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57 MTBIOLOGY TODAY | JULY    ‘15 57

and     slender stalk.  Due     to  its     weight  a   small   cup-shaped  

depression  is   formed around  the  female flower. If   male   

flowers,    floating    on  the     surface,    come    close   to  the     female  

flowers they     tumble down    into     the     depression.

• The anthers strike the exposed stigma, and the pollen mass

is transferred to it. After pollination the stalk of the pistillate

flower undergoes spiral torsion. The coils draw closer and
tighten, bringing the pollinated flowers under water once

again. The fruits mature in submerged condition.

Entomophily

• Pollination by insects is called entomophily. Many insects
such as bees, wasps, ants, hover bees, moths, butterflies,
grasshopper, locusts, beetles, dragon flies, bugs, thrips are
involved in pollination.

• Main features of insect-pollinated flowers are :

• Nectar is produced by nectar glands of flower which
  
  attracts the pollinators for feeding.


• Some flowers produce edible pollen grains which
  
  attract the pollinators to visit the flowers e.g., Rosa,
  
  Clematis, Magnolia.


• Flowers are fragrant and emit scent and odour e.g.,
  
  Jasminum, Cestrum etc.


• Flowers are bright coloured, they have coloured
  
  leaves, petals, sepals, stamens or sometimes stigma,
  e.g., leaves in Euphorbia pulcherrima, bracts in
  Bougainvillea.


• Small flowers occur in groups and thus,
  become more conspicuous.


• Stigma also secretes some
  
  exudates which make
  stigma sticky. This
  
  sticky stigma
  can get pollens
  easily.


• The pollen
  
  grains are
  
  spiny, heavy
  
  and surrounded
  
  by a yellow oily
  
  sticky substance
  
  called pollenkit.
  
  
  • To sustain animal visits, the flowers have to provide
    rewards to the animals.Nectar and pollen grains
    are the usual floral rewards. In some species floral
    
    rewards are in providing safe places to lay eggs; e.g.,
    
    Yucca, Amorphophallus.
    • Insect pollinated plants show different mechanisms for
    pollination such as:
  
  
  
  


• In Salvia (sage plant), a member of family labiatae,
  pollination occurs by bees and there is a special
  
  mechanism called “turn pipe mechanism” or “lever
  mechanism” of pollination.


• ‘Fly trap mechanism’ of pollination occurs in
  Aristolochia. Flies are attracted by foul-rotten tobacco
  like odour.


• ‘Trap door mechanism’ of pollination occurs in Ficus
  species (having hypanthodium inflorescence).


• In orchid (Ophrys speculum) pollination occurs by
  act of pseudocopulation by the wasp called Colpa.
  
  Appearance and odour of Ophrys flower is similar to
  
  female wasp and hence male wasps are mistaken and
  they land on Ophrys flowers to perform act of pseudo-
  copulation and thus pollination takes place. This plant-
  insect relationship is useful only to plant.


• The plant Yucca has developed an obligate symbiotic
  
  relationship with the moth Tageticula, both the species,
  moth and the plant, cannot complete their life cycles
  without each other. The moth deposits its eggs in
  
  the locule of the ovary and the flower, in turn, gets
  
  pollinated by the moth. The larvae of the moth come
  
  out of the eggs as the seeds starts developing.

Ornithophily

•    Pollination    of   flowers    by   birds  is   called     ornithophily.

Ornithophilous  flowers have     tubular    (Nicotiana glauca), 

cup-shaped  (Callistemon)   or  urn-shaped  (some   members 

of   Ericaceae) corollas.

•        Ornithophilous flowers are 

large    sized, brightly    coloured,

usually odourless   and     they    

produce a    large   amount

of mucilagenous 

nectar  for  drinking

by   birds. E. g. ,

Strelitzia reginae

is pollinated    by 

sun  birds,     Bignonia

by   humming    birds,

Aloe vera   by   sun     bird   

and Salmelia    by   crow    and    

mainas.