The fusion of male gamete with female gamete is called fertilisation. First of all, fertilisation was discovered by Strasburger (1884) in the Monotrapa plant.Fertilisation in Flowering Plant
This process is completed in the following steps:
(i) Porogamy: In this, pollen tube enters into the ovule through the micropyle. It is known as porogamy. It is found in most of the Angiosperms (Capsella).
(ii) Chalazogamy: In this method, the pollen tube enters into the ovule through the chalaza. This method is discovered in Casuarina by Treub (1891).
Examples: Betula and Juglans (walnut).
(iii) Mesogamy: In this method, pollen tube enters into the ovule either through integuments - Cucurbita or through the funiculus - Pistacia and Populus.
Table: Differences between Pollination and Fertilisation
“Double fertilisation is a complex process which involves the fusion of one female gametophyte with two male gametes”
Double fertilisation is a chief trait of flowering plants. In the phenomena, one female gamete unites with two male gametes. One of the male gametes fertilizes the egg resulting in the formation of a zygote and the other unites with 2 polar nuclei for the formation of an endosperm.
Double fertilisation provides stimulus to the plant resulting in the ovarian development to fruits and development of ovules into the seed. When the haploid male gametes and female gametes fuse, the diploid state of the plant is restored.
The process of double fertilisation is explained below:
Double Fertilisation in Angiosperms
Angiosperms are flower-bearing plants and are the most diverse group of terrestrial plants. The flowers form the reproductive part of angiosperms with separate male and female reproductive organs. Each contains gametes – sperm and egg cells, respectively.
Pollination helps the pollen grains to reach stigma via style. The two sperm cells enter the ovule-synergid cell. This proceeds to fertilisation.
In angiosperms, fertilisation results in two structures, namely, zygote and endosperm, hence the name “double fertilisation.”
Double fertilisation is a complex process where out of two sperm cells, one fuses with the egg cell and the other fuses with two polar nuclei which result in a diploid (2n) zygote and a triploid (3n) primary endosperm nucleus (PEN) respectively.
Since endosperm is a product of the fusion of three haploid nuclei, it is called triple fusion. Eventually, the primary endosperm nucleus develops into the primary endosperm cell (PEC) and then into the endosperm.
The zygote becomes an embryo after numerous cell divisions.
Development of Embryo in Angiosperms
Once fertilisation is completed, embryonic development starts and no more sperms can enter the ovary. The fertilized ovule develops into a seed, and ovary tissues develop a fleshy fruit which encloses the seed.
After fertilisation, the zygote divides into the upper terminal cell and lower basal cell. The basal cell develops into suspensor, which helps in the transport of nutrients to the growing embryo. The terminal cell develops into pro-embryo.
Following are the different stages involved in the development of an embryo.
Stages of Embryonic Development In Angiosperms
The significance of double fertilisation is as follows:
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