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5.7.1 - Structure of a Dicotyledonous Seed

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Introduction to Dicotyledonous Seeds

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Teacher
Teacher

Today, we will explore the structure of dicotyledonous seeds. Who can tell me how many cotyledons are found in these seeds?

Student 1
Student 1

There are two cotyledons in dicotyledonous seeds.

Teacher
Teacher

Correct! These cotyledons serve essential roles, including nutrient storage. Can anyone explain what a seed coat is?

Student 2
Student 2

The seed coat is the outer protective layer of the seed.

Teacher
Teacher

Exactly! The seed coat has two layers: the outer **testa** and the inner **tegmen**. This structure protects the seed and aids in its survival.

Student 3
Student 3

What does the hilum and micropyle do?

Teacher
Teacher

Great question! The **hilum** is a scar where the seed was attached to the fruit, and the **micropyle** is a small opening that allows for water absorption during germination.

Teacher
Teacher

To summarize: dicotyledonous seeds have two cotyledons, a protective seed coat, a hilum, and a micropyle. Remember, the hilum looks like a little scar on the seed!

Components of a Seed

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Teacher
Teacher

Now, let's talk about the embryo of the dicotyledonous seed. What parts does it consist of?

Student 4
Student 4

It consists of the radicle, cotyledons, and an embryonal axis.

Teacher
Teacher

Excellent! The **radicle** is the part that develops into the root. What role do cotyledons play, Student_1?

Student 1
Student 1

They store food and help in nutrition for the seedling.

Teacher
Teacher

Absolutely right! In many seeds, the cotyledons are quite fleshy. This leads us to an interesting point about endosperm. What can someone tell me about it?

Student 2
Student 2

Endosperm provides additional food supply to the growing plant.

Teacher
Teacher

Exactly, some seeds like castor have endosperm, making them **endospermic**, while others, such as beans, do not, hence they are classified as **non-endospermous**.

Teacher
Teacher

Key takeaway: the seed structure, particularly the embryo and cotyledons, is vital for the seed’s survival and germination process.

Endospermic vs. Non-endospermous Seeds

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Teacher
Teacher

Let's elaborate on endospermic and non-endospermous seeds. Who can provide an example of an endospermic seed?

Student 3
Student 3

An example is the castor seed.

Teacher
Teacher

Correct! And how about non-endospermous seeds? Any examples?

Student 4
Student 4

Beans and peas are examples.

Teacher
Teacher

Exactly! The key difference is that non-endospermous seeds like beans utilize the food stored in their cotyledons for nourishment. Can anyone tell me why this is important for their development?

Student 1
Student 1

It helps the seedling get started in its early stages until it can produce its food through photosynthesis.

Teacher
Teacher

Correct! To recap, endospermic seeds have additional food storage in the form of endosperm, which is crucial during the initial stages of growth.

Introduction & Overview

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Quick Overview

This section covers the structure of a dicotyledonous seed, describing its key components, including the seed coat, embryo, and availability of endosperm.

Standard

The section details the structural organization of a dicotyledonous seed, which includes an outer seed coat made of two layers (testa and tegmen), the presence of hilum and micropyle, and the embryo comprised of a radicle, cotyledons, and embryonal axis. The function and significance of endospermic versus non-endosperm seeds are also discussed.

Detailed

Structure of a Dicotyledonous Seed

Dicotyledonous seeds, characterized by their two cotyledons, display a distinct structure integral to their development. The outer layer of a seed is known as the seed coat, which consists of two parts: the testa (outer layer) and the tegmen (inner layer). The hilum is a scar on the seed coat where the seed was attached to the fruit, while above it lies the micropyle, a small pore crucial for water absorption during germination.

Internally, the seed houses the embryo, which is established from the fertilized ovule. The embryo comprises the embryonal axis, the radicle (future root), and cotyledons (the seed leaves), which are pivotal for the nutrient storage used during the seedling's early growth. The cotyledons are often fleshy and filled with reserve food materials.

Additionally, the presence of endosperm may vary; some seeds like castor have endospermic tissues, while others such as bean, gram, and pea do not possess active endosperm at maturity and are termed non-endospermous. This section emphasizes the structural significance of these features, showcasing how they contribute to the seed's viability and germination process.

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Audio Book

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Seed Coat

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The outermost covering of a seed is the seed coat. The seed coat has two layers, the outer testa and the inner tegmen.

Detailed Explanation

The seed coat serves as a protective layer for the seed. It has two parts: the outer layer called the testa and the inner layer called the tegmen. This structure is important because as a seed develops, it needs to be shielded from environmental hazards such as pests, pathogens, and extreme weather conditions that could harm the embryo inside.

