· Seed is defined as Matured integumented megasporangium.
· Study of seed is Spermology.
· Ovules at maturity or ripened ovary is called a seed.
· A typical seed is made up of seed coat, embryo and endosperm.

A typical seed has the following parts:
1. Seed coat: protective covering of seed and is made up of:
    a) Outer: Testa, which is usually hard.
    b) Inner: Tegmen, which is thin and papery.
- In some cases like Litchi, Asphodelus, etc a sort of third integument or aril is present, which forms an additional covering of the seed.
- Some seeds like castor (Ricinus communis) have a spongy outgrowth near the micropyle, which is known as caruncle and it absorbs water during seed germination
[Caruncle develops from outer integument].
- There is a small opening at one end of the seed coat, called a micropyle through which water enters the seed. [Micropyle is also present in ovule]
- The stalk of the seed with which the seed is attached to the fruit wall is called a funiculus.
- Smallest or minute seeds are found in orchis which is the lightest in the plant kingdom and are called dust seeds.
- Largest seeds are of double ‘coconut’ (Lodoicea maldivica), which are bilobed.

Fig: Typical seed structure

Fig: Showing monocot seed

1. Embryo:
- An embryo is a young plant enclosed in a seed coat.
- It has 2 parts.
a. Cotyledons:
- These are the leaves of the embryo, sometimes they store food and becomes fleshy.
- The cotyledons are hinged to an axis called tigellum at a point called the cotyledonary node.
b. Tigellum:
- It is the main axis of the embryo.
- One end of tigellum is pointed and protrudes out of the cotyledons opposite to the micropylar end called radicle (rudimentary root).
- The other end of tigellum is feathery plumule (first apical bud of the shoot).
- The portion of the axis above the cotyledonary node is called epicotyl and below the cotyledonary node is called the hypocotyl.

2. Endosperm:
It is food-laden tissue.
-  In some seeds, the endosperm is present until maturity called endospermic (albuminous) seeds.  
Eg: Dicots: Castor, papaya, cotton & Monocots: Wheat, maize, coconut, rice.
- In some plants endosperm is consumed in young stages by developing cotyledons called non-endospermic (exalbuminous) seeds. Eg: Dicots: Pea, gram, bean, tamarind, cucurbits & Monocots: Pothos, Vallisneria.
Note: Pea seed is dicotyledonous and non-endospermic.
- Castor seed is dicotyledonous, endospermic and perispermic.
- Maize seed (monocot) has an aleurone layer where cells contain abundant protein (aleurone grains).
- The single cotyledon of monocot is called a scutellum (shield-shaped)


· Germination includes all the changes that take place from the time when a dry, viable seed starts to grow when placed under suitable conditions of germination to the time when seedling becomes established on the substratum.
The condition necessary for Germination:
A. External factors:
1. Water:
- When protoplasm absorbs the water, the seed resumes vigorous physiological activities, and hence the embryo bursts through the seed coat.
2. Oxygen:
- In germinating seed respiration and other physiological activities are more vigorous, hence oxygen is essential.
3. Temperature:
- Seeds usually germinate between 0-50 degrees centigrade and the optimum temperature is usually 25-30 degrees centigrade.
B. Internal factors:
- Foods and growth regulators.
- Completion of the rest period.
- Viability (Germinating capacity)
- Note: Light and soil are not necessary factors for seed germination.

Types of germination:
1. Hypogeal germination:
- The epicotyl grows first, the cotyledons remaining embedded Under the soil.
- Hypogeal germination is shown by most of the monocots (E.g; wheat, maize, rice, coconut) and some dicots (eg; pea, gram, broad bean (Vicia faba), mango, cycas, etc)
2. Epigeal germination:
- Hypocotyl grows quickly pushing the seed out of the soil.
- The seed coat is cast off and the cotyledon opens like two leaves.
- Mostly occurs in dicots like mustard, castor, French Bean onion, cucurbits, Tamarindus, pinus etc.
Note: a special type of germination called vivipary occurs in some mangrove plants like Rhizophora, Breiquira, Ceriops
- Integument of ovule form seed coat.
- Outer integument forms testa and tegmen(in castor, bean etc.) develops from the inner integument.
- The first part to emerge from the germinating seed is the radicle.

Dispersion of Seed

1. Autochory:
- By plant itself. eg; Geranium, Impatience, Bauhinia.

2. Dispersal by wind or Anemochory: Some adaptations are:
a) Light, minute and powdery seeds E.g; Orchids and Grasses.
b) Winged seed: eg; Moringa, Cinchona, Pinus, Oroxylum.
c) Parachute mechanism: Appendage of some fruits and seeds act as a parachute, due to which fruits and seeds remain in the air for a longer period and disperse at a good distance. Eg; Tridex. 
The appendages are as follow:
i) Pappus: plants of family Compositae has persistent, hairy calyx called pappus. Eg; Tagetes, Sonchus.
ii) Coma: These are one or more tufts of hairs attached to the seed. Eg; Calotropis (one coma), Alstonia (two coma).
iii) Hairs: present in cotton seeds.
iv) Persistent hairy style: Eg; Narvelia, Clematis.
v) Ballon shaped appendages: eg; swollen calyx of physalis, swollen ovary of Cardiospermum.

3. Censor mechanism:
- Seeds of some fruits come out from very minute aperture (pores), these pores are so minute that only some seeds come out of pore at a time. eg; papavera, Argemone, Aristolochia, Antirrhinum.
4. Hydrochory: Dispersion by water. Eg; coconut.
5. Zoochory: E.g. Rafflesia, Begonia.
6. Myrmechory:
Dispersion by ants. E.g; Trilium and Anemone.

Note: In Camel’s foot climber, the dispersal of seeds is by explosive mechanism.

Previous Post Next Post

Main Tags