In this article, we propose to discuss the conditions necessary for germination and types of germination in plants.
Germination is the awakening of the dormant embryo. In all mature Angiospermic seeds the embryo lies in a dormant state when its physiological activities come to a minimum.
Even its respiration is so slow as to be detectable only by sensitive instruments. As soon as the necessary conditions are satisfied this dormancy is broken and the phenomenon of germination begins.
Like all growth processes, the process of germination is irreversible, i.e., when germination has once commenced it cannot go back and the seed cannot be brought back to the dormant state.
Under germination are included all changes that take place from the time when the dry seed is placed under suitable conditions to the time when the seedling becomes established on the substratum.
Conditions Necessary for Germination:
In order that germination may begin, certain conditions are to be fulfilled. Some of these conditions are external while others are internal.
(A) External Factors:
Water is of primary importance in germination. No seed can germinate unless it is thoroughly moistened. Actual submergence under water is not necessary and may even be harmful as it may choke oxygen supply. Water absorbed by the protoplasm enables the resumption of vigorous physiological activities.
Digestion, -respiration or conduction cannot go on without water. The swelling of the embryo enables it to burst through the seedcoats which again, are softened by water absorption. Oxygen cannot get through the testa unless it is moist.
Oxygen is necessary for the seed as for any other living organ. The need of oxygen is even greater during germination as respiration and all physiological activities are more vigorous at this stage. Germination can go on for some time even without oxygen but it is soon checked.
(3) Optimum temperature:
Like all physiological activities germination is affected by temperate. There is a certain minimum and a certain maximum beyond which germination cannot take place. Within this range there is a certain optimum temperature where germination is most satisfactory.
This range varies from plant to plant. Seeds are not usually expected to germinate below 0°C and above 50°C and the optimum often lies between 25-30°C. Some seeds do not germinate well in any given temperature but prefers a fluctuation of temperature during the germinating period.
Light is not considered as an essential factor since’ germination takes place even without light. But, recent experiments have shown it to be of greater importance than what was hitherto thought.
Its pronounced effect on germination cannot be minimised. Light affects the germination of different seeds in different ways. It was previously thought that light retards germination in most cases.
But experiments by Kinzel (Germany) with about one thousand species of plants showed that the germination of 70% of seeds is favoured by light, in about 26% light inhibits germination while about 4% of the seeds are indifferent to light.
Tobacco, Rumex, mistletoe (Viscum album) seeds do not germinate in darkness while Datura and tomato seeds do not germinate well in light. Sometimes, some special treatment of seed (e.g., removal of seedcoat) may do away with this influence of light.
(B) Internal Factors:
A normal seed is expected to the internally capable of germination.
The following factors are of importance in determining this internal capacity:
(1) Food and auxins:
All normal seeds contain a supply of food ‘which is necessary for the growing embryo and the young seedling. It has already been seen that this food may be contained in the cotyledon (mainly protein and starch to pulses; oil in mustard, groundnut, sesame, linseed, cucurbits, sunflower, etc.), endosperm (starch and a little protein in cereals; oil in castor, coconut, etc.; cellulose in date, areca-nut, etc.; mucilage in mallow or Malva sp., etc.), perisperm (black pepper, cardamom, etc.) or testa (pomegranate). Auxins are growth-promoting substances whose presence is essential for growth during germination.
(2) Completion of resting period (dormancy):
Many Angiospermic seeds cannot germinate as soon as they are formed. They have to undergo a period of dormancy or resting period. The period of dormancy varies from plant to plant. It may be a few days or some months.
Most cereals are capable of germination immediately after harvesting while some other seeds do not-germinate till after a year. Some plants do not need any resting period. The period of rest may be necessary for various reasons.
In some seeds the seedcoat is so hard that it takes time to wither. In this case the period of dormancy may be cut short by breaking open the seedcoat.
In others, the embryo may take time to be fully differentiated or some after-ripening may be necessary for the seed. Different treatments may hasten this ‘after-ripening.
Seeds retain their viability (capacity to germinate) for a definite period of time after which the embryo becomes dead for all practical purposes. Viability test is necessary to ascertain the germinating capacity of any seed. Conditions of storage (temperature, humidity, etc.) and circumstances in which the seed matured often determine the period for which the seed remains viable. Seeds usually keep well when kept dry, cold and free from insects or fungi.
Proper drying of seed is extremely important in the retention of viability although there are a few seeds (e.g., willow, poplar, maple) which fail to germinate if too dry. Weak or immature seeds lose their viability quickly. Hard-coated seeds often remain viable for a long time. Even when everything is satisfactory, it is found that the period during which the seed remains viable varies in different plants. The period may vary from one growing season to many years.
The longest authentic record of this period of viability is that of some lotus (Nelumbo nucifera) seed found within peat at the dried up bottom of a lake in Manchuria whose age has been calculated to be some eight hundred years.
Stories of 5,000 year old wheat or other cereal grains found in tombs in Egypt or Mohenjodaro proving viable are without any foundation. Seeds that old, are found to be in a carbonised condition. When viability is apparently lost, it may sometimes be restored by different treatments. This shows that viability may be lost even when the embryo is not really dead.
Changes during Germination:
When all the necessary conditions are satisfied, the first change noticed is swelling of the seed by rapid imbibition and osmosis of water. This may cause a bursting of the seed-coat. Absorption of water causes a vigorous resumption of physiological activities by the protoplasm. There is rapid respiration and copious secretion of enzymes which causes digestion of stored food. Insoluble food is rendered soluble and complex food made simple.
This simple food solution is diluted by water and conducted towards the growing epicotyl, hypocotyl, radicle and plumule. Food is translocated from perisperm to endosperm, from endosperm to cotyledon and from cotyledon to the growing organs according as which of them are present in the seed. Assimilation of this food by the growing organ enables growth and the seedling soon assumes its ultimate shape.
When the seed is placed in the soil and growth has become vigorous the radicle is the first organ to grow vigorously. It comes out through the micropyle and fixes the seed to the soil. After this, either the hypocotyl or the epicotyl begins to grow. When the hypocotyl grows first, it pushes the cotyledonary node and all other parts of the seed (with or without the seedcoat) out of the soil and the mode of germination is called epigeal or epigeous.
When the epicotyl grows first only the plumule is pushed out of the soil while the cotyledonary node, cotyledons and all other parts remain under the soil.
This type of germination is called hypogeal or hypogeous. In Monocots, however, the hypocotyl does not grow but the epigeous and hypogeous nature is determined by the growth of the cotyledon itself.
Types of Germination:
A. Epigeal Germination:
Epigeal germination is shown by some dicotyledonous plants and a few monocots. Common examples of this type of germination are found in:
Dicotyledonous exalbuminous: Cucurbits, mustard, tamarind, French bean (Phaseolus vulgaris), Lablab (Dolichos lablab), sunflower.
Dicotyledonous albuminous: Castor.
Monocotyledonous albuminous: rare, found in onion.
Monocotyledonous exalbuminous: Alisma plantago.
(1) Gourd (Cucurbita Maxima):
As germination begins, the straight radicle comes out and fixes the seed to the soil with the secondary roots coming out of the radicle. The hypocotyl next grows so quickly that it forms a loop which comes out of the soil and pulls out the rest of the seed.
Frequently, the seedcoat gets caught to a peg-like projection at the base of the hypocotyl so that it is cast off easily and the cotyledons are brought out in the air.
Next, the cotyledons open out like two leaves, become green, large and thin so that they look and behave like ordinary leaves in every way though differing in form from normal cucurbita leaves. The plumule within the cotyledons becomes exposed and soon grows into, the aerial shoot.
(2) Mustard (Brassica spp.):
The seedcoat is thinner, the two cotyledons are much oily and the radicle is curved in the tiny seed. The stages of germination are essentially the same as in the cucurbits.
(3) Tamarind (Tamarindus Indica):
The testa in this case is very hard. Nevertheless, the radicle comes out first after the testa is burst and fixes the seed by forming the root system. The hypocotyl now grows fast and soon pulls out the two large and thick cotyledons.
The plumule then grows out into the aerial shoot. The cotyledons turn greenish, gradually shrivel up and finally drop off as the food matter within them is used up. The cotyledons, although turning greenish, never look like ordinary leaves as they do in Cucurbita.
(4) & (5) Lablab (Dolichos lablab) and French Bean (Phaseolus vulgaris):
Both of them germinate like tamarind. The fleshy cotyledons do not become leafy but behave like tamarind cotyledons. Lablab is the common flat bean of the plains.
(6) Castor (Ricinus Communis):
The shell-like testa first bursts near the caruncle and the radicle grows out. Subsequent growth of the hypocotyl pulls out of the soil the two thin cotyledons enclosed in the endosperm. The testa is cracked and is soon cast off but the cotyledons do not come out of the endosperm until the latter is almost consumed by the former.
The cotyledons then open up and become green and leafy while the plumule slowly develops into the leafy shoot. The remnant of the endosperm withers and drops off.
(7) Onion (Allium Cepa):
Epigeous germination is very rare among the Monocots. Onion is one of the very few examples. In this case the radicle as well as the base of the curved cotyledon (scutellum) grows out of the seed.
The radicle penetrates the soil while the other end of the cotyledon remains within the endosperm and sucks the food material. The base of the cotyledon grows further, turns green and pushes the seed out of the soil.
The plumule is not visible so long, as it is covered by the base of the cotyledon in the form of a sheath just above the radicle. The plumule now pierces this cotyledon sheath and forms the first cylindrical foliage leaf.
Meanwhile, adventitious roots develop from above the radicle forming a fibrous root system which is characteristic of monocots.
It should be noted in this case that the seed is pushed out of the soil not by the growth of the hypocotyl as in the other cases but by that of the base of the cotyledon itself.
(8) Peperomia Peruviana:
This is a dicot (Piperaceae) with endosperm and perisperm showing a peculiar type of germination. During germination one of the cotyledons remains hypogeal within the endosperm sucking the food material from the latter as well as from the perisperm while the other cotyledon becomes epigeal and green.
B. Hypogeal Germination:
Hypogeal germination is shown by some dicotyledons and by most of the monocotyledons. Common examples are:
Dicotyledonous exalbuminous: Pea, gram, broad bean (Vicia faba), scarlet runner bean (Phaseolus multiflorus), mango, jack-fruit.
N.B. In pea and runner bean sometimes a tendency to epigeous germination is noticed.
Monocotyledonous albuminous: Rice, maize, wheat, coconut, date, areca-nut, fan (palmyra) palm.
(1) Pea (Pisum Sativum):
The radicle comes out first, penetrates the soil and forms a root system by giving out secondary branches. It is the epicotyl which grows first here.
It arches out and carries the plumule above ground. The plumule soon forms the aerial shoot. The cotyledons, therefore, remain under soil throughout.
(2) & (3) Gram (Cicer Arietinum) and Broad Bean (Vicia Faba):
The mode of germination is -essentially the same as in pea. The cotyledons remain underground and are gradually used up.
(4) Mango (Mangifera Indica):
The seed is covered by the hard endocarp. Absorption of water causes swelling and rupture of the endocarp and the seedcoat. The radicle comes out and forms a root system.
The epicotyl then grows, comes out through a slit in the cotyledons and takes the plumule out of the soil while the cotyledons remain within the endocarp below. The first leaves are copper-coloured which gradually become green.
(5) Jack-fruit (Artocarpus Heterophyllus):
Germination is hypogeous. Although the two unequal cotyledons remain underground, they develop a green colour.
(6) Rice (Oryza Sativa):
A day or two after the seed is placed in moist soil, the coleorhiza pierces the base of the fruit and appears as a glistening knob. The radicle next penetrates the soil after splitting the coleorhiza.
The coleoptile comes out now. (If the seed remains submerged in water, sometimes the coleoptile may come out first, the emergence of the radicle being delayed.) Immediately after the emergence of the radicle two other roots grow from its base and these are called seminal roots.
The radicle and seminal roots give rise to” secondary branches but, as opposed to the dicotyledons, the radicle does not form the root system. The base of the coleoptile and the mesocotyl now lengthen somewhat and the plumule soon comes out piercing the -coleoptile.
Meanwhile, adventitious roots are formed from the base of the plumule (top of the mesocotyl) or from the lowermost nodes of the stem. These adventitious roots form the fibrous root system of the mature plant. The pierced coleoptile soon withers away.
(7) Maize or Corn (Zea Mays):
The mode of germination is essentially the same. The radicle emerges first by piercing the fruit wall and the coleorhiza. The coleoptile follows. The coleoptile and plumule develop as in rice. Three seminal roots develop from above the radicle (one opposite scutellum and two others from slightly above that point). In exceptional cases the number of seminal roots may vary from 0 to 10.
The radicle and the seminal roots with their branches persist throughout the life of the plant and are not short-living as was previously thought. The adventitious roots are formed from the lowermost nodes above the mesocotyl.
(8) Wheat (Triticum Spp):
Germination as in rice and maize. Seminal roots number 4 to 5. The number of seminal roots is sometimes found to be variable. Adventitious roots, as in others, develop from above the mesocotyl though some of them may develop on the coleoptile. The branched radicle and the seminal roots probably persist throughout the life of the plant along with adventitious roots higher up.
(9) Coconut (Cocos Nucifera):
The small embryo below one eye on the shell on the top of the endosperm is undifferentiated at first. During germination the lower (actually the upper end as the fruit remains in a hanging upside down position) end of the embryo forms the cotyledon which begins to grow as a spongy structure inside the endosperm.
This spongy cotyledon increases in size as it absorbs the food material stored within the endosperm. The upper end of the embryo develops through the eye carrying the radicle and the plumule. The plumule pierces the fibrous pericarp and emerges like a horn. This develops the aerial shoot even before the roots have come in contact with the soil. The radicle fails to develop any further but several adventitious roots grow from the base of the plumule. The seedling becomes established when the adventitious roots penetrate the soil.
(10) Date (Phoenix Sylvestris):
The stony seed is formed mainly of a cellulose endosperm on one side of which is the small embryo. During germination the cotyledon begins to grow.
The base (sheath and stalk) of the cotyledon grows out by forcing open the soft tissue above the embryo (which often comes out in the form of a lid) while the upper part remains inside the endosperm gradually increasing in size and absorbing more and more of the reserve cellulose transforming the latter into sugar.
The base of the cotyledon which penetrates the soil is like a sheath enclosing the axis at its extremity.
This sheath may be called the cotyledonary sheath. The radicle pierces the coleorhiza at the lower end and forms a more or less strong primary root system in the soil which is stronger than in other monocots although it does not form the main root system.
Next, the plumule bursts out from one side of the sheath anchdevelops aerial leaves. The upper part of the cotyledonary sheath acts as a stalk for the part of the cotyledon which is still inside the endosperm sucking the nutrients.
Adventitious roots are later given out from the base of the aerial shoot and form the main root system of the growing plant.
(11) & (12) Palmyra or Fan Palm (Borassus Fiabellifer) and Betel- or Areca-Nut (Areca Catechu):
Both of these have got hard cellulose endosperms. The betel-nut endosperm is also ruminated. The manner of germination is practically the same as in date palm.
Vivipary means germination of the seed within the fruit while still attached to the mother plant. In the animal world, mammals are viviparous as the embryo differentiates into the young individual while still in the mother’s womb.
Birds lay eggs and ordinary plants develop seeds and not seedlings on the parent body, so they are not viviparous. Vivipary, however, is found in a number of plants. In the vegetable Sechium edule of Cucurbitaceae (locally called squash), common in Indian hill towns, it is often seen that the seed germinates while inside the fruit still attached to the mother plant.
So is the case of coconut. Paddy grains germinate on the mother plant if they get sufficient moisture. Such vivipary is always dependent on excessive moisture in the atmosphere or within the fruit (e.g., lemon and oranges, tomatoes, melons) and on the absence of any period of dormancy. Vivipary may also take place through vegetative organs, e.g., bulbils of Agave (discussed in connection with ‘bud’).
Beside the above, a special kind of vivipary is noticed in the mangrove plants found in estuarine tidal shores of the tropics, e.g., the Sundarbans in the Gangetic delta and similar mangrove formations in the estuaries of different Indo-Burmese rivers.
The seed embryos in these plants do not have any resting stage and continue to grow uninterruptedly inside the fruit. The radicle first comes out of the fruit and then the hypocotyl begins to grow very vigorously so that it looks like a club which is usually 2 to 9 Inches long and may sometimes attain 18 inches.
The length varies according to the species of plant and may be determined by the depth of the water below. The plumule also grows somewhat while the cotyledons remain inside the fruit acting as haustoria.
Some fast growing mangrove plants (e.g., Sonneratia caseolaris of Sonneratiaceae) do not show any vivipary while in a few others, e.g., Avicennia (Verbenaceae) and Aegiceras (Myrsinaceae), the type of vivipary is rudimentary.
In markedly viviparous plants the process of germination is slow. Common examples are Rhizophora mucronata, Ceriops decandra, Bruguiera gymnorhiza and Kandelia candel all belonging to the family Rhizophoraceae. Of these, Rhizophora grows in the deepest water and shows the largest hypocotyl while Ceriops comes next.
When the hypocotyl grows very heavy the fruit gets detached from the plant or, in some cases, the axis (i.e., radicle, hypocotyl and plumule) gets detached from the cotyledons and, because of the heaviness and the shape of the hypocotyls, it falls vertically down like a dart so that the radicle penetrates the soil below the shallow water.
It soon forms a root system while the plumule grows safely above the water level. Sometimes, if the water be too deep, the viviparous fruit may float with the hypocotyl hanging down and get rooted where the depth of water is just right enabling the radicle to get fixed to the soil.
This is a peculiar natural adaptation particularly suiting this type of plants .