Life Process

There are many kinds of organisms on Earth, and every one of them gives birth to its own kind. Animals do it by laying eggs, by giving birth to young, or by some asexual method. Plants do it with seeds, with spores, or asexually. Reproduction is the biological process by which an organism produces new individuals of its own kind. Through it, each kind of living thing carries on from one generation to the next.

Importance of reproduction

• a.It keeps a kind of organism from dying out (extinction): new individuals replace the old.
• b.New births balance the deaths in a population.
• c.It is the start of every new generation.
• d.It keeps up the variety of living things (biodiversity) in an ecosystem.

Advanced organisms have male and female reproductive organs. They make male and female gametes, and the two gametes fuse; the fused cell is called a zygote, and it grows into a seed or a baby animal. But many organisms reproduce without any gametes fusing: a new individual grows from a broken piece, from a bud, or from a special reproductive unit. Reproduction by one parent alone, without the fusion of gametes, is called asexual reproduction.

There are six types of asexual reproduction:

• a.fission
• b.budding
• c.fragmentation
• d.regeneration
• e.sporulation
• f.parthenogenesis

(a) Fission

Fission is making new organisms by splitting the parent’s body. It happens mostly in single-celled organisms: prokaryotes such as bacteria (kingdom Monera) and eukaryotes such as amoeba (kingdom Protista). After fission the parent no longer exists: it has become its own young.

Binary fission (binary: in two). A full-grown cell splits into two halves. First its nucleus divides, then its cytoplasm. A cell can split along its length (longitudinal binary fission, as in Euglena) or across it (transverse binary fission, as in Paramecium): the line along which a cell is cut is its axis. Amoeba, Paramecium, Euglena, bacteria, archaea and blue-green algae reproduce by binary fission.

Multiple fission. The cell divides into more than two pieces at once, and each piece becomes a new individual. Plasmodium (the malaria parasite) does this inside our red blood cells; Chlamydomonas, many other protists and some algae do it too.

(b) Budding

In budding, a small outgrowth, a bud, appears on the parent’s body (or cell). It grows into a complete organism and then breaks off. Yeast, hydra, Obelia, corals and sponges reproduce by budding; jellyfish do it at one stage of their life.

In yeast, a bud often forms before the last one has broken off, so you see little chains of yeast cells. Corals never let go: their buds stay joined, and over thousands of years whole reefs grow this way.

(c) Fragmentation

Some simple plants break into pieces, pushed by water, wind or animals, and each piece grows into a new plant. Growing whole new plants from broken pieces in their natural home is called fragmentation. Spirogyra, the slimy green thread of ponds, does this; so do Marchantia (a liverwort), some ferns, lichens and the bread mold Mucor.

(d) Regeneration

Cut a planarian (a small flatworm of streams) into pieces, and each piece grows into a whole new worm. Growing a whole new body from a cut or broken piece is called regeneration. The piece must stay alive in its natural surroundings while it regrows. The parent also grows back what it lost. Regeneration is common in simple animals: hydra, planaria, starfish and tapeworms; an earthworm can regrow lost segments.

(e) Sporulation

Ferns, mushrooms, Marchantia and Mucor make spores. A spore is a tiny reproductive unit, usually a single cell with a tough coat. Spores are made in capsules called sporangia (singular: sporangium); making spores inside sporangia is called sporogenesis. When the sporangium bursts, the ripe spores drift away on the wind. If one lands somewhere damp, with plenty of food (rotting leaves, an old slice of bread), it grows into a new plant or fungus. Reproduction by spores is called sporulation.

(f) Parthenogenesis

In some animals, an egg that has not been fertilized develops into a new individual. This is parthenogenesis (Greek: “virgin birth”). In honey bees, the drones (males) hatch from the queen’s unfertilized eggs; male ants and wasps do too. Some kinds of aphids, termites, grasshoppers, stick insects and even a few lizards can reproduce this way.

Plants have something similar. A banana, a pineapple, or a seedless cucumber grows a fruit without fertilization, so the fruit has no seeds (or only tiny, empty ones). Fruit forming without fertilization is called parthenocarpy, and growers like it, because people like seedless fruit.

