Unit vi embryology

As per SGBAU Bsc Botany Part II Semester III Syllabus

1. UNIT VI EMBRYOLOGY By , Pranav Vijay Gadkar , Assistant Professor, Department Of Botany, BNB College Digras .

2. INTRODUCTION What is Embryology ? The Branch of biology which deals with the formation , early growth and development of living organism. What we study in Plant Embryology? Plant embryology studies not only embryonic development but also the formation of gametes in generative sphere and fertilization. Comparative plant embryology studies embryological processes in various species in order to obtain data for systematics and phylogeny.

3. FLOWER – REPRODUCTIVE STRUCTURE Flower is defined as highly modified & condensed reproductive shoot of plant.

4. MALE REPRODUCTIVE STRUCTURE STAMEN-

5. STRUCTURE OF ANTHER

6. MICROSPOROGENESIS FORMATION OF POLLEN GRAINS- Meiosis I Meiosis II Pollen Mother Cell (2n) Tetrad of haploid pollen cell Pollen Grains Mature Pollen grain Exine Intine Germpore Haploid Nucleus

7. Development of Male Gametophyte Exine Germpore Nucleus Intine Generative Cell Tube Cell Generative Cell Tube Nucleus Pollen tube Male Gamet Tube Nucleus A) Pollen Grain (Microspore) B) 2 Celled Pollen grain C) Germinating Pollen grain D) Male gametophyte

8. Types of Ovules

9. Structure of Anatropous Ovule

10. Megasporogenesis and Development of Female Gametophyte

11. Monosporic Embryo Sac

12. Double Fertilization and Triple Fusion

13. Development of endosperm where repeated free-nuclear divisions take place; if a cell wall is formed it will form after free-nuclear divisions. Commonly referred to as liquid endosperm. Coconut water is an example of this.

14. Development of endosperm where a cell-wall formation is coincident with nuclear divisions. Coconut meat is cellular endosperm. Acoraceae has cellular endosperm development while other monocots are helobial .

15. Development of endosperm Where a cell wall is laid down between the first two nuclei, after which one half develops endosperm along the cellular pattern and the other half along the nuclear pattern.

16. Development of Dicot Embryo

17. Two cell stage Following fertilization, the zygote and endosperm are present within the ovule, as seen in stage I of the illustration on this page. Then the zygote undergoes an asymmetric transverse  cell division  that gives rise to two cells - a small apical cell resting above a large basal cell .These two cells are very different, and give rise to different structures, establishing  polarity  in the embryo. apical cell The small apical cell is on the top and contains most of the  cytoplasm , the aqueous substance found within cells, from the original zygote. It gives rise to the  hypocotyl ,  shoot apical meristem , and  cotyledons . Basal cellThe large basal cell is on the bottom and consists of a large  vacuole  and gives rise to the  hypophysis and the  suspensor .

18. Eight cell stage After two rounds of longitudinal division, and one round of transverse division, an eight-celled embryo is the result. Stage II, in the illustration above, indicates what the embryo looks like during the eight cell stage. According to Laux et al., there are four distinct domains during the eight cell stage. The first two domains contribute to the embryo proper. The  apical embryo domain , gives rise to the shoot apical meristem and cotyledons. The second domain, the  central embryo domain , gives rise to the hypocotyl , root apical meristem , and parts of the cotyledons. The third domain, the  basal embryo domain,  contains the hypophysis . The hypophysis will later give rise to the radicle and the root cap. The last domain, the  suspensor , is the region at the very bottom, which connects the embryo to the endosperm for nutritional purposes.

19. Sixteen cell stage Additional cell divisions occur, which leads to the sixteen cell stage. The four domains are still present, but they are more defined with the presence of more cells. The important aspect of this stage is the introduction of the protoderm , which is meristematic tissue that will give rise to the epidermis. [7]  The protoderm is the outermost layer of cells in the embryo proper. [

20. Globular stage- The name of this stage is indicative of the embryo's appearance at this point in embryogenesis; it is spherical or globular. Stage III, in the photograph above, depicts what the embryo looks like during the globular stage. 1 is indicating the location of the endosperm. The important component of the globular phase is the introduction of the rest of the primary meristematic tissue. The protoderm was already introduced during the sixteen cell stage. According to Evert and Eichhorn , the ground meristem and procambium are initiated during the globular stage.The ground meristem will go on to form the  ground tissue , which includes the pith and cortex. The procambium will eventually form the  vascular tissue , which includes the xylem and phloem.

