Line 1: In angiosperms, the seed is the final product of sexual reproduction.
Explanation: In the world of flowering plants (angiosperms), sexual reproduction culminates in the formation of a seed. This seed is essentially a fertilized ovule (egg) that has undergone a remarkable transformation.
Line 2: It is often described as a fertilized ovule. Seeds are formed inside fruits.
Enrichment: Imagine the ovule as a tiny plant egg nestled within the ovary of a flower. After successful fertilization, this ovule starts developing and maturing. The surrounding ovary also undergoes changes, eventually giving rise to the fruit, the protective enclosure for the developing seed(s).
Line 3: A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.
Explanation: A typical seed is like a tiny package containing three key components:
Seed coat(s): This tough outer covering protects the delicate embryo inside. It can be single or double layered, depending on the plant species.
Cotyledon(s): These are the seed leaves, often filled with stored food reserves like starches, proteins, or oils. They nourish the seedling during germination until it can establish its own root system and start producing food through photosynthesis. The number of cotyledons is a distinguishing feature, with dicots having two (di-) and monocots having one (mono-). (Refer to Figure 2.15a if available for illustration)
Embryo axis: This is the heart of the seed, containing the embryonic root (radicle) and shoot (plumule). It’s like a miniature plant waiting for the right conditions to sprout.
Line 4: The cotyledons (Figure 2.15a) of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes).
Enrichment: Look at Figure 2.15a (if provided). The cotyledons appear plump and fleshy due to the packed food reserves they hold. These reserves are crucial for the baby plant’s initial growth after germination. Legumes (beans, peas) are a good example of seeds with thick, food-laden cotyledons.
Line 5: Mature seeds may be non-albuminous or ex-albuminous. Nonalbuminous seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).
Explanation: Seeds can be classified based on the presence or absence of endosperm, a food storage tissue that develops within the seed. Non-albuminous seeds, like peas and peanuts, have no leftover endosperm because the developing embryo utilizes it all for its growth.
Line 6: Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor).
Enrichment: In contrast, albuminous seeds like wheat and corn retain some endosperm even after embryo development. This leftover endosperm provides additional nourishment for the germinating seedling.
Line 7: Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm.
Explanation: This line introduces a less common feature: the perisperm. In some seeds (black pepper, beet), a leftover tissue called the nucellus (part of the ovule) persists along with the endosperm. This persistent nucellus is referred to as the perisperm.
Line 8: Integuments of ovules harden as tough protective seed coats (Figure 2.15a).
Enrichment: The integuments, the outer layers of the ovule, transform into the seed coat. Refer to Figure 2.15a (if available) to see how these integuments become the tough, protective outer covering of the seed.
Line 9: The micropyle remains as a small pore in the seed coat. This facilitates entry of oxygen and water into the seed during germination.
Explanation: The micropyle, a tiny opening in the ovule, persists as a small pore in the seed coat. This pore plays a vital role during germination by allowing water and oxygen to enter the seed, initiating the growth process.
Line 10: As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass). The general metabolic activity of the embryo slows down.
Enrichment: As the seed matures, it undergoes dehydration, losing a significant amount of water. This drying process reduces the metabolic activity of the embryo.
Line 11: The embryo may enter a state of inactivity called dormancy, or if favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate.
Explanation: With reduced metabolic activity, the embryo can enter a resting phase called dormancy. This dormancy allows seeds to withstand harsh environmental conditions and ensures germination only when favorable conditions (moisture, oxygen, and suitable temperature) are present.
Line 12: As ovules mature into seeds, the ovary develops into a fruit, i.e., the transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
Enrichment: This line highlights the synchronized development of seeds and fruits. As the ovules within the ovary mature into seeds, the ovary itself undergoes significant changes, transforming into the fruit, the protective and often fleshy enclosure for the seeds.
Line 13: The wall of the ovary develops into the wall of fruit called pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc.
Explanation: The ovary wall becomes the pericarp, the fleshy or dry wall of the mature fruit. Fruits come in a wide variety of forms: fleshy and juicy (guava, orange) or dry and indehiscent (groundnut, mustard). The type of pericarp can be related to the fruit’s dispersal strategy.
Line 14: Many fruits have evolved mechanisms for dispersal of seeds.