Significance of asexual reproduction

• a.No male and female gametes need to be made or brought together.
• b.There is no male and female: one parent is enough.
• c.It can usually start from ordinary body cells, not special reproductive cells.
• d.The organism grows a particular reproductive unit (a bud, a spore, a piece) that grows on into a new body.
• e.The young are genetically identical to the parent and to each other (copies, or clones); any differences come only from their surroundings.

A flowering plant has two kinds of parts. The flower is its reproductive part: it makes the seeds, through pollination and fertilization. All the other parts, the root, stem and leaf, are vegetative parts. Asexual reproduction from a vegetative part is called vegetative propagation.

It can happen by itself in nature (natural vegetative propagation), or people can do it on purpose, with tools and skill (artificial vegetative propagation).

Natural vegetative propagation

(a) Root

The roots of some plants, like rose, sissoo and guava, carry buds. If a piece of such a root is separated from the stem, its buds grow into new shoots. Sweet potato, asparagus and dahlia store plenty of food and water in their swollen roots, and these roots give rise to new shoots with leaves: a sweet potato sprouts them anywhere on its skin, while a dahlia or asparagus root sprouts only from the buds at its top, where it joins the old stem, so each piece planted needs a bit of that. Roots then grow from the base of each new shoot, and it becomes a separate plant.

(b) Underground stem

Some stems grow under the soil and are stuffed with stored food. A potato looks like a root, but it is really a stem: it has buds (its eyes), and only stems have buds and nodes. There are five kinds of underground stem, and every one can grow new plants:

1. Sucker. A side shoot with its own bunch of roots grows from the base of the main stem underground and comes up as a new plant beside it. Mint, banana, bamboo and ground raspberry spread this way.
2. Rhizome. A thick stem that lies flat, just under the surface, full of water and food. It has many buds, and every season some of them grow into new shoots. Ginger, turmeric, calamus root (bojho) and many ferns have rhizomes.
3. Corm. A short, swollen, roundish stem at the base of the shoot, full of food and water, with a few buds that grow into new plants. Taro (pindalu) and gladiolus have corms.
4. Bulb. A very short stem disc wrapped in thick, juicy leaf bases, with buds between them. Onion, garlic and tulip have bulbs; each clove of garlic grows into a new plant.
5. Tuber. A swollen tip of an underground stem. A tuber has small hollows called eyes (aankha); each holds buds that sprout into new shoots. Potato and yam grow from tubers.

(c) Aerial stem

Some stems above the ground have buds at their nodes that grow new shoots and roots when they touch moist soil. A piece of sugarcane with a node grows into a whole new plant, which is why farmers plant cut pieces of cane, not seeds. Dubo (Bermuda grass), which is used in pujas, creeps along the ground and roots at every node, so given the chance it soon covers a whole field.

(d) Bulbils

In some plants, buds at the nodes, at the base of a leaf, or in place of flowers swell with food and fall off. These fleshy buds are called bulbils. Each grows roots and a shoot and becomes a new plant. Agave (kettuke), githa (air yam, whose bulbils are eaten in Nepal), some lilies, onion and garlic make bulbils. The slips of a pineapple, small leafy shoots that grow just below the fruit, are often counted as bulbils too.

(e) Leaf

The leaves of some plants can grow new plants. Bryophyllum (pattharchatta, also called kalanchoe), begonia, Sansevieria (snake plant) and crassula (jade plant) all do. Each notch on the edge of a bryophyllum leaf has a tiny bud. On an old leaf, or one that falls on damp soil, the buds grow into baby plants with their own roots and shoots, a whole row of them round the leaf.

Gardeners and farmers grow new plants from vegetative parts on purpose, with tools and skill, to get more and better crops and fruit. This is artificial vegetative propagation. The methods are cutting, layering, gootee, grafting and tissue culture.

Why not just sow seeds? A seed is made by two parents, so the plant that grows from it is a mix, and may be quite different from the tree you liked. A mango seed from a sweet tree might give sour fruit. A piece of the tree itself grows into a copy of it. And because the new plant starts from a part that is already grown up, it bears fruit years sooner than a seedling.

