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MTBIOLOGY TODAY | JULY   ‘15 55

pollination

• The transfer of pollen grains from anther to the stigma is

called pollination.

• Pollination in angiosperms generally takes place at 2-celled
stage (rarely 3-celled stage) of microspores or pollen, i.e.,
pollen having vegetative cell and generative cell.

• If the pollen grains are transferred to the micropyle of the
ovule directly, the pollination is called direct pollination,
e.g., gymnosperms. Since the ovules are enclosed in the
ovary in angiosperms, the pollination is called indirect
pollination.

• Pollination is of two types – self pollination and cross
pollination.

Self pollination

• The transfer of pollen grains from anther to stigma of

same flower or genetically similar flower is called self

pollination. Self pollination is of two types – autogamy
and geitonogamy.

• Autogamy is the transfer of pollen grains from anther to

stigma of same flower.

• Geitonogamy is the transfer of pollen grains from the anther

of one flower to the stigma of another flower present in

the same inflorescence or in the same plant.

• Geitonogamy occurs between bisexual flowers or unisexual

flowers of the same plant.

• Geitonogamy is genetically equivalent to self pollination
but ecologically it is cross pollination.

•    Flowers    show    many    contrivances    (devices)   to  ensure  self    

pollination such     as:

• In some plant, flowers are bisexual and both sexes
  
  mature at the same time (homogamy).


• In some cases, flowers are bisexual and cleistogamous,
  i.e., remain closed (e.g., Commelina benghalensis).


• In some cases, pollination occurs in bud condition
  
  before the opening (anthesis) of flower, (e.g., pea,
  
  wheat, etc.)

Advantages of self pollination

•    The     advantages of   self    pollination    are:

• It maintains the parental characters or purity of the
  
  race indefinitely.


• Self pollination is used to maintain pure lines for
  hybridisation experiments.


• The plant does not need to produce large number of
  
  pollen grains.


• Flowers need not to develop devices for attracting
  
  insect pollinators.


• It ensures seed production.


• Self pollination eliminates some bad recessive
  characters.
  Disadvantages of self pollination
  • The disadvantages of self pollination are:


• New useful characters are seldom introduced.


• Vigour and vitality of the race decreases with prolonged
  
  self pollination.


• Immunity to diseases decreases.


• Variability and hence adaptability to changed
  environment is reduced.
  cross pollination
  • Cross pollination (or allogamy or xenogamy) is the
  
  process when pollens are transferred from anther of one
  flower to stigma of flower of a genetically different plant
  
  of same species.
  • Dichogamy ( flowers maturing at different times),
  dicliny or unisexuality, herkogamy (presence of natural
  and physical barrier between androecium and gynoecium),
  heterostyly and self sterility are the main reasons or
  
  adaptations for cross pollination in bisexual flowers.

1. What are the functions of tapetum?


2. For its pollination, the orchid Ophrys speculum has picked up one of the most selective attraction. How
   is this insect-plant relationship beneficial only to the plant?


3. How do synergids help in the nutrition of embryo sac?