Chapter 6: Seeds — Structure and Germination
In 2005, a scientist in Israel planted a seed that had been waiting for about two thousand years.
The seed had been recovered decades earlier from excavations at Masada, an ancient hilltop fortress overlooking the Dead Sea. It was a date palm seed — hard, brown and completely dry. It had lain in the rubble of a storeroom through twenty centuries of hot, dry desert air. Radiocarbon dating of fragments from the same batch placed the seeds at roughly two thousand years old.
Nobody expected anything to happen. The seed was soaked in water, potted in soil, and kept warm and watered. Within a few weeks a shoot pushed up through the surface. The seedling was nicknamed Methuselah, and it grew into a date palm tree.
Think about what that means. For two thousand years, something inside that seed was alive. Not growing, not visibly breathing, not using sunlight — but alive, and still able to build a whole tree once it was given water, warmth and air.
A seed is not a lifeless object. It is a young plant, packed with its own food supply, sealed inside a protective coat, and switched almost completely off. This chapter is about what is inside that package, and about what happens on the day it switches back on.
Diagram
Fig. 6.0-1 — A Two-Thousand-Year-Old Seed Sprouts Line drawing in two panels: on the left, a single hard, oval date palm seed shown whole against a plain background; on the right, the same seed in a pot of soil with a young palm seedling emerging, showing a slender shoot with one narrow first leaf above the soil and roots below. Label seed, first leaf, root. Caption panels A and B.
6.1 What Is A Seed?
Every flowering plant you have ever seen began as a seed. Before we can follow what happens inside a seed when it starts to grow, we need to be clear about what a seed actually is.
Three words are used loosely in everyday speech — fruit, seed and grain. In Biology each one names a different structure, formed from a different part of the flower. Sorting them out now will save a great deal of confusion later.
6.1.1 Fruit, Seed And Grain
Inside a flower sits the ovary, and inside the ovary sit one or more ovules. Once fertilisation has taken place, both of these change. Nothing else in the flower matters for our purpose here.
A fruit is the ripened ovary of a flower. The wall of the ovary thickens into the fruit wall, which encloses and protects the seeds. A mango and a pea pod are both fruits, however different they look.
The fruit has two jobs. It shields the developing seeds from injury, and it helps them travel away from the parent plant — carried by an animal, by wind or by water.
A seed is the ripened ovule. Within it lies a young plant, and around it lies a tough coat that guards that young plant against knocks, drying and attack. A bean seed and a pea are seeds.
A grain is a special case, and it is a fruit rather than a seed. In a grain, the fruit wall and the seed coat are fused into a single protective layer that cannot be peeled apart. Maize and wheat are grains.
The three are compared in Table 6.1.
What Becomes What
The whole relationship can be held in one line, set out in FC 6.1. Read it as two parallel changes that happen together after fertilisation.
Flowchart
FC 6.1 — What The Flower’s Parts Become After Fertilisation Fertilised flower → branches into two: (a) ovule → seed → contains the young plant and its food store; (b) ovary → fruit, whose wall (from the ovary wall) encloses the seed or seeds
So a mango fruit is a ripened ovary, and the hard stone inside it holds the ripened ovule. A grain of wheat is both at once, because its two coverings have grown together.
6.1.2 More About The Seed
A seed forms only after fertilisation. An unfertilised ovule does not become a seed; it simply withers. This is why a flower that is never pollinated sets no seed at all.
At fertilisation, the nucleus of a male gamete fuses with the nucleus of the egg cell. The single cell produced by that fusion divides again and again to form the embryo — the tiny living plant inside the seed.
The Three Parts Of The Embryo
The embryo is not a shapeless lump. It already carries, in miniature, the beginnings of the plant it will become.
The radicle is the part that will grow into the root. The plumule is the part that will grow into the shoot. The cotyledons, or seed leaves, are the thick leaf-like structures attached to the embryo axis.
Because the future root and the future shoot are already laid down, a germinating seed does not have to decide which way to grow. The plan is built in before the seed ever leaves the parent plant.
A Packed Lunch For The Embryo
The embryo cannot make its own food yet. It has no green leaves and, buried in soil, no light to use. Its early growth must therefore run on food that was packed into the seed in advance.
In many seeds that food is stored in a separate tissue called the endosperm. In others there is no endosperm, and the cotyledons themselves swell with stored food. Either way, the store is used up in the first days of growth.
Waiting For The Right Moment
Inside a dry seed the embryo is inactive. It is not growing, and it is using its food store extremely slowly. It will stay that way until water, warmth and air reach it.
That inactivity is what allows seeds to survive conditions no leafy plant could face — bitter cold, months of drought, long storage in a granary. Some seeds are known to remain in this resting state for a hundred years or more and still grow when finally sown.
6.2 Classification And Structure Of Seeds
Seeds are astonishingly varied on the outside. Yet if you open enough of them, the same few parts turn up again and again, arranged in the same way.
This section first sorts seeds into groups, and then looks closely at two of them — a bean seed and a maize grain — because between them they show almost everything a seed can contain.
6.2.1 Seeds Vary In Size
Some seeds are so small that a single one is hard to see. Poppy seeds are of this kind. The dark specks on the surface of a strawberry are of similar size, although each speck is really a one-seeded fruit rather than a bare seed.
Others are far larger. The stone of a mango or a plum holds a seed you can hold between two fingers.
The largest seeds of all belong to the coconut and, larger still, to the double coconut, a palm whose single seed can weigh many kilograms.
Size, shape and outer appearance therefore vary enormously from plant to plant. The basic internal structure of most seeds, however, is much the same.
6.2.2 Types Of Seed Based On Endosperm
Seeds can be sorted by what happens to the endosperm — the food store laid down inside the seed — by the time the seed is mature. In some seeds it is still there. In others it has already been absorbed.
An albuminous seed, also called an endospermic seed, keeps its endosperm. Its cotyledons stay thin and membranous, because the food is not stored in them. Cereals, millets and palms are monocot albuminous seeds; poppy and custard apple are dicot albuminous seeds.
An exalbuminous seed, also called a non-endospermic seed, has no endosperm left at maturity. The food has been transferred into the cotyledons, which become thick and fleshy. Vallisneria, orchids and Amorphophallus are monocot exalbuminous seeds; gram, pea, mango and mustard are dicot exalbuminous seeds.
The difference is only in where the food sits, not in whether food is present. Every seed carries a food store; an exalbuminous seed has simply moved it into the cotyledons in advance. Table 6.2 sets the two types side by side.
6.2.3 Monocotyledonous And Dicotyledonous Seeds
Seeds can also be sorted a second way, by how many cotyledons the embryo carries. This is the division you will meet most often.
A monocotyledonous seed contains one cotyledon. Maize and the grasses are examples.
A dicotyledonous seed contains two cotyledons. Pea, gram and bean are examples, and a soaked gram splits neatly into two halves for exactly this reason.
The two schemes cut across each other, as FC 6.2 shows. A seed has both a number of cotyledons and a state of endosperm, so every seed belongs to one group in each scheme.
Flowchart
FC 6.