The Seed: Parts of Seed, Structure of Monocot and Dicot Seed, Practice Problems and FAQs

Did you know that the breakfast cereal, the yummy pasta, the rice we have for meals, and many such plant based foods start their life as seeds from different grasses? Seeds help in the birth of new plants. The largest seed in the world is the double coconut? It can measure up to 50 cm around the middle! But where do these seeds come from? How are they able to give birth to new plants? Did you know that not all seeds are the same? While in non-flowering plants seeds remain naked, in flowering plants they are always enclosed within fruits. Even for flowering plants, the seeds can be of different types based on their internal structure. Interesting right? Come, let us help you understand more about seeds.

Table of Contents

• Development of a Seed
• Parts of Seed
• Structure of a Dicot Seed
• Structure of a Monocot Seed
• Difference Between Monocot and Dicot Seeds 
• Practice Problems of Parts of Seed
• FAQs of Parts of Seed

Development of a Seed

The ovules mature into seeds after fertilisation. In flowering plants (angiosperms) the seeds are enclosed within the mature ovary, i.e, the fruit. The seed encloses the embryo which eventually grows into a new plant.

Parts of a Seed 

A seed consists of -

• Seed coat
• Cotyledon(s)
• Embryo

  

• A mature seed may or may not have an endosperm which is a nutritive tissue.

Seed Coat

It is the outermost covering of a seed. It develops from the integuments of the ovule. The seed coat usually consists of two layers. A hard and leathery outer layer called testa. A thin and papery inner layer called tegmen. The seed coat protects the seed from environmental factors.

Cotyledon

Cotyledon is the embryonic seed leaf and is also called seed leaf. It stores food for the growing embryo. After the seed germinates, the cotyledons emerge, enlarge, and become green.

Based on the number of cotyledons in a seed, seeds can be of two types -

• Monocotyledonous seeds - Possess a single cotyledon, e.g, maize, wheat, etc.
• Dicotyledonous seeds - Possess two cotyledons, e.g, beans, gram, etc.

Embryo

The embryo consists of an embryonal axis which has a radicle at the lower end, which eventually develops into the root system, and a plumule at the other end, which eventually develops into the shoot system.

Endosperm

It is a triploid nutritive tissue which has stored food for the developing embryo. It is formed by the fusion of one of the haploid male gametes with the diploid central cell within the embryo sac.

Based on whether the mature seed possesses an endosperm, seeds can be of two types -

• Endospermic or albuminous seeds.

• Non-endospermic or exalbuminous seeds.

Endospermic or albuminous seeds

Mature seeds in which the endosperm persists and is not completely used up during embryo development are called endospermic or albuminous seeds. 

Example of dicot endospermic seed - castor.

Example of monocot endospermic seed - rice, maize.

Non-endospermic or Exalbuminous Seeds

Mature seeds in which the endosperm does not persist and is completely used up during embryo development are called non-endospermic or exalbuminous seeds. Example of dicot exalbuminous seed - beans, grams, peas, etc.

Example of monocot exalbuminous seed - orchid seeds.

Structure of a Dicot Seed

A typical dicot seed such as a bean seed has the following parts -

• Hilum - It is a scar which marks the site of attachment of the seed to the ovary wall.
• Micropyle - It is a pore that lies just below the hilum. Absorption of water and exchange of respiratory gases occurs through the micropyle.
• Seed coat - Hard protective covering of the seed which consists of an outer testa and inner tegmen.
   

  

Opening up the seed exposes two massive and fleshy cotyledons which store food for the growing embryo.

The cotyledons are laterally attached to an embryonal axis which consists of a radicle and a plumule. The region between the radicle and the point of attachment of the cotyledons is known as hypocotyl. The region between the plumule and the point of attachment of the cotyledons is known as epicotyl.

Structure of a Monocot Seed

In a typical monocot seed such as a maize the seed coat is membranous and fused with the fruit wall. Endosperm is bulky and stores food in the form of starch.

The outer layer of the endosperm is proteinaceous and is known as the aleurone layer. A thin layer called epithelium separates the embryo from the endosperm. Embryo is small and situated in a groove at one end of the seed. It has one large and shield shaped cotyledon which is in a reduced state and is known as scutellum. Short embryonal axis has a plumule and a radicle.

The plumule is covered by a sheath called the coleoptile. The radicle is covered by a sheath called coleorhiza.

Difference Between Monocot and Dicot Seeds

Practice Problems of Parts of a Seed 

1. Study the given diagram and identify the option which correctly labels the unknown parts.

a. A - Coleoptile, B - Coleorhiza, C - Radicle, D - Plumule
b. A - Coleorhiza, B - Coleoptile, C - Plumule, D - Radicle
c. A - Coleoptile, B - Plumule, C - Radicle, D - Coleorhiza
d. A - Coleoptile, B - Radicle, C - Coleorhiza, D - Plumule

Solution: The part of the embryo that grows into the shoot system is known as the plumule (B) and the part of the embryo that develops into the root system is known as the radicle (C). The plumule is covered by a sheath called the coleoptile (A). The radicle is covered by a sheath called coleorhiza (D). Thus, the correct option is c.

2. While observing the longitudinal section of a maize seed, Rahul saw a large shield shaped structure within the seed. Which of the following structures did he see?

a. Scutellum
b. Coleorhiza
c. Plumule 
d. Coleoptile

Solution: The single cotyledon in monocot seeds such as maize is a large, shield shaped structure which is in a reduced state and is known as the scutellum. Thus, the correct option is a.

3. Which of the following produce non-albuminous seeds ? 

a. Maize
b. Wheat
c. Castor
d. Pea

Solution: Pea is an example of a dicotyledonous non-endospermic or exalbuminous seed. Endosperm is the nutritive tissue that is found in seeds. Mature seeds in which the endosperm does not persist and is completely used up during embryo development are called non-endospermic or exalbuminous seeds. Thus, the correct option is a.

FAQs of Parts of a Seed 

Question 1. How is an embryo different from a seed?

Solution: A fertilised ovule develops into a seed. Seed has the embryo within it which contains a plumule and radicle. Embryo is formed from the zygote which is formed when the egg is fertilised by the male gamete. Plumule and the radicle develop into a shoot and the root, respectively. The embryo remains in a dormant state or an inactive state within the seed, until it is exposed to favourable conditions. Under favourable conditions, the embryo becomes active and the seed germinates. 

Question 2. The plumule develops into which part of the plant ?

Solution: Plumule is a part of the seed embryo which develops into shoots, bearing first leaves. 

Question 3. What is coleoptile?

Solution: The coleoptile is a characteristic feature of monocots. The protective sheath of a plumule is coleoptile in monocot seed.

Question 4. What are albuminous seeds?

Solution: Mature seeds in which the endosperm persists and is not completely used up during embryo development are called endospermic or albuminous seeds. 

Example of dicot endospermic seed - castor.

Example of monocot endospermic seed - rice, maize.

Question 5. What is the aleurone layer?

Solution: In a typical monocot seed such as a maize the seed coat is membranous and fused with the fruit wall. Endosperm is bulky and stores food in the form of starch. The outer layer of the endosperm is proteinaceous and is known as the aleurone layer. 

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