What is Seed Germination
Germination is the beginning of growth and development of the dormant embryo within the seed, which consists of various changes till its final development into seedling.
An embryo in a seed lies dormant i.e. showing no sign of life ; it grows into seedling after being awakened to life, this growth of embryo is not sudden but gradual which consists of a series of changes.
Conditions necessary for Germination :
The following conditions are necessary for the germination of seeds:—
(a) Water or Moisture — It is always found that no seed germinates if kept perfectly dry or subjected to dry state. Water is essential to bring about the vital activities like metabolism of the dormant embryo, to dissolve various salts and to hydrolyse organic substances stored in the cotyledons, to help the embryo to come out easily by softening the seed coat etc.
(b) Supply of oxygen or Air — Seeds fail to germinate if deprived of air i.e. oxygen supply. It is observed that seeds kept always immersed in water (from which air was expelled by previous boiling and then cooling) do not germinate. Constant supply of oxygen is required for the respiration in germinating seeds.
(c) Suitable temperature — It has been observed that if a moist seed is subjected to very low temperature, germination is retarded or if a water-soaked seed is subjected to high temperature it fails to germinate, because the embryo within the seed is perished. To carry out the vital activities of the protoplasm, suitable temperature is required.
The optimum temperature for germination of seeds of plants of tropical countries varies from 20°C to 25°C. In temperate countries optimum temperature may be very low. Most seeds fail to germinate below 4°C and above 50°C.
(d) Light — It has no direct effect over germination as germination takes place also in dark, but light facilitates and hastens embryo to its perfect growth into seedling ; so light indirectly influences germination. Light favours germination of seeds of many epiphytes.
(a) Food and Auxin contents — For growth and development of seedlings different kinds of food like carbohydrates, fats and proteins (that are stored in different parts of seeds) are required. Some growth promoting substances like auxins, hetero-auxins are also formed in seeds at the time of germination which controls the growth and development of seedlings during germination.
(b) Completion of dormancy — Some seeds germinate immediately after maturation — in some other cases seeds delay to germinate showing a period of rest or inactivity though external conditions are favourable. This state of inactivity of seeds is called dormancy. This may be due to (1) impermeability of seed coats to water and oxygen, (2) tough seed coats, (3) immature and dormant embryos and (4) inhibitors i.e. some chemicals present in seeds which inhibit germination.
The period of dormancy varies in seeds of different plants. Dormancy may be broken by rupturing the seed coats, by temperature and light treatment etc. Normally seeds germinate when the period of dormancy is over.
Stages of Seed Germination :
Series of physiological and metabolic changes other than physical and growth changes occur during germination, these are :
1. Intake of water through the micropyle and its imbibition by different tissues of seed, resulting in an increase in volume of seed.
2. The pressure exerted by the swelling of seed causes rupturing of the seed coat.
3. Dry seed coat (impermeable to gases in dry condition) also imbibe water and becomes more or less permeable to oxygen and carbon dioxide.
4. Hydration of cell walls and protoplasm is brought about by water, the enzymes present in the cotyledon cells become active and help to digest the reserved food which is converted into soluble simple form. Next this simple soluble food is conducted towards epicotyl, hypocotyl, radicle and plumule. Digested foods are then assimilated into the protoplasm of the cell.
5. Soluble carbohydrates are partly consumed in respiration — at the time of germination, soluble carbohydrates (derived from fats) and soluble sugars (derived from carbohydrates) are consumed in respiration which is vigorous and of anaerobic type at least in the early stage.
6. Due to assimilation, growth of the embryo takes place — as a result seedling with shoot and root system develops.
Types of Seed Germination :
There are three types of germination, e.g. (i) Hypogeal, (ii) Epigeal, and (iii) Viviparous types.
I. Hypogeous or Hypogeal germination
In this type of germination the cotyledon(s) remain in the same place where the seed was placed first, — neither they are moved upward nor are they transformed into first pair of leaves of the seedling. The radicle is the first to come out piercing the seed coat near the micropyle, it gradually grows and always bends down to the earth; the plumule in the meantime grows upwards.
