Unit 4
Embryology
Chapter 4.1
EmbryologyClass 12 Biology – Embryology Notes PDF
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Introduction
The chapter connects flower reproduction with pollen formation, embryo-sac development, pollination, double fertilization, endosperm formation and embryo development.
1. Microsporogenesis and Pollen Grain
Inside a young anther, pollen mother cells undergo meiosis and produce haploid microspores. This process is called microsporogenesis. Each microspore develops into a pollen grain or male gametophyte.
Structure of Pollen Grain
- Outer wall: exine, usually thick and resistant.
- Inner wall: intine, thinner and delicate.
- Germ pores are regions where exine is thin or absent.
- The mature pollen grain contains a vegetative/tube cell and a generative cell; the generative cell forms male gametes.
Fig. 1 – T.S. of Anther
Fig. 2 – Structure of Pollen Grain
2. Megasporogenesis and Embryo Sac
The megaspore mother cell inside the ovule undergoes meiosis to form megaspores. Usually one functional megaspore survives and develops into the female gametophyte or embryo sac.
Fig. 3 – Structure of a Typical Ovule
Fig. 4 – Mature Embryo Sac
3. Pollination
Self-pollination
Pollen is transferred to the stigma of the same flower or another flower on the same plant.
Cross-pollination
Pollen is transferred between flowers of different plants of the same species.
Pollination may be aided by wind, water, insects, birds or other animals. After compatible pollen reaches the stigma, a pollen tube grows through the style toward the ovule.
4. Double Fertilization
Angiosperms show a unique process called double fertilization. Two male gametes enter the embryo sac through the pollen tube.
- One male gamete fuses with the egg to form a diploid zygote. This is syngamy.
- The other male gamete fuses with the two polar nuclei/secondary nucleus to form the primary endosperm nucleus. This is triple fusion.
- Syngamy + triple fusion together constitute double fertilization.
Fig. 5 – Double Fertilization
5. Endosperm
The primary endosperm nucleus divides and forms endosperm, a nutritive tissue that supports the developing embryo. Depending on the pattern of nuclear and cellular development, endosperm may show different developmental types.
6. Embryo Development
The zygote develops into the embryo. In a typical dicot embryo, the mature structure includes two cotyledons, an embryonal axis, plumule and radicle.
Fig. 6 – Dicot Embryo
7. Quick Revision & Exam Points
- Define microsporogenesis and megasporogenesis.
- Draw and label a pollen grain.
- Draw and label a mature embryo sac.
- Define pollination and distinguish self- and cross-pollination.
- Explain double fertilization.
- Differentiate syngamy and triple fusion.
- What is endosperm?
- Draw a dicot embryo.
- Microsporogenesis → microspores from pollen mother cell.
- Megasporogenesis → megaspores from megaspore mother cell.
- Embryo sac = female gametophyte.
- Pollen grain = male gametophyte.
- Syngamy forms zygote.
- Triple fusion forms primary endosperm nucleus.
- Both together = double fertilization.
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