56 MTBIOLOGY TODAY | JULY ‘15Advantages of cross pollination
• The advantages for cross pollination are:
- Cross pollination introduces genetic recombinations
and hence variations in the progeny. - Cross pollination increases the adaptability of the
offspring towards changes in the environment. - The plants produced are usually larger and the offspring
have characters better than the parents due to the
phenomenon of hybrid vigour. - The plants produced are more resistant to diseases.
- New and more useful varieties can be produced through
cross pollination. - The defective characters of the race are eliminated and
replaced by better characters.
Disadvantages of cross pollination
• The disadvantages of cross pollination are:
- It is highly wasteful because plants have to produce
a large number of pollen grains and other accessory
structures in order to suit the various pollinating
agencies. - Some undesirable characters may creep in the race.
- The very good characters of the race are likely to be
spoiled. - As external agency is involved so the chance factor
is always there.
agents of pollination
• The transfer of pollen grains from plant to plant in cross
pollination takes place through biotic agencies (living
agencies) or through abiotic agencies (non-living
agencies).
Anemophily
• The pollination taking place by means of wind is known as
anemophily. E.g., Zea mays (maize), grasses, mulberry,
date palm, Salix, Cannabis etc.
• Main features of wind pollinated flowers are :
- Non-essential whorls such as calyx, corolla, bracts and
bracteoles are not showy. - Flowers are devoid of scent and nectar.
- In case of unisexual flowers, the male flowers are more
in number than female flowers and in case of bisexual
flowers, the stamens are generally numerous. - Both the stigmas and anthers are exserted.
- Plants produce a very large quantity of dusty pollen
grains. - Pollen grains are dry, very small sized, non-sticky and
unwettable, so that they can be carried by wind to
long distances. - Stigma is hairy, feathery (grasses) or branched to catch
the wind borne pollens. - Anemophily is associated with reduction in the number
of ovules per ovary. Biological justification for this
reduction is that chances for windborne pollen reaching
the right stigma are remote. If the flowers of wind-
pollinated plants were to produce many ovules most
of them would have remained unfertilized. In grasses
there is just one ovule per ovary.
- In case of unisexual flowers, the male flowers are more
Hydrophily
• Pollination which occurs with the help of water is called
hydrophily.
• Main features of water pollinated flowers are :- Inconspicuous and small flowers.
- Odour, nectar and colour is absent in flowers.
- Flower whorls, if present then coated by wax.
- Pollen grains are small, light weighted, non-sticky and
coated by wax. - Stigma is unwettable but sticky.
• Hydrophily is of two types— hypohydrophily and
epihydrophily.
• Hypohydrophily occurs below the surface of water, e.g.,
Ceratophyllum, Najas, Zostera.
• Zostera is submerged marine perennial plant where the
pollens are exceptionally long (upto 250 mm) and needle-
like. They look more like pollen tubes. The specific gravity
of these pollens is almost the same as that of sea water.
Consequently, they can freely float in water at any depth.
The stigma is also very long in this plant. If a pollen comes
in contact with the stigma, it coils around the latter.
• Epihydrophily takes place over the surface of water, e.g.,
Potamogeton, Vallisneria, Myriophyllum, Ruppia maritima.
• Vallisneria (ribbon weed) is a classic example of this type. It
is a submerged dioecious plant. The flowers are borne under
water. Upon attaining maturity the male flowers get detached
from the parent plant and float on the surface of water. The
anthers project beyond the margins of the sepals.
• The pistillate flowers also develop under water. At the time
of pollination they are brought to the surface by thin, long