Hypogeal germination Examples :
Among dicotyledonous exalbuminous seeds, gram (Cicer arietinum), pea (Pisum sativum), mango (Mangifera indica), etc., and among dicotyledonous albuminous seeds water lily (Nymphaea esculenta) are the examples of hypogeous germination.
Monocotyledonous exalbuminous seeds like Pothos, Amorphophallus, etc., and monocot albuminous seeds like maize (Zea mays), rice (Oryza sativa), etc. are the typical examples of this type of germination.
II. Epigeous or Epigeal germination
Here the radicle comes out first, bends down to the earth to form tap root as usual, the consequent growth and development of plumule to form shoot system is delayed a bit. The hypocotyl grows faster and forms a loop which straightens up, as a result the cotyledons are carried upwards.
Epigeal Germination Examples :
Dicotyledonous exalbuminous seeds: bean (Dolichos lablab), tamarind (Tamarindus indica), gourd (Cucurbita maxima), etc.
Dicotyledonous albuminous seeds: castor (Ricinus communis), Basella sp., etc.
Monocotyledonous exalbuminous seed — Alisma plantago.
Monocotyledonous albuminous seeds — onion (Allium cepa), hoghla (Typha elegans) etc.
III. Vivipary or Viviparous germination
Several genera of mangrove plants i.e. halophytes show this phenomenon. Here the embryo of seed undergoes development (i.e. germination) without any resting period with the fruit still attached to and nourished by the parent plant.
In species of Ceriops, Rhizophora of the family Rhizophoraceae, the embryo grows out not only from seed but emerges out from the fruit, so that the hypocotyl and the radicle grow and project out to a considerable length from the fruit—finally the seedling breaks, falls vertically on the salty mud and gets embedded in it, and thrusts the radicle in the muddy saline soil. The cotyledons serve as sucking organs.
In Aegiceras sp. (Myrcinaceae) the embryo comes out of the seed, but remains within the fruit, it is green and has a large plumule.
Germination of different kinds of Seeds and Grains :
I. HYPOGEAL TYPES
A. Dicotyledonous exalbuminous seeds
1. PEA (Pisum sativum, Papilionaceae) — Radicle is the first to come out, it gradually grows and always bends down to the soil forming the root system. Usually it forms the main root with secondary branches. In the meantime the plumule grows upwards due to the elongation of epicotyl and forms the shoot. The cotyledons remain under the soil, the function of which is the storing of food to be utilised at the time of germination.
2. Gram (Cicer arietinum, Papilionaceae) — Like pea, cotyledons remain in the same place where the seed was placed first—the storing food of cotyledons is utilised at the time of germination.
3. Mango (Mangifera indica, Anacardiaceae) — The seed is provided with seed coat and hard endocarp—endocarp and seed coat rupture due to swelling as a result of the intake of water. The radicle first comes out and forms the taproot system: next, the epicotyl grows and comes out and as a result the plumule is taken out of the soil through a slit of the shoot system. The cotyledons remain as it is within the endocarp which remain on or within the soil, thus the germination is hypogeal.
B. Monocotyledonous albuminous seeds
4. MAIZE OR INDIAN CORN (Zea mays, Gramineae) — Here the single cotyledon or scutellum neither comes out of the seed during germination nor forms a green structure but, remains all along attached to the vast endosperm functioning as sucker. At first the radicle is to come out piercing the fruit wall and coleorhiza and immediately after it, several other roots (generally 3) develop from a region above the radicle—those roots are called seminal roots.
Radicle and seminal roots may persist forever but adventitious roots develop very soon from the base of the plumule above the mesocotyl. In the meantime coleoptile and mesocotyl elongate, and the plumule breaks through the coleoptile and develops into shoot.
