Question

The production of the mature female gametophyte (embryo sac) via the process of megagametogenesis is unique to Angiosperms.

The process of double fertilization is also a defining Angiosperm characteristic. Describe double fertilization that occurs in the mature embryo sac and indicate the results of such fertilization .

And finally, name and describe two advantages that the Angiosperms enjoy over the Gymnosperms (and everyone else) that has allowed them to conquer the plant world! (2@5 marks each).

Answer 1

The angiospermic megagametophyle takes part in double fertilization

Normally Megagametophyte contains-

2 ends, Micropile and Chalazal end

Micropylar end contains 3 cells- 2 synergids and one egg cell

The chalazal end contains 3 antipodal cells

2 polar nuclei are located centrally in megagametophyte

Microgametophyte-

A haploid microgametophyte is produced in anther by meiosis. Later on the haploid microgametophyte undergoes meiosis and forms 2 cells (generative cell and tube cell). Generative cell again undergoes mitosis and produces two sperm cells.

Tube cell helps in emergence of pollen tube once the pollen falls on stigma.

Pollen tubes enters the megagametophyte through micropyle. Synergids direct the pollen tube towards egg cell.

One sperm cell fuses with egg and forms diploid zygote

The second sperm cell fuses with 2 polar nuclei and forms triploid endosperm.

Since, two sperm cells fuse one by one with egg cells and polar nuclei. This process is called double fertilization.

Unlike Gymnosperms in case of angiosperms endosperm formation only happens after fertilization. So, it saves the plant regarding wasteful; production of endosperm before fertilization happens (Gymnosperms). Because in case of gymnosperms endosperm is formed brfore hand (haploid) and it fertilization fails to happen then the rich endosperm remains unutilized. Angiosperms made it sure by double fertilization that endosperm (triploid) will only be formed after fertilization. So, it is an evolutionary advantage in terms of cellular economy.

Angiosperms have several advantages over other classes plants-

1. Angiosperms are the only class which produce flowers. Flowers help in cross pollination by attracting different pollinators by offering them nectar. Cross pollination ensures production of better recombinants and checks inbreeding depression.

2. In angiosperms ovule is covered inside walls (Ovary ). After fertilization it produces fruit.

Ovary is converted to fruit

Ovule is converted to seed..

Fruits attract different animals and birds. Different organisms eat fruit and release the seed. Thereby, seed dispersal happens. Production of fruit is another key aspect of angiosperms which ensures seed dispersal to very distant places

.

Gymnosperms and other neither produce floewer nor fruit.