Seeds and Seed Germination PDF

## Section (1) **Prepared by:** prof. dr. Galal Eisa **dr. Safaa Mourad** ### Seeds and Seed Germination #### Seed definition: It is a fertilized ovule, which contains an embryo. It is a mature fertilized integumented ovule. #### Ovule Formation: Ovules are the female reproductive structures fou...

## Section (1) **Prepared by:** prof. dr. Galal Eisa **dr. Safaa Mourad** ### Seeds and Seed Germination #### Seed definition: It is a fertilized ovule, which contains an embryo. It is a mature fertilized integumented ovule. #### Ovule Formation: Ovules are the female reproductive structures found in plants. They are essential for the production of seeds and the continuation of the plant’s life cycle. ##### Formation * **Meiosis:** Ovules develop from specialized cells called megaspore mother cells. These cells undergo meiosis, a type of cell division that reduces the chromosome number by half. This process produces four haploid megaspores. * **Megaspore Development:** Typically, only one of the four megaspores survives and develops further. This surviving megaspore undergoes three mitotic divisions to form a structure called the female gametophyte. ###### Figure show Ovule Formation <br> #### Ovule Structure: The female gametophyte consists of seven cells: * **Egg cell:** The female gamete that will fuse with a pollen grain to form a zygote. * **Synergid cells:** Cells that assist in pollen tube growth and fertilization. * **Antipodal cells:** Cells of unknown function. * **Central cell:** A large cell containing two polar nuclei. ###### Figure show ovule structure <br> #### Location Ovules are found within the ovary of a flower. The ovary is part of the pistil, the female reproductive organ of a flower. #### Function The primary function of an ovule is to produce a seed after fertilization. When a pollen grain lands on the stigma of a flower and germinates, it forms a pollen tube that grows down through the style to reach an ovule. The male gamete from the pollen grain travels through this tube and fuses with the egg cell in the ovule, resulting in the formation of a zygote. The zygote then develops into an embryo, while the ovule itself develops into a seed coat. ### Embryo Formation in Plants: (The Beginning of a New Life) Embryo formation is a critical stage in the life cycle of plants. It’s the process where a fertilized egg cell (zygote) develops into a miniature plant. This tiny plant, enclosed within a seed, is the foundation for the next generation. <br> ### Seed formation: Seed formation is a crucial process in the life cycle of plants. It’s the culmination of sexual reproduction, where a plant embryo is enclosed in a protective covering along with a food reserve. This package, known as a seed, is essential for the plant’s survival and propagation. ###### Figure show double fertilization <br> #### The Process 1. **Pollination:** This is the transfer of pollen grains from the anther (male part) of a flower to the stigma (female part) of another or the same flower. This can be done by wind, water, insects, or other animals. 2. **Fertilization:** Once the pollen grain reaches the stigma, it germinates, forming a pollen tube that grows down through the style to reach the ovule. The male gamete from the pollen grain travels through this tube and fuses with the female gamete (egg cell) in the ovule. 3. **Embryo Formation:** The fertilized egg cell, now called a zygote, begins to divide and develop into an embryo. The embryo consists of the cotyledons (seed leaves), the shoot, and the root. 4. **Endosperm Formation:** In angiosperms, a second fertilization event occurs, where a second male gamete fuses with the polar nuclei in the ovule. This results in the formation of the endosperm, a tissue that stores nutrients for the developing embryo. 5. **Seed Coat Formation:** The ovule wall develops into a protective seed coat, or testa, which surrounds the embryo and endosperm. 