Examples & Analogies

Think of the seed coat like a tough wrapping around a fragile product you order online. The outer wrapping protects the item from damage during shipping, similar to how the seed coat protects the embryo before it germinates.

Hilum and Micropyle

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The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle.

Detailed Explanation

The hilum is where the seed was connected to the fruit, essentially marking the point of attachment. The micropyle, located above the hilum, is a tiny opening that plays a crucial role during germination. It serves as a channel for water and gases to enter the seed, initiating the germination process.

Examples & Analogies

You can think of the hilum like a small hole you might find on a sealed bag after it’s been tied. It’s the point that attaches it to the larger container, and the micropyle is like a small air vent that allows air to flow in when you’re ready to open the bag.

Embryo Composition

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Within the seed coat is the embryo, consisting of an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food materials.

Detailed Explanation

The embryo is the future plant that will grow once the seed germinates. It consists of the embryonal axis, which will develop into the stem and roots, and two cotyledons, which provide nutrition until the plant can produce its own food through photosynthesis. The fleshy cotyledons store essential nutrients that help the embryo start growing.

Examples & Analogies

Imagine the embryo as a baby bird inside an egg. The cotyledons would be like the yolk, providing all the necessary nutrients for the baby bird until it’s strong enough to leave the egg and find food on its own.

Radicle and Plumule

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At the two ends of the embryonal axis are present the radicle and the plumule.

Detailed Explanation

The radicle is the part of the embryo that will develop into the root, while the plumule will develop into the shoot and eventually form the leaves and stems of the plant. This differentiation is crucial for the seed's successful growth into a mature plant, as roots and shoots have distinct roles in nutrient uptake and photosynthesis respectively.

Examples & Analogies

Think of the radicle and plumule like a person getting ready to start a journey. The radicle represents the person’s feet (roots) that dig into the ground for support, while the plumule is like their head and arms (shoot) reaching out towards the sky, eager to explore and grow.

Endosperm in Seeds

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In some seeds such as castor, the endosperm formed as a result of double fertilisation is a food storing tissue and called endospermic seeds. In plants such as bean, gram, and pea, the endosperm is not present in mature seeds and such seeds are called non-endospermous.

Detailed Explanation

Endosperm is a tissue that provides nutrition to the developing embryo within the seed. In endospermic seeds (like castor), this tissue remains and nourishes the embryo after germination. In contrast, non-endospermous seeds (like bean and pea) utilize all of their stored nutrients in the cotyledons, so there is no endosperm present at maturity.

Examples & Analogies

Consider endospermic seeds as a meal kit with leftovers. The endosperm is like the leftover ingredients that help the baby plant grow, while non-endospermous seeds are like a meal that uses up all the ingredients at once, leaving no extra for later.

Definitions & Key Concepts

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Key Concepts

  • Seed Coat: The outer protective layer that has two components, the testa and tegmen.

  • Hilum: The scar marking the point of attachment of the seed to the fruit.

  • Embryo: The part of the seed that contains the radicle and cotyledons.

  • Radicle: The embryonic root that will develop into the plant's root system.

  • Cotyledons: The fleshy seed leaves that provide nutrients at germination.

  • Endosperm: A tissue present in some seeds that serves as a nutritional reserve.

Examples & Real-Life Applications

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Examples

  • A castor seed is an example of an endospermic seed.

  • Beans and peas are examples of non-endospermous seeds.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Seeds with coats, a hilum in sight, / Radicles grow down to roots so bright.

📖 Fascinating Stories

  • Once there was a mighty seed wearing a coat. Inside, it held its tiny parts, a radicle and cotyledons, waiting for their moment to sprout into life.

🧠 Other Memory Gems

  • Remember CHeRCo: Cotyledons help Radicle grow from the Coat of the seed.

🎯 Super Acronyms

SEED

  • Shell (seed coat)
  • Embryo (includes cotyledons and radicle)
  • Endosperm (nutritional reserve
  • if present)
  • Development (the process of germination).

Flash Cards

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Glossary of Terms

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  • Term: Seed Coat

    Definition:

    The outer protective layer of a seed, composed of the testa and tegmen.

  • Term: Hilum

    Definition:

    The scar on the seed coat through which the seed was attached to the fruit.

  • Term: Micropyle

    Definition:

    A small pore in the seed coat that allows for water absorption during germination.

  • Term: Radicle

    Definition:

    The part of the embryo that develops into the root.

  • Term: Cotyledon

    Definition:

    Seed leaves that store nutrients to support the seedling.

  • Term: Endosperm

    Definition:

    A food-storing tissue found in some seeds, crucial for seed development.