Biology_Today_-_October_2016

Homoeostasis
s Phenomenon of maintaining constant internal environment despite changes
in external temperature is called homoeostasis.

s Endothermal animals show homoeostasis 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. Water

s Water is an important factor which influences life of organisms. Productivity and distribution of land plants depend on availability
of water. Rain and precipitation determine vegetation of an area.

Xerophytes

Plants of dry habitats which are faced with the problem

of more water loss through transpiration than is the

water available from soil, e.g., Acacia, Casuarina, etc.

On the basis of moisture availability

Mesophytes

Plants of moist habitats with luxuriant vegetative

growth. Spines and thorns are absent. Most crop

plants, vegetable and fruit plants are mesophytes.

Hygrophytes

Plants of wet areas with soft stems of moderate

height, large thin leaves with hydathodes for

excreting excess water in guttation.

Terrestrial plants

Aquatic Animals

In rivers and streams, animals obtain

most of their food from organic materials

coming from outside the water.

Desert Animals

Animals of arid areas either conserve water or depend on food as major source

of water. Some of these animals eat only seeds. E.g., Kangaroo rat. Desert

animals conserve water by producing solid faeces and excreting solid urine.

Animals

Stenohaline

Organisms restricted to narrow range

of salinity tolerance. E.g., Shark

Euryhaline

Organisms tolerant to wide

range of salinity. E.g., Salmon.

Types of organisms based on salinity tolerance

3. Light

s Various life processes are influenced by intensity, duration and quality of light.
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 enchanced 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),
e.g., butterflies, most birds, most mammals. (ii)
Nocturnal (active during night), e.g., rat, owl,
cockroach.

Growth

It is favoured by increased availability of food, moderate light intensity and red

light. Blue light favours moderate but normal growth. High light intensity reduces

growth but increases development of mechanical tissues.

Movement

Small photosynthetic organisms show positive phototaxis in moderate light, e.g.,

Chlamydomonas, Euglena, Volvox. Plant shoots bend towards the source of light.

It is positive phototropism. Flowers of some plants open or close in response

to light and darkness. The phenomenon is called photonasty. Nyctinasty is

folding of leaves in response to darkness. Planaria and earthworm generally

show negative phototaxis.

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, vegetative growth.

Germination

A number of seeds are sensitive to light. They are called photoblastic seeds.

Positively photoblastic seeds germinate only in the presence of light, e.g.,

Viscum, Lactuca, Rumex. Negatively photoblastic seeds do not germinate in

presence of light, e.g., onion, tomato.

Life Processes