Enrichment: Fruits play a crucial role in seed dispersal, ensuring the spread of the plant species to new locations. Fruits have evolved diverse mechanisms for dispersal, such as wind dispersal (winged fruits of maple), animal dispersal (fleshy fruits eaten by animals), and water dispersal (coconut with a fibrous husk).
[Suggestion]: You can research different fruit dispersal mechanisms to gain a deeper understanding of this fascinating adaptation.
Line 15: Recall the classification of fruits and their dispersal mechanisms that you have studied in an earlier class.
Explanation: This line prompts you to revisit your previous knowledge about fruit classification and dispersal mechanisms. Understanding these mechanisms can help you appreciate the intricate relationship between fruits and seed propagation.
Line 16: Is there any relationship between number of ovules in an ovary and the number of seeds present in a fruit?
Enrichment: In most cases, there is a correlation between the number of ovules in an ovary and the number of seeds in a fruit. If all the ovules are fertilized and develop into seeds, the number of seeds will match the number of ovules. However, some ovules might not be fertilized or may not develop properly, leading to fewer seeds than ovules.
Line 17: In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.
Explanation: As the ovary transforms into the fruit, other flower parts (sepals, petals, stamens) typically wither and fall off. Their role in reproduction is complete, and the focus shifts to fruit and seed development.
Line 18: However, in a few species such as apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits (Figure 2.15b).
Enrichment: This line introduces the concept of false fruits. In some plants (apple, strawberry), the fleshy part we consider the fruit actually develops from the thalamus, the flower base, in addition to the ovary. These structures that involve tissues beyond the ovary are classified as false fruits. (Refer to Figure 2.15b if available for illustration)
Line 19: Most fruits however develop only from the ovary and are called true fruits.
Explanation: For clarification, the passage emphasizes that the majority of fruits develop solely from the ovary and are categorized as true fruits.
Line 20: Although in most of the species, fruits are the results of fertilisation, there are a few species in which fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. Banana is one such example.
Enrichment: This line introduces parthenocarpy, a fascinating phenomenon where fruits can develop without fertilization. In some plants like banana, fruit development can be triggered by hormonal changes without requiring seed formation.
Parthenocarpic fruits can also be induced commercially by applying growth hormones to plants. These fruits are often seedless, making them more desirable for consumption.
Line 21: Seeds offer several advantages to angiosperms. Firstly, since reproductive processes such as pollination and fertilisation are independent of water, seed formation is more dependable.
Enrichment: Seeds offer a significant advantage to angiosperms compared to their ancestors who relied on water for fertilization. Seed formation is independent of water, making reproduction more reliable and less restricted by environmental conditions.
Line 22: Also seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
Enrichment: Seeds are excellent dispersal units. Many fruits have evolved mechanisms for dispersal by wind, animals, or water, allowing seeds to reach new locations far from the parent plant. This dispersal capability promotes colonization of new habitats and increases the plant’s geographic range.
Line 23: As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own.
Enrichment: The stored food reserves within cotyledons or endosperm provide crucial nourishment for the germinating seedling. This internal food source sustains the young plant until it develops its own root system and can start producing its own food through photosynthesis.
Line 24: The hard seed coat provides protection to the young embryo.
Enrichment: The tough seed coat acts as a shield, protecting the delicate embryo within from harsh environmental conditions such as drying, extreme temperatures, and mechanical damage. This protection allows seeds to survive various challenges until germination occurs under favorable conditions.
Line 25: Being products of sexual reproduction, they generate new genetic combinations leading to variations.
Enrichment: Sexual reproduction, involving the fusion of sperm and egg cells, leads to genetic variation in the offspring. Seeds, being products of sexual reproduction, carry this genetic diversity. This variation within a population allows for better adaptation to changing environments and increases the chances of survival for the species.
Line 26: Seed is the basis of our agriculture.
Enrichment: Seeds are the foundation of agriculture. They allow us to cultivate desired crops, store them for planting in future seasons, and develop new varieties through selective breeding. Seeds are essential for food security and sustainable agricultural practices.
Line 27: Dehydration and dormancy of mature seeds are crucial for storage of seeds which can be used as food throughout the year and also to raise crop in the next season.
Enrichment: Dehydration and dormancy are key features that enable seed storage. Dry seeds have reduced metabolic activity and can be stored for extended periods without spoilage. This allows us to preserve seeds for consumption as food throughout the year and use them for planting in the next growing season.
Line 28: Can you imagine agriculture in the absence of seeds, or in the presence of seeds which germinate straight away soon after formation and cannot be stored?