5. RICE GRAIN (Oryza sativa, Gramineae) — Like maize but radicle and seminal roots do not persist, instead adventitious roots develop from the base of the plumule and form the fibrous root system. First coleorhiza pierces the base of the grain and then radicle comes out to the soil piercing the coleorhiza.
6. PALM TYPE — This type of germination is the characteristic of Palmae e.g. coconut (Cocos nucifera), fan palm (Borassus flabellifer), date palm (Phoenix sylvestris), etc.
In this type of germination the cotyledon at first begins to grow. The basal part of cotyledon emerges out of the seed by forcing open the soft tissue above the embryo, and grows to a long semi-hollow tubular structure which penetrate the soil; the axis of the embryo is slender and small, carried off along with this long tubular structure known as cotyledonary sheath and is encased by it near the tip.
The upper part of the cotyledon also grows into a spongy mass lying attached to the endosperm functioning as sucking and digestive organs; later the radicle pierces the radicle sheath and grows to form primary root system in the soil. Next the plumule bursts out from one side of the sheath and form aerial leaves. The radicle does not persist to form tap root but in its place numerous adventitious roots come out from the base of the shoot.
II. EPIGEAL TYPES
A. Dicotyledonous exalbuminous seeds
1. TAMARIND (Tamarindus indica, Caesalpinaceae) — The hard testa bursts and radicle comes out first, bends down to the soil and forms root system. Now the hypocotyl grows faster and pulls out the two fleshy cotyledons above the soil. The plumule then grows out into the aerial shoot. The cotyledons remain attached to the elongated vertical hypocotyl for pretty long time, then become greenish, shrivel up gradually and finally fall off after the utilization of food matter.
2. GOURD (Cucurbita maxima, Cucurbitaceae) — Like tamarind the radicle grows out first and fixes the seed to the soil by the root system developing from the radicle. The hypocotyl grows rapidly, forms a loop which straightens up and pulls out the seed. Finally the seed coat gets detached and the cotyledons are exposed and become leaf-like green bodies; then they are turned into a pair of seedling leaves. Now the plumule which lies hidden between the two cotyledons is exposed and forms the aerial shoot.
B. Dicotyledonous albuminous seed
3. CASTOR (Ricinus communis, Euphorbiaceae) — Here the testa is hard and shell-like which bursts near the caruncle and the radicle grows out through it. The two leafy cotyledons together with endosperm are pushed upwards and come out of the soil by the rapid growth of the hypocotyl. Next, the testa cracks and falls off, and the cotyledons open up to form green leafy structures after the endosperm is consumed by the cotyledons. The plumule gradually develops into a leafy shoot. Finally, the cotyledonary leaves wither and fall off.
C. Monocotyledonous albuminous seed
4. ONION (Allium cepa, Liliaceae) — This is an albuminous seed as the endosperm is stored within the seed; embryo is curved and embedded within the endosperm.
The radicle comes out through the sharp end of the seed and goes into the soil. The cotyledon elongates, as a result the base of the curved cotyledon grows out of the seed forming a loop—it turns green and forms the first leaf-like structure; further growth pushes out the seed from the soil. The coiled end of the cotyledon still remains within the seed and absorbs food material from the endosperm.
Further growth of the cotyledon results in the straightening of the green leafy cotyledon bearing the seed at the apex. The plumule, which is not visible and remains covered by the base of the cotyledon in the form of sheath now pierces the sheath, and comes out in the form of a cylindrical foliage leaf. In the meantime adventitious roots develop from the base of the bulb which is actually formed from the base of the plumule sheath.
D. Monocotyledonous exalbuminous seeds
5. In Alismaceae, e.g. species of Alisma, Sagittaria, etc. the seeds are exalbuminous, the food for the nourishment of the embryo is stored either in the single cotyledon or in the large hypocotyl. Here the radicle comes out first, after breaking through the seed coat, it does not grow to form the main root. The hypocotyl after emerging from the seed grows upwards with the single cotyledon being carried at its tip; circle of fibrous roots spring from the base of the hypocotyl.