Ans. Sexual Reproduction in Flowering Plants
In angiosperms, parent plant is diploid sporophyte generation:They produce flowers which have the reproductive structures of the plant
Structure of Flower
Flower is the reproductive structure in this generation. The flower components are arranged in the form of whorls. The flower is-actually a condensed shoot with the nodes present very close to each other The different parts of the flower are attached to the nodes All the structures present at one node are collectively called the
Petal
Reproduction
whorl. The outer two whorls in a flower, (calyx and corolla) are the non-reproductive whorls while the inner two whorls, (androecium and gynoecium)are the reproductive whorls. Figure
i) Calyx
It is the outermost whorl. It is usually green in colour. Its individual units (leaflets) are called sepals.
Function: Sepals protect the inner whorls at bud stage.
ii)Corolla is the next inner whorl and is often coloured brightly. Its individual units (leaflets) are called petals.
Function: They *erve to attract bees; birds, etc. which are the agents of pollination.
Hi) Androecium
It 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 (pollen grains) are produced through meiosis. Each microspore germinates into the male gametophyte generation. During this, the nucleus of microspore undergoes mitosis and produces two nuclei i.e. a tube nucleus and a generative nucleus. The generative nucleus again undergoes mitosis and produces two sperms. All these structures are the male gametophyte generation of plant.
iv) Gynoecium
Fourth whorl i.e. gynoecium is the female reproductive part of flower. Its units are called carpels (or pistils). Each carpel is made up of the basal ovary, middle sytle and upper stigma. Inside ovary, there are one to many ovules. Inside each ovule, one haploid macrospore is produced through meiosis. Macrospore germinates into the femate gametophyte generation. During this, macrospore undergoes mitosis and produces an egg cell and some associated, structures (e.g. fusion nucleus). Egg cell and associated structures are the female gametophyte generation of plant.
Pollination
When pollen grains mature, they are transferred to stigma. It is called pollination.
Double Fertilization
On reaching the stigma, the tube nucleus of pollen grain constructs 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.
Formation Of Seeds and Fruits
Zygote develops into embryo and endosperm nucleus develops into endosperm tissue (food of the growing embryo). Ovule changes into fruit. When seeds mature, they are dispersed. If seeds get suitable conditions, their embryos develop into new plants (the diploid saprophytes of the next generation). Figure 14.13