Enrichment: This line prompts you to consider the challenges of agriculture without seeds. Without seeds for storage, planting would be limited to specific times of the year, and food security would be significantly compromised. Seeds that germinate immediately would be difficult to manage and wouldn’t allow for planning and controlled cultivation.
Line 29: How long do the seeds remain alive after they are dispersed? This period again varies greatly. In a few species the seeds lose viability within a few months. Seeds of a large number of species live for several years. Some seeds can remain alive for hundreds of years.
Enrichment: Seed longevity varies greatly depending on the species. Some seeds lose viability quickly, while others can remain viable for years or even centuries. This variation allows for diverse germination strategies and ensures the survival of some seeds even under unfavorable conditions for extended periods.
Line 30: There are several records of very old yet viable seeds. The oldest is that of a lupine, Lupinus arcticus excavated from Arctic Tundra. The seed germinated and flowered after an estimated record of 10,000 years of dormancy.
Enrichment: This line provides a fascinating example of seed longevity. The lupine seed, estimated to be 10,000 years old, highlights the remarkable dormancy capabilities of some seeds. This ability allows plants to persist through long periods of harsh conditions and germinate when suitable conditions arise.
Line 31: A recent record of 2000 years old viable seed is of the date palm, Phoenix dactylifera discovered during the archeological excavation at King Herod’s palace near the Dead Sea.
Enrichment: This line adds another example of exceptional seed longevity. The 2000-year-old date palm seed demonstrates the potential for long-term seed viability under certain conditions.
[Activity Suggestion]: You can research different seed dormancy mechanisms and their ecological significance to gain a deeper understanding of this adaptation.
Line 32: After completing a brief account of sexual reproduction of flowering plants it would be worth attempting to comprehend the enormous reproductive capacity of some flowering plants by asking the following questions:
Enrichment: This line transitions from seed function to exploring the remarkable reproductive potential of flowering plants. The following questions will guide you in this exploration.
Line 33: How many eggs are present in an embryo sac?
Explanation: In most flowering plants, each embryo sac typically contains only one egg cell. This single egg, if fertilized, will develop into the embryo of the seed.
Line 34: How many embryo sacs are present in an ovule?
Explanation: Normally, one embryo sac develops within each ovule. However, some ovules might have irregularities or developmental defects leading to the formation of multiple embryo sacs.
Line 35: How many ovules are present in an ovary?
Explanation: The number of ovules present in an ovary varies greatly depending on the plant species. Some ovaries may contain only one ovule, while others might house hundreds. The number of ovules can influence the potential number of seeds a single fruit can produce.
Line 36: How many ovaries are present in a typical flower?
Explanation: A flower can have a single ovary (e.g., pea) or multiple ovaries that are fused together (e.g., hibiscus). The number of ovaries in a flower contributes to the overall potential seed production capacity of that flower.
Line 37: How many flowers are present on a tree? And so on…
Enrichment: This line encourages you to consider the exponential increase in reproductive potential as you move through these levels. A single flower with multiple ovaries, each containing multiple ovules, and each ovule harboring one egg cell, translates to a significant number of potential seeds if all ovules are fertilized and develop into seeds. Imagine the vast number of seeds a large tree with numerous flowers could produce!
Line 38: Can you think of some plants in which fruits contain very large number of seeds?
Enrichment: This line prompts you to identify plant examples that showcase high seed production. Think about orchid pods that contain thousands of tiny seeds, or fruits of parasitic plants like Orobanche (broomrape) and Striga that also produce numerous small seeds.
Line 39: How tiny is the seed of Ficus? How large is the tree of Ficus developed from that tiny seed. How many billions of seeds does each Ficus tree produce? Can you imagine any other example in which such a tiny structure can produce such a large biomass over the years?
Enrichment: This section highlights the incredible disparity between the size of a seed and the eventual size of the mature plant it gives rise to. Consider the tiny seed of a Ficus tree, which can grow into a massive tree with a vast canopy. Each Ficus tree can produce billions of seeds, showcasing the immense reproductive potential of these plants.
[Reflection Point]: This passage encourages you to appreciate the remarkable adaptations of flowering plants for sexual reproduction. Seeds, with their protective coats, stored food reserves, and dormancy capabilities, ensure successful dispersal and germination across diverse environments. The high number of seeds produced by some plants further enhances their chances of survival and propagation.
