If you want to read about the ‘Structure of Seed‘, then this article is for you because we are going to cover everything about ‘what is the structure of seed ?‘.
In the life of flowering plants, seeds are very important structures as they help in the formation of new plants. A seed is a mature ovule after fertilization.
The ovule develops rapidly after fertilization using the food reserves. In addition to the development of new plants, seeds also help in the dispersal of a species as well as overcoming unfavourable conditions.
So lets discuss about the structure of seed…
What is the Structure of Seed ?
All seeds have some common features. Though, they vary in size, shape, colour and some other features. A typical seed is differentiated into three parts i.e.
- Seed coat
- Stored food.
The seed coat
Each seed is covered by a protective covering called seed coat. It develops from the integument of ovule. The seed coat is differentiated into an (i) outer tough, protective testa-and (ii) inner thin tegmen.
The testa protects the seed from dessication, bacteria, fungi and insects. On the seed is present a scar called hilum. It represents the point of attachment of the seed with its stalk.
Adjacent to the hilum is present a minute opening called micropyle. Absorption of water as well as exchange of gases take place through the micropyle during germination of seed.
The micropyle is not easily seen in a dry seed. But, if a seed, soaked in water is pressed gently, some water and air-bubbles ooze out through this pore and it can be easily located.
An embryo is defined as a young miniature plant along with cotyledons, enclosed within the seed coat. The embryo is differentiated into (A) cotyledons and (B) embryo axis.
(A) Cotyledons – Also called seed leaves, they are either two (in dicotyledonous plants) or one (in monocotyledonous plants) in number.
They are food laden, fleshy structures attached to the embryo axis. They provide nourishment to the embryo in the early stages of development. In endospermic seeds cotyledons are not massive e.g., castor.
(B) Embryo axis – It is the young or miniature plant differentiated into the following parts :
- Mesocotyl – It is a part of the embryo axis where cotyledons are attached.
- Epicotyl – It is a portion of the embryo axis between the mesocotyl and plumule.
- Hypocotyl – It is a portion of the embryo between mesocotyl and radicle.
- Plumule – It is the feathery or leafy end of the embryo axis that grows into shoot system of the plant.
- Radicle – It is the pointed lower end of the embryo axis that develops into the root system of a plant.
Food storage in the seeds
The seeds contain stored food material to be used during early development. The food is stored either in the cotyledons or in a special food storage tissue called the endosperm.
The seeds in which food is stored in the cotyledons and contain no endosperm are called non-endospermic or exalbuminous e.g. bean, gram, pea, alisma e.g., castor, cotton, maize, wheat, rice.
Those seeds which contain stored food in the endosperm are called endospermic or albuminous seeds.
Types of seeds
Depending upon the number of cotyledons and presence or absence of endosperm, seeds are classified as shown in figure below.
Structure of bean seed-a dicotyledonous exalbuminous seed
Bean seeds are kidney-shaped being convex on one side and concave on the other. The following external features are seen in the bean seed :
1. Seed coat – seed coats that develop from the two integuments of the ovule. These two coats are: Bean seed is surrounded by two
(a) Testa – It is the outer, smooth, thick and coloured protective layer. It protects the seed from fungi, bacteria and insects.
(b) Tegmen – It is thin, white, membrane-like inner seed coat. It is very firmly fused with the testa. It is often very difficult to separate it from the testa.
Also read about cell theory.
2. Hilum – On the concave side of the seed is present a scar called hilum. It is the point from where the seed is attached to the pod with the help of a short stalk called funiculus. On maturity, the funiculus detaches and leaves behind this scar.
3. Micropyle – Adjacent to the hilum is present a very minute pore called micropyle. Absorption of water and exchange of gases take place through the micropyle during germination of seeds.
4. Raphe – It is a ridge-like structure present around the median groove. It represents the portion of the stalk (funiculus) which remains fused with testa.
To study the internal structure of bean seed, it is soaked in water for a few hours. The soaking make the seed coats soft and easy to remove. On removing the seed coat we find the embryo.