Advantages of artificial vegetative propagation

• i.The breed and qualities of the plant can be chosen as we need: we copy a plant that is known to be good.
• ii.The new plants bear fruit sooner.
• iii.Many identical plants can be grown from one parent plant in a short time.
• iv.Plants with no viable seeds (banana, seedless grapes, pineapple) can still be grown.

(a) Cutting

Growing new plants by planting cut pieces of a stem, root or leaf is called cutting. Raspberry, ground raspberry and blackberry can be grown from pieces of root; sugarcane, rose and lemon from pieces of stem. For a stem cutting, choose a twig about a year old, and make sure each piece has at least two nodes: one goes in the soil to make roots, and one stays above to make the shoot. After a few days, roots and a shoot start to grow from the nodes.

(b) Layering

Layering is done on plants with long, soft branches near the bottom of the stem, such as jasmine, grapevine and strawberry. The branch chosen is called the layer. A ring of bark 2–3 cm wide is peeled off it, and that part is bent down into the soil and covered, with the tip still above ground. The soil is kept moist. New roots grow from the peeled, buried part while the branch is still fed by its parent. Then the branch is cut from the parent and planted somewhere else.

(c) Gootee

Gootee (गुटी) is air layering: layering done up in the air, for branches too stiff to bend down. Take a branch at least a year old of litchi, mango, guava, orange or pomelo (bhogate), and peel a ring of bark 2–3 cm wide. Cover the bare part with a ball of moist soil (or moss) mixed with minerals and a rooting hormone, and wrap it tightly in a plastic sheet to keep it moist and free of infection. After a month or two, roots grow into the ball from the peeled part. The branch is then cut off below the ball and planted.

(d) Grafting

Grafting is joining parts of two closely related plants so that they grow as one. It is used to make fruit plants that bear good fruit early.

• The stock (rootstock) is the lower part, with roots: a young plant about a year old, grown from seed, cut off 20–30 cm above the ground. It gives strong roots.
• The scion (or graft) is a twig from a fruit tree of good quality and yield, with a few buds (nodes), with or without a few half-cut leaves.

The cut ends of the two are shaped so that they fit tightly, like a key in a lock, and the living layers under their bark touch, so that water and food can flow from one to the other. The joint is sealed airtight with grafting wax or grafting tape. In a few weeks the wounds heal and the two parts grow into one plant. It soon bears fruit just like the tree the scion came from.

In Nepal, the Nepal Agricultural Research Council (NARC) and many private nurseries prepare thousands of grafted saplings of mango, orange, pear, apple, avocado and other fruits every year. Farmers who plant them get good fruit in a few years, which has helped fruit farming in Nepal grow.

(e) Tissue culture

Tissue culture is the most modern way of growing new plants. A tiny piece of the parent plant, a few cells or a bit of tissue from a shoot tip, called the explant, is placed on a jelly of nutrients, the culture medium, in a clean glass jar. (A medium is a mixture of the nutrients that cells or microorganisms need to grow.) Everything is kept germ-free, and the light and temperature are controlled in the lab.

1. The explant grows into a small, pale-green lump of cells that are all alike, called a callus.
2. Plant hormones added to the medium make the callus form shoots and roots: a tiny plant, a plantlet.
3. The plantlets are moved into pots, toughened up in a greenhouse, and planted out.

One small piece of a plant can give hundreds of thousands of identical, disease-free plantlets in a few months. Labs in Nepal grow banana, potato, orchids, strawberries and flowers this way.

1. Choose the best answer

2. Differentiate between

3. Give reasons

4. Answer these questions

Many-celled plants and animals usually reproduce by making gametes: male gametes made by male reproductive organs, and female gametes made by female ones. Reproduction by the fusion of a male gamete and a female gamete of the same species is called sexual reproduction. The fused cell is the zygote, and it grows and develops into a new individual.

• Organisms with only male or only female organs are unisexual. In humans and cows, the male and female organs are in separate individuals. (A maize plant has separate male and female flowers: each flower is unisexual, though the plant has both.)
• Organisms with both male and female organs in the same body are bisexual, or hermaphrodite. A mustard flower has both; so does an earthworm.

