Parts Of A Seed

Seeds Definition

Seeds are units of reproduction in flowering plants developed after fertilization, encapsulating the embryo, endosperm, and seed coat. They provide a very effective means of perpetuation of plant species by helping plants reproduce, disperse, and further ensure genetic diversity. The seeds also provide plants with the capability to undergo dormancy, thus surviving adverse conditions.

Role Of Seeds In Plant Reproduction

Seeds become the most important vehicle of reproduction in plants, as they are dispersed and germinate to produce new plants that would carry the further continuity and spread of species in different environments. Another important function is in providing genetic variation which is very important for plant adaptation and evolution.

Introduction to the Structure and Function of Seeds:

The main constituents of a seed include seed coat, embryo, and endosperm. The seed coat is responsible for protecting the seed from injuries and pathogens; the embryo gives rise to a new plant; and the endosperm contains most of the energy-rich nutrients needed by the growing embryo for its growth and development.

Types Of Seeds

There are broadly two kinds of seeds. They are mainly classified based on the number of cotyledons present.

Monocotyledons (Monocots)

• One cotyledon, generally thin and grassy in appearance.

• Parallel leaf venation with veins running in straight lines directly across the length of the leaf.

• Fibrous root system with many thin roots extending out from the plant stem base

• Vascular bundles are scattered around the stem, with no particular pattern.

Examples

• Wheat, corn, rice, barley, lilies, onions

Dicotyledons (Dicots)

• Two cotyledons, seed leaves, usually broad and visible as the seed germinates.

• Net venation is a network of interconnecting veins in the leaf.

• Taproot type root system, where one primary root grows downward and smaller roots branch off laterally from this main root.

• Vascular bundles in a ring in the stem, produce an evident pattern.

• Bean, pea, tomato, oak, rose

Structure Of Seed

The structure of the seed is described below:

Seed Coat (Testa)

• The outer protective layer is mainly composed of cellulose and lignin and so provides mechanical strength.

• Other layers may be formed, for example, the tegmen, which adds to protection.

• Physically protects the embryo from mechanical damage, desiccation and attack by microorganisms.

• May act to regulate the amount of water taken up during germination, preventing germination until suitable conditions are present for the seed to grow into a healthy plant.

Types Of Seed Coats

• Thin: Water and air can pass through, but they germinate relatively quickly.

• Thick: More highly protective but the seed may need special conditions to break dormancy.

• Hard: Physically very resistant; germination accomplished by mechanical or chemical force.

• Soft: It forms a minimal physical barrier to germination and is most common in seeds that begin germinating shortly after dispersal.

Embryo

• Plumule: This is the embryonic shoot that will produce the plant's stem and leaves.

• Radicle: This is the embryonic root that will become the plant's primary root.

• Cotyledons: Seed leaves that feed the developing seedling.

Plumule

• That part of the embryo will develop into the stem and often bends after the cotyledons have been raised from beneath the earth's surface.

• May push the cotyledons up out of the soil.

Radicle

• That part of the embryo becomes the primary root of the plant.

• Anchors the seedlings in the soil and start to absorb water and mineral elements.

Cotyledons

• The seed leaves supply food to the developing embryo until the true leaves have developed to the point of carrying on photosynthesis satisfactorily.

• In monocots, this single cotyledon is known as the scutellum.

• In dicots the two cotyledons store and sometimes even produce nutrients for the developing plant.

Monocot And Dicot Cotyledon Comparison

• Monocots have one cotyledon that may participate in nutrient absorption.

• Dicots have two cotyledons that typically store food.

Endosperm

• Nutritive tissue that develops during seed formation, provides nourishment to the growing embryo.

• Acts as a food reserve that is rich in carbohydrates, proteins, and lipids.

Types Of Endosperms

• Nuclear: Nucleus-free division with no formation of cell wall initially.

• Cellular: The cell walls are formed just after the nuclear division.

• Helobial: A combination of Nuclear and cellular types; This is common in monocots. Nutritional value

Process Of Germination Of Seeds

The germination process is explained below:

Stages Of Seed Germination

• Imbibition: water absorption of the seed causes it to swell, breaking the seed coat.

• Metabolism Activation: Biochemical processes initiate after the activation of enzymes and respiration.

• Emergence of Radicle and Plumule: The radicle grows downwards into the soil, while the plumule grows upwards towards light.

Factors Affecting Germination

• Water: Metabolic processes require water, which further functions to soften the seed coat.

• Temperature: Species have temperature optima; some temperatures have to be present for enzymatic activity.

• Oxygen: Cellular respiration requires oxygen for energy generation.

• Light: Induces germination in some species, and may act as a signal for some seeds.

Special Adaption Of Some Seeds

The special adaption of some seeds is given below:

Seed Dormancy

• Causes: impermeable seed coat; Hormonal: Growth-inhibiting hormone; lack of a suitable environment, temperature, and water.

• Dormancy release: Natural: weathering, microbial, action passage through the alimentary canal of an animal.

• Artificial Scarification: mechanical scratching or breaking of seed coat; Stratification: exposure of seeds to cold temperature.

Seed Dispersal Mechanism

• Water: Seeds that are buoyant, thus capable of floating, for example, coconut

• Animal: Seeds with hooks or the fruit is edible, for example, burrs or berries

• Explosion: Seeds explosively shot out of the pod, for example, touch-me-not or squirting cucumber

Comparative Table: Monocots vs Dicots

Importance Of Seeds In Agriculture And Ecology

The details are given below:

Role in Crop Production

• Seeds form the basis of food crops.

• They are selected for particular attributes and bred

• Food security is guaranteed through the consistent and reliable production of crops.

Seed Banks And Conservation

• Seed banks store seeds for conserving diversity or against plant extinction, thereby protecting genetic information in seed-preserved form.

• This is important for basic and applied research on plant species and their reintroduction in the ecosystem.

Ecological Impact

• As an agent for the plant's reproduction, seeds maintain the viability of plant species and, hence, are very important in maintaining biodiversity.

• New plant taxa become established in new sites since seeds become dispersed and germinate, a route toward achieving ecological equilibrium.

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