5. Embryo – The embryo consists of the following parts :
(a) Cotyledons – The bulk of embryo consist of two fleshy structures called cotyledons. These store the food and provide nourishment to the developing embryo axis. The bean seeds do not contain endosperm, and thus are non-endospermic.
(b) Embryo Axis – In between the two cotyledons is present, a young miniature plant called embryo axis. It is differentiated into :
- Mesocotyl – part of the embryo axis where cotyledons are attached laterally.
- Epicotyl – portion of the embryo axis between mesocotyl and plumule.
- Plumule – the feathery or leafy end of the embryo axis that later grows into shool system.
- Hypocotyl – portion of the embryo axis between mesocotyl and radicle.
- Radicle- The pointed lower end of the embryo axis that later grows into root system of the plant.
Structure of maize seed (Zea mays a Monocotyledonous albuminous seed)
Maize is one seeded fruit called caryopsis. In maize seed the fruit wall (pericarp) is fused with the seed coat. The maize (corn) grain is a flat, almost triangular oblong structure. Externally it is differentiated in two unequal areas i.e.,
- A large, yellowish, upper area which marks the position of endosperm.
- Small whitish lower area that contains the embryo.
The maize grain consists of the following parts :
1. Fused seed coat and pericarp – The maize grain is covered externally by a thin but firm, yellowish membranous layer. It is formed by the fusion of seed coat and fruit wall or pericarp.
2. Hilum and micropyle – These are not distinguishable in maize grain. The internal structure of maize grain is studied în a longitudinal section. It shows two distinct regions i.e. the upper large endosperm and the lower smaller embryo. These are separated by a prominent epithelial layer.
3. Endosperm – It forms the bulk of the grain and stores starch as reserve food material. The endosperm is surrounded by a special one-cell-thick layer called aleurone layer. It is rich in proteins.
4. Embryo – The embryo lies obliquely in the lower small part of maize grain. It consists of: –
- Cotyledon- The maize grain contains a single, papery, shield-shaped cotyledon called scutellum. The scutellum does not contain food. It is closely pressed against the endosperm.
- Plumule- It consists of the growing tip of the shoot along with few embryonic leaves. It is covered by a conical cap or plumule sheath called coleoptile.
- Radicle – It lies at the base of the grain and is covered by root cap. It is surrounded by a protective root sheath called the coleorhiza.
Questions & Answers related to Structure of Seed
Q. What is called Seed ?
Ans. A fertilized and mature ovule is called seed.
Q. What is called Embryo ?
Ans. A seed is covered by two seed coats (testa and tegmen), contains reserve food and a miniature plant called embryo.
Q. What are the components of Embryo ?
Ans. Embryo consists of one (monocotyledonous) or two (dicotyledonous) seed leaves called cotyledons and an embryonal axis.
Q. How Embryonal Axis is differentiated ?
Ans. The embryonal axis is differentiated into mesocotyl, epicotyl, hypocotyl, plumule and radicle.
Q. How Root and Shoot Systems are formed ?
Ans. The upper leafy part of the embryonal axis is called plumule which forms the future shoot system and lower pointed part is called radicle that forms the future root system.
Q. Where is the food store for the Germination of Seed ?
Ans. Food is stored for the early germination of seeds either in the cotyledons (exalbuminous) or in the endosperm (albuminous or endospermic seeds).
Q. What happens to Seed in the Dormancy Period ?
Ans. Seeds after shedding off from the parent plant, usually undergo a dormant period. During dormancy they may not germinate even if provided with all the favourable conditions.
Q. What is the main structure of seed plant ?
Ans. A normal seed is differentiated into three structures i.e.
- Seed coat
- Stored food.
Title : Structure of Seed – Different Parts & Types of Seed with Diagram
This article mainly covered the topics related to the Structure of Seed’. The following topics are also covered briefly in this article :
- What is the Structure of Seed ?
- Different Parts of the Seed.
- Types of Seed.
- Diagrams related to germination of seed.