Pollination and Fertilisation
Gynoecium and Formation of Female Gametophyte
Female reproduc ve part of a flower is called gynoecium.
Each pis l consists of three parts viz.
S gma: receives pollen grains
Style: part below s gma
Ovary: basal part that contains placenta
Structure of Megasporangium and Megasporogenesis
Megasporangium
It is commonly known as ovule.
Structure of a typical megasporangium
Nucellus is a nutritive tissue; micropyle is the opening of the ovule
from where the pollen tube enters
Embryo sac is located inside the nucellus. It is also known as the
female gametophyte.
Megasporogenesis is the process of formation of the megaspore from the
megaspore mother cell by the process of meiosis.
After megasporogenesis, four haploid megaspores are formed. Out of the four
megaspores, one functional megaspore develops into the female gametophyte
or embryo sac.
A typical megaspore embryo sac is a 7-celled and an 8-nucleate structure, as
shown below:
Pollination
It is the process of transfer of pollen grains from the anther to the stigma of the
pistil.
Pollination is divided into three types:
Autogamy: Pollination that occurs within the same flower
Geitonogamy: Pollination that occurs between two flowers of the same
plant
Xenogamy: Pollination that occurs between the flowers of different
plants. Xenogamy brings genetic variation.
Self pollination
Homogamy and cleistogamy are two contrivances for self pollination.
Cleistogamy– Flowers are closed; therefore, the stigma and the anther are not
exposed to be pollinated by pollens from different flower.
Homogamy- Anther and stigma of the bisexual flower mature at same time
Self pollination is necessary to ensure the continuity of the race and to preserve
parental characteristics. But repeated self pollination leads to loss of vigour in
plants.
Cross pollination
Unisexuality, self sterility, dichogamy, herkogamy, heterostyly are some
of the contrivances for cross pollination to take place.
Dichogamy means that stamens and carpels mature at different times.
Protogyny means gynoecium maturing earlier and protoandry means
androecium maturing earlier.
Herkogamy means some sort of barrier is introduced between stamen
and pistil in bisexual flower so that self pollination is prevented.
Heterostyly means that flowers are borne in different styles or forms.
Cross pollination leads to a healthier and viable offspring and positive
variations are introduced in the progeny. Disadvantages of the process
include wastage of pollen in course of transfer from one flower to other,
and dependence on pollinators.
Agents of cross pollination
Wind: Flowers pollinated are small and lack bright colour; pollen grains
are small, dry and light in weight and occur in abundance; example,
grass. Pollination by wind is called anemophily.
Water: Flowers pollinated are not colourful and emerge above the water
level; example, Vallisneria, Hydrilla. Pollination by water is called
hydrophily.
Animals: Flowers pollinated are brightly coloured and contain large
amounts of nectar to attract the pollinators; example, Yucca .Pollination
by insects is called entomophily
Pollen–pistil interaction leads to the acceptance or the rejection of a pollen by
the stigma.
Fertilisation
The fertilisation in an angiosperm is called double fertilisation as it involves
two steps.
Syngamy: Fusion of one male gamete with an egg cell to form a zygote
Triple fusion: Fusion of another male gamete with two polar nuclei
located inside the central cell of the embryo sac to form a triploid
primary endosperm nucleus
After Fertilization,
Fertilized ovule forms seed.
Floral parts such as sepals, petals, stamens, style, and stigma fall off.
Ovary grows, enlarges, and ripens to become fruit.
Fruits can be fleshy and juicy (examples – apples, mangoes) or dry and hard
(example − nuts or peas).
Fruit – Fruit is the mature ovary.
It has two main parts pericarp (It further contains outer hard epicarp, fleshy,
edible mesocarp and innermost endocarp).
Endocarp is the part that covers the seed.
Fruit helps to protect the plant from animals or extreme climatic conditions.
It also helps in seed dispersal and performs the function of storage of food.
Seed – Seed is the mature ovule.
Outer covering of seed is called seed coat.
A seed is made up of one or two cotyledons and a seed axis.
Plumule of the axis develops into shoot and radicle into root.
Under proper conditions seed germinates to form a new plant.