6. **Fruit Formation:** In many plants, the ovary of the flower develops into a fruit, which encloses the seeds and aids in their dispersal. <br> ### Seed Structure A typical seed consists of: * **Embryo:** The developing plant. * **Endosperm:** The food storage tissue. * **Seed Coat:** The protective outer layer. ###### Figure show development of male gametophyte <br> #### Parts of a Seed ###### Figure show Hypogeal Germination <br> ### Types of seeds #### Based on Structure: * **Monocotyledonous (Monocots):** These seeds have a single cotyledon (seed leaf). Examples include corn, wheat, and rice. * **Dicotyledonous (Dicots):** These seeds have two cotyledons. Examples include beans, peas, and sunflowers. #### Based on Germination Patterns: * **Albuminous:** These seeds have a reserve of endosperm (a tissue that stores nutrients) that nourishes the embryo during germination. Examples include wheat, rice, and corn. * **Exalbuminous:** These seeds lack endosperm, and the nutrients are stored in the cotyledons themselves. Examples include peas, beans, and ground nuts. <br> ### Importance of Seeds * **Reproduction:** Seeds are the primary means of reproduction for many plants. * **Dispersal:** Seeds can be dispersed by various means, allowing plants to colonize new areas. * **Dormancy:** Seeds can remain dormant for long periods, enabling plants to survive unfavorable conditions. * **Food Source:** Seeds are a vital food source for many animals and humans <br> ### Seed Germination: Seeds are the reproductive units of plants. They contain a plant embryo, which is a miniature plant in a dormant state, along with a food reserve (endosperm) and a protective outer layer (seed coat). Seed germination is the process by which a seed, after a period of dormancy, begins to grow into a new plant. It’s a crucial stage in the life cycle of a plant. #### Conditions for Germination: * **Water:** Water is essential for softening the seed coat, hydrating the embryo, and providing the necessary medium for metabolic processes. * **Temperature:** Most seeds have a specific temperature range within which they can germinate. Some seeds require a cold period (vernalization) before they can germinate. * **Oxygen:** Oxygen is necessary for the respiration of the embryo. * **Light:** While some seeds require light for germination, others are indifferent to light. #### Stages of Germination: 1. **Imbibition:** The seed absorbs water, causing it to swell and soften the seed coat. the radicle begins to appear 2. **Epicotyl or Hypocotyl Emergence:** The radicle or embryonic root growth increases. 3. **Epicotyl or Hypocotyl elongates in a curved position to avoid friction with the soil.** 4. **Final stage or Photosynthesis:** The epicotyl straightens after it appears above the surface of the soil and the prophyll leaves appear, which are two small, seated leaves. After that, the normal leaves appear, alternate leaves of the plant, which are able to photosynthesis. Secondary roots also appear on the main root. #### Factors Affecting Germination: * **Seed quality:** The viability and dormancy of the seed can affect germination. * **Soil conditions:** The soil’s moisture content, pH, and nutrient composition can influence germination. * **Competition::** The presence of other plants or weeds can compete with seedlings for resources. #### Dormancy: Some seeds have a dormancy period, which prevents them from germinating until conditions are favorable. This can be due to factors such as a hard seed coat, the presence of inhibitors, or the need for a cold period. #### Importance of Seeds * **Reproduction:** Seeds are the primary means of reproduction for many plants. * **Dispersal:** Seeds can be dispersed by various means, allowing plants to colonize new areas. * **Food source:** Seeds are a valuable food source for many animals and humans. * **Economic importance:** Many crops are grown for their seeds, such as grains, legumes, and oilseeds. #### Types of Germination There are two main types of seed germination: epigeal and hypogeal. #### Epigeal Germination In Epigeal germination, Hypocotyl (stem under the cotyledons) elongates faster than Epicotyl; the epicotyl remains unchanged in length. The hypocotyl grows in an upward curved position carrying with it the two cotyledons (seed leaves) and the plumule. The cotyledons (seed leaves) are pushed above the ground surface. #### Hypogeal Germination In Hypogeal germination cotyledons remain in the ground. In Hypogeal germination, Epicotyl (stem above the cotyledons) elongates faster than Hypocotyl, the Hypocotyl remains unchanged in length. The Epicotyl grows in an upward curved position carrying with the plumule, it pushing the shoot above the ground while the cotyledons remain under the soil surface. ###### Figure show Hypogeal Germination