  
  
  • Having an insulating coat in the form of thick skin, scales,
    hairs, feathers and subcutaneous fat.
  
  
  • Changing cutaneous circulation, constricting superficial blood
    vessels in cold and dilating them in hot weather.
  
  
  • Migration to warmer areas in winter and cooler areas in summer.
  
  
  
  

2. Light

• Various life processes are influenced by intensity, duration and quality of light.

Germination

• Most of seeds are sensitive to light. They are called
  photoblastic seeds. Positively photoblastic seeds
  germinate only in the presence of light, HJ9LVFXP
  /DFWXFDand5XPH[. Negatively photoblastic seeds do not
  germinate in presence of lightHJ, onion, tomato, etc.

Movement

• Small photosynthetic organisms show positive phototaxis
  in moderate light, HJ&KODP\GRPRQDV(XJOHQD9ROYR[,
  etc. In positive phototropism, plant shoots bend towards
  the source of light. Flowers of some plants open or close
  in response to light and darkness known as photonasty.
  Nyctinasty is folding of leaves in response to darkness.
  3ODQDULD and earthworm generally show negative phototaxis.

Life Processes

Growth

• It is favoured by increased availability of food, moderate
  light intensity and red light. Blue light favours moderate
  and normal growth. High light intensity reduces growth but
  increases development of mechanical tissues.

Photosynthesis

• The amount of photosynthesis depends upon the
  quality, intensity and duration of light. Photosynthetic
  yield is maximum at equator and tropical areas.

Transpiration

• Stomata generally open in light and close in darkness,
  because of it light promotes transpiration. Transpiration
  is further enhanced by heating effect of light.

Pigmentation

• Animals develop dark colour in dim light and light
  colour in bright light. In humans, prolonged exposure
  to light causes tanning or darkening of skin. Some
  animals show seasonal colour changes.

Daily rhythm

• Most animals are active during a particular period of
  the day. (i) Diurnal : Active during the daytime, HJ,
  butterflies, most birds, most mammals. (ii) Nocturnal:
  Active during night, HJ, rat, owl, cockroach.
  (iii) Auroral : Active at dawn or early morning, HJ,
  %XEDOFXV. (iv) Vesperal : Active at the time of dusk
  or sunset, HJ, Rabbit. (v) Crepuscular : Active during
  the dawn and dusk HJ, fireflies and deer.

Photoperiodism

• Response of organism to number and duration of day
  lengths. Organisms show three types of response to light
  duration - short day, long day and day neutral. Seasonal
  activities of organisms controlled by photoperiodism are
  bird migration, hibernation, flowering and vegetative growth.