Characteristics of sexual reproduction

• a.Male and female gametes are formed, and they fuse to make a zygote.
• b.The organism has male and female reproductive organs, either in the same body or in different bodies.
• c.It keeps the ability to reproduce going from one generation to the next.
• d.The young are better able to adapt to a changing environment.
• e.The young show variation: each one is a new mix of its two parents, unlike its brothers and sisters.
• f.Because of this variation, sexual reproduction makes evolution of living things much more likely.

In flowering plants, the flower is the reproductive part. It has male and female parts. Sexual reproduction in a flowering plant happens in two steps: pollination, then fertilization.

The parts of a flower

Whorl	Parts	Job
Calyx	the green sepals	protects the flower in bud
Corolla	the colored petals	attracts insects and birds
Androecium (male)	the stamens: each an anther on a thin filament	the anther makes pollen grains
Gynoecium (female)	the pistil (or carpels): stigma on top, style in the middle, swollen ovary at the base	the ovary holds the ovules, each with an egg cell

Each stamen ends in one anther, usually with two lobes; inside it are pollen sacs full of powdery pollen grains. When the pollen is ripe, the anther bursts open and the pollen is shed.

(A) Pollination

The transfer of pollen grains from an anther to a stigma of the same kind of plant is called pollination.

A pollen grain is far too small to see one by one: dust-sized, a few hundredths of a millimeter across. It has a tough wall of two layers. The outer layer is often covered in spikes, ridges or a sticky oil, which help it stick to an insect and to the stigma. Inside, it carries the cells that will make the male gametes. Each kind of plant has pollen of its own shape, so a scientist can tell from a pollen grain which plant it came from.

There are two types of pollination: self-pollination and cross-pollination.

(a) Self-pollination

Self-pollination is the transfer of pollen to a stigma of a flower on the same plant. It has two forms:

• Autogamy: pollen falls on the stigma of the same flower. No helper is needed.
• Geitonogamy: pollen reaches the stigma of another flower on the same plant. This needs a carrier, an insect or the wind, but the genes are still the plant’s own.

Self-pollination is common in small, dull flowers without nectar or scent, which don’t need to attract insects. Rice (paddy), wheat, peas and groundnut usually self-pollinate; in a pea, the flower pollinates itself before it even opens.

Advantages of self-pollination

• a.The good qualities of the parent plant are kept in its young.
• b.It nearly always produces seeds.
• c.It is easy: no pollinator is needed.

Disadvantages of self-pollination

• a.It brings no new characters into the young.
• b.Over many generations of only self-pollination, the plants slowly become weaker and make fewer seeds.
• c.Bad characters in the line can’t be corrected by mixing with other plants.

(b) Cross-pollination

Cross-pollination is the transfer of pollen from a flower of one plant to a flower on a different plant of the same species. It usually happens in colorful, scented flowers with nectar, and in flowers whose anthers and stigma ripen at different times, so that they can’t pollinate themselves. Something has to carry the pollen: a pollinating agent.

Pollinating agent	Name	The flowers are…	Examples
wind	anemophily	small, dull, no scent; lots of light, dry pollen; feathery stigmas	maize, rice, wheat, grasses (and pine’s cones)
water	hydrophily	small; pollen floats on or under the water	Vallisneria, Hydrilla
insects	entomophily	bright, scented, with nectar; sticky pollen	mustard, sunflower, pumpkin
birds	ornithophily	often red or orange, tube-shaped, with lots of nectar, little scent	coral tree (phaledo), silk cotton (simal), hibiscus

The birds are small nectar-drinkers: in Nepal, sunbirds and tailorbirds (fisto); in the Americas, hummingbirds. Mango and litchi flowers are visited by flies and bees, and also shed pollen on the wind. Many plants use more than one agent.

Advantages of pollination

• a.Cross-pollination produces new plants with new, often better, combinations of qualities.
• b.Plants from cross-pollination are better able to adapt to their environment.
• c.It produces healthier seeds that germinate better.

