Flowers

Sexual Reproduction in Flowering Plants:

In angiosperms, parent plant is diploid sporophyte generation. Flower is the reproductive structure in this generation. The flower components are arranged in the form of whorls. The outer two whorls in a flower are the non-reproductive whorls while the inner two whorls are the reproductive whorls.

Calyx is the outermost whorl. It is usually green in colour. Its individual units are called sepals. Sepals protect the inner whorls at bud stage. Corolla is the next inner whorl and is often coloured brightly. Its individual units are called petals. They serve to attract bees, birds, etc. Which are the agents of pollination.

Third whorl i.e. androecium is the male reproductive part of flower. Its units are called stamens. Each stamen has a thread-like filament at the free end of which anther is attached. Anther has pollen-sacs in which haploid microspores are produced through meiosis. Each microspore germinates into the male gametophyte nuclei. The  generative nucleus again nucleus and two sperms. All these structures are the male gametophyte generation of plant.

Fourth whorl i.e. gynaecium is the female reproductive part of flower. Its units are called carpel’s. Each carpel is made up of the basal ovary, middle style and upper stigma. Inside ovary, there are one two many ovules. Inside each ovule, one haploid macrospore is produced through meiosis. Macrospore germinates into the female gametophyte generation. During it, macrospore undergoes mitosis and produces an egg cell and some associated structures. Egg cell and associated structures are the female gametophyte generation of plant.

When pollen grains mature, they are transferred to stigma. It is called pollination. On reaching the stigma, the tube nucleus of pollen grain constructs a pollen tube. The pollen tube contains a tube nucleus and two sperms. The tube grows through style and ovary and enters ovule. Here, it bursts and releases the sperms. Both sperms enter the female gametophyte. One sperm fuses with egg and forms a diploid zygote. The other sperm fuses with diploid fusion nucleus and forms a triploid (3N) nucleus called endosperm nucleus. Since the process of fertilization involves two fusions, it is called double fertilization.

Zygote develops into embryo and endosperm nucleus develops into endosperm tissue. Ovule then becomes seed and ovary changes into fruit. When seeds mature, they are dispersed. If seeds get suitable conditions, their embryos develop into new plants.