21. Heart stage- According to Evert and Eichhorn , the heart stage is a transition period where the cotyledons finally start to form and elongate. [7]  It is given this name in eudicots because most plants from this group have two cotyledons, giving the embryo a heart shaped appearance. The shoot apical meristem is between the cotyledons. Stage IV, in the illustration above, indicates what the embryo looks like at this point in development. 5 indicates the position of the cotyledons. Pro embryo stage -This stage is defined by the continued growth of the cotyledons and axis elongation.In addition, programmed cell death must occur during this stage. This is carried out throughout the entire growth process, like any other development.However , in the torpedo stage of development, parts of the suspensor complex must be terminated.The suspensor complex is shortened because at this point in development most of the nutrition from the endosperm has been utilized, and there must be space for the mature embryo. After the suspensor complex is gone, the embryo is fully developed.Stage V, in the illustration above, indicates what the embryo looks like at this point in development

22. Development of Monocot embryo

23. Seeds and their significance The distribution and dominance of angiosperm on earth is due to seeds Success of seed as propagule is due to many characteritics Dormancy Viability Reserve food Protective coat Dispersal Edible fruits

24. Suspended Animation ( Dormancy) State or a condition in which seeds are prevented from germinating even under the favourable environmental conditions for germination. The main reason behind these conditions is that they require a period of rest before being capable of germination. These conditions may vary from days to months and even years. These conditions are the combination of light, water, heat, gases, seed coats and hormones.

25. Causes of Seed dormancy There are certain major causes for the seed dormancy. Listed below are the few reasons for the seed dormancy. Light Temperature Hard Seed Coat Period after ripening Germination inhibitors Immaturity of the seed embryo Impermeability of seed coat to water Impermeability of  seed coat to oxygen Mechanically resistant seed coat Presence of high concentrate solutes

26. Types of Seed Dormancy The seed dormancy is of following types: Innate dormancy It is the condition of seeds which is incapable of germination even if conditions suitable for seedling growth are supplied. This inability to germinate may be due in certain species to the embryo being immature at the time of dispersal. Enforced dormancy It is the condition of seeds which is incapable of germination due to an environmental restraint which includes, an adequate amount of moisture, oxygen, light and a suitable temperature. Induced dormancy This type of seed dormancy occurs when the seed has imbibed water, but has been placed under extremely unfavourable conditions for germination. Finally, seed fails to germinate even under more favourable conditions.

27. Methods of Breaking Seed Dormancy The different methods of breaking dormancy are mentioned below: The natural breaking of Seed Dormancy Nature of dormancy stops when the embryo gets appropriate environment such as adaptive moisture and temperature. The seed coat that exists in many species becomes permeable due to the rupturing of smoothing action of natural agents like microorganism, temperature, and abrasion by the digestive tract of birds and animals that feed on these seeds. Other natural methods include: Completion of the over-ripening period. Leaching of inhibitors present in the seed coat. Inactivation of inhibitors by the supply of cold, heat, and light. Leaching of the excess and highly concentrated solutes from the seeds. Production of growth hormones which can neutralize the effect of inhibitors.

28. Artificial Overcoming of Seed Dormancy Some of the artificial methods used for breaking seed dormancy are listed below: Action with hot water for termination of waxes, surface inhibitors, etc. Rupturing of seed coats by filing, chipping, or threshing through machines. Exposure to heat, cold or light, depending upon the type of seed dormancy. By applying Hydraulic pressure for 5 to 20 minutes in order to weaken the tough seed coats. Seed coats are treated with concentrated sulphuric acid for removing all traces of the mineral acid.

29. Treatment to break dormancy in seeds There are separate treatments to overcome dormancy, and they are further divided into the following groups: Seed coat treatment These treatments make a hard seed coat permeable to water or gases either by softening or cracking. This process is called scarification. The treatment can be either chemical or physical in nature. Embryo treatments Stratification:  The incubation of seeds at an appropriate low temperature over a moist layer before transferring to a temperature suitable for germination. High-temperature treatment:  Incubation at 40-50 °C for a few hours to a few days may have an effect in overcoming dormancy in some species. For instance, rice seeds treated with hot water at 40°C for at least 4 hours. Chemical treatments Plant growth regulators  or other chemicals can be used in induced germination growth regulators.

30. Seed Dormancy pathway

31. Seed germination

32. Importance of Seed Dormancy It follows the storage of seeds for later use by animals and man. It helps in the dispersal of the seeds through the unfavourable environment. Dormancy induced by the inhibitors present in the seed coats is highly useful to desert plants. Allows the seeds to continue to be in suspended animation without any harm during cold or high summer temperature and even under drought conditions. Dormancy helps seeds to remain alive in the soil for several years and provides a continuous source of new plants, even when all the mature plants of the area have died down due to natural disasters