Disadvantages of pollination

• a.Cross-pollination depends on pollinating agents; without them, it may not happen at all.
• b.It is hard to keep a plant’s good qualities exactly as they are: the young are mixtures.
• c.The young may also get bad, unwanted characters from the other parent.

(B) Fertilization

The fusion of a male gamete with a female gamete of the same species is called fertilization. In flowering plants, it happens inside the ovule, which sits inside the ovary.

A pollen grain that lands on the right stigma takes in water and sugar from the stigma’s sticky fluid, and germinates: it grows a long, thin pollen tube. The tube grows down through the style towards the ovary, like a root growing through soil. On the way, the pollen grain’s contents move into the tube and form two male gametes. The tube enters an ovule through a tiny opening, the micropyle, and releases both male gametes inside it.

• One male gamete fuses with the egg cell (the female gamete). They form the zygote, which grows into the embryo, the baby plant.
• The other male gamete fuses with the two polar nuclei in the large central cell of the ovule (in many plants these have joined into one secondary nucleus). This makes the endosperm nucleus, which grows into the endosperm, the food store of the seed.

Two fusions happen in each ovule, so this is called double fertilization. It is found only in flowering plants. Afterwards, the ovule becomes the seed and the ovary becomes the fruit; the petals, stamens, stigma and style usually wither and fall.

In most animals, males and females are separate. When they become adults, they can reproduce. Each has special organs for making gametes: the male has testes, which make sperms, and the female has ovaries, which make eggs or ova (singular: ovum). Sperms and eggs are the male and female gametes. Making gametes in the testes and ovaries is called gametogenesis. (In plants, gametes are made in the anthers and the ovules.)

The two gametes look nothing alike.

• A sperm is tiny and built to swim: an oval head holding the nucleus, with a cap called the acrosome that helps it get into the egg; a middle piece packed with mitochondria, which give it energy; and a long tail that whips to drive it forward.
• An ovum is large, round and still. It has a nucleus (with a nucleolus), a lot of cytoplasm with stored food, and a plasma membrane, surrounded by a clear jelly layer, the zona pellucida, and a ring of small cells, the corona radiata.

A man makes millions of sperms every day; a woman’s ovaries usually release just one ovum about once a month.

Fertilization in animals

After mating (a male and a female coming together so that his sperms reach her eggs), the sperms swim towards the egg. Many sperms may reach it, but only one fertilizes it: as soon as one sperm gets in, the egg’s outer layer changes and shuts all the others out. The nucleus of the sperm fuses with the nucleus of the egg. This is fertilization. The fertilized egg is now a zygote, carrying genes from both parents. The zygote is the first cell of the new body. It divides again and again into an embryo, which grows into a baby. The time a zygote takes to develop into a baby ready to be born or hatched is called the embryonic period (in mammals, the pregnancy or gestation period: about 9 months in humans and cows, about 2 months in dogs).

There are two kinds of fertilization in animals.

• Internal fertilization happens inside the female’s body, after mating. In mammals, the sperm meets the egg in the oviduct (in humans, the fallopian tube). Insects, spiders, reptiles, birds and mammals have internal fertilization.
• External fertilization happens outside the female’s body, in water. The female lays a large number of eggs in water, or on wet plants or stones, and the male releases sperms over them. Most fishes and amphibians (frogs, toads) do this.

External fertilization	Internal fertilization
outside the body, in water	inside the female’s body
no mating; eggs and sperms are shed into water	after mating
very many eggs (a frog lays thousands)	far fewer eggs
eggs and young are unprotected; most are eaten	the egg is protected; more young survive
most fishes, frogs, toads	insects, reptiles, birds, mammals

1. Choose the best answer

2. Differentiate between

3. Give reasons

4. Answer these questions

After fertilization, the zygote inside the ovule divides again and again and becomes the embryo, a tiny baby plant. When the embryo is complete, the whole fertilized ovule has become a seed. A seed, then, is a fertilized, ripened ovule with an embryo inside it, which can germinate into a new plant when it gets the right conditions. Just as babies are part of the life cycle of animals, seeds are part of the life cycle of flowering plants.

Seeds come in every size and shape: as fine as dust (orchids), small and round (mustard), flat (pumpkin), wrinkled (dry peas), winged (pine), hairy (simal), or huge (coconut).

Dispersal of seeds

Seeds grow inside fruits. A fruit may hold one seed (mango) or many (papaya). When the fruit ripens, the seeds separate from it. If they all fell right under the parent, the seedlings would fight it and each other for light, water and space. So many plants send their seeds away. The carrying of seeds from the parent plant to other places is called dispersal of seeds. It is an adaptation of flowering plants that lets them reach new places and settle there. Seeds are dispersed in five main ways:

• a.by wind
• b.by water
• c.by animals
• d.by gravity
• e.by bursting of pods

(a) Dispersal by wind

Seeds carried by the wind are small and light, and many have hairs like a parachute, wings, or little air-filled pouches, which lower their density and keep them floating in the air for a long time. When the fruit ripens and opens, the seeds lift off and the wind carries them far away. Simal (red silk cotton), aak (Calotropis), kans grass, dandelion, cotton, maple and pine seeds travel by wind.

(b) Dispersal by water

Plants of rivers, ponds, wetlands and seashores (hydrophytes, and plants of river banks) often have seeds or fruits that are waterproof, light and able to float. When they ripen and drop into the water, the current carries them far away. Lotus, water lily, mangroves, coconut and cattail are dispersed by water.

(c) Dispersal by animals

Many fruits are tasty, and are eaten by monkeys, deer, squirrels, rats, rabbits, birds and fruit bats (flying foxes). Animals help in two ways.

• Eaten and passed out. The animal swallows small seeds with the fruit. The seeds have hard coats, so they are not digested, and they pass out in the animal’s droppings (egested) far away, often with a little manure. Jamun, harro, barro, amala, guava, pear, kaphal and ainselu (raspberry) are spread like this. Other animals eat the flesh of a bigger fruit and drop the hard seed somewhere else.
• Hooks and bristles. Some seeds have hooks or spikes that stick to animals’ fur, or to your socks. They fall off later, far from the plant. Kuro (cobbler’s pegs, Bidens) and bhede kuro (cocklebur) are dispersed this way.

People are the biggest seed carriers of all: we have taken the seeds of cereals, pulses, fruits and vegetables all round the world. Maize, potatoes, tomatoes and chillies all came to Nepal from the Americas.

(d) Dispersal by gravity

Some fruits and seeds are big and heavy. When ripe, they simply fall to the ground under the parent plant. The fruit rots, and the seed germinates if conditions are right. Mango, apple and jackfruit fall this way (and are often carried farther by animals that eat them).

(e) Dispersal by bursting of pods

In some plants, the ripe fruit (a pod) dries in the sun and its walls pull unevenly as they dry, until it bursts open with a snap and throws the seeds out around the parent plant. Mustard, okra (ladies’ finger), beans, peas, ipil-ipil, dhaturo (thorn apple), laal gedi (rosary pea) and balsam (Impatiens) are dispersed like this.

Structure of a seed

A seed has three main parts: the seed coat, the embryo and the endosperm.

(a) Seed coat

The seed coat is the outer covering of the seed. It has two layers: a thick, tough outer layer, the testa, and a thin inner layer, the tegmen. On it you can see the hilum, a scar where the seed was attached to the fruit, and next to it a tiny hole, the micropyle, through which water enters when the seed is soaked. Litchi and tamarind have hard, shiny seed coats. (A coconut’s hard shell is not its seed coat but part of the fruit wall.) The seed coat protects the seed from injury, drying, insects and germs, and keeps it from germinating during bad conditions.

(b) Embryo

The embryo is the most important part of the seed: it is the baby plant, the part that grows when the seed germinates. It has three parts:

• the radicle, which grows into the root system;
• the plumule, which grows into the shoot system (stem and leaves);
• the cotyledon(s), the seed leaves. They protect the radicle and plumule, and in many seeds they store starch and protein for the young plant. In seeds like bean, they come above the ground and turn green, making food by photosynthesis for a few days: the plant’s first leaves.

A monocot seed (maize, rice) has one cotyledon; a dicot seed (bean, gram, mango) has two; and a gymnosperm (pine) has two or more, in a ring. In a monocot, a protective sheath, the coleoptile, covers the plumule; the first leaf grows out through it.

(c) Endosperm

The endosperm lies just under the seed coat. It stores food (starch and protein) for the embryo until the young plant can make its own food. In monocots such as maize, rice and wheat, the endosperm takes up most of the seed, and the cotyledon only absorbs food from it and passes it to the embryo. Most dicot seeds (castor is an exception) have no endosperm left when they are ripe: the embryo has already moved the food into its two fat cotyledons. That is why a bean is mostly cotyledon, and a grain of rice is mostly endosperm.

Germination of seed

A ripe seed can stay alive without growing for months or years if it is kept dry. This resting state, in which the seed stays unchanged and doesn’t grow, is called dormancy. It lets seeds wait out winter or drought, and lets farmers store seed until the next sowing. Different seeds stay alive for different lengths of time; well-stored seeds of many crops keep for several years.

When a seed gets enough water, air (oxygen) and warmth, it absorbs water and swells; chemical reactions start in its cells, and the embryo wakes up. The radicle comes out first and grows down into the root system. Then the plumule grows up into the shoot system. The growth of an embryo into a young plant, with its own root and shoot, is called germination.

There are two ways the young shoot comes up:

• In bean and gourds, the stem below the cotyledons (the hypocotyl) grows into a hook, pulls the cotyledons above the ground, and they turn green: epigeal germination.
• In maize, gram and pea, the cotyledons stay underground, and only the plumule grows up (in maize, through its coleoptile): hypogeal germination.

The young plant that grows from a germinated seed, until it is ready to plant out, is a seedling. A sprouting seed mobilizes its food: enzymes turn its stored starch and protein into sugars and smaller molecules, and new vitamins are made. That is why sprouted grains and pulses are easy to digest and nutritious.

Factors affecting germination

Germination depends on things inside the seed (a living embryo, enough stored food, dormancy broken) and on its surroundings. The outside factors are water, air, temperature and light.

(a) Water

Seeds are dried well in the sun before they are stored. In a dry seed, almost no chemical activity (metabolism) goes on. When it is soaked, the seed absorbs plenty of water through its micropyle and coat, swells, its dormancy breaks, and its cells become active. The water also softens the seed coat so the radicle can break through. That is why many seeds are soaked for about a day before sowing.

(b) Air

Germinating seeds need oxygen. The growing embryo needs a lot of energy, which its cells get by respiration: they use oxygen in their mitochondria to break down stored food. A dormant seed respires so slowly that it hardly uses any food or oxygen; once it germinates, it needs plenty. A seed buried in waterlogged soil, or kept under water, often fails to germinate or rots, because it can’t get air.

(c) Temperature

Seeds don’t germinate if it is too cold or too hot: their enzymes work only in a range of temperature. Many seeds germinate best at about 16–24 °C; crops of hot places such as maize and rice prefer it warmer, about 25–30 °C. That is why each crop is sown in its own season. A suitable temperature also helps break dormancy. Seeds buried in soil are kept moist, warm (the soil is warmed by the sun) and supplied with air.

(d) Light

Most seeds can germinate in the dark, under the soil. But as soon as the shoot comes up, the seedling needs light to make food by photosynthesis. Some small seeds (lettuce, many weeds and forest trees) do need light to germinate, so they wait in the soil until it is dug or a gap opens in the forest. That is why the seeds dropped on a dark forest floor may wait for years.

Importance of germination of seeds

• a.Germination produces the next generation of plants, so it is needed for the plant’s kind to carry on.
• b.Good germination of seeds on farms means a better harvest, which helps farmers, consumers and the country.
• c.Sprouted seeds are easier to digest and have more of some vitamins, so it is good to sprout grains and pulses before eating them.
• d.Good germination of the seeds of grasses and trees in forests and pastures helps to keep biodiversity.

1. Choose the best answer

2. Differentiate between

3. Give reasons

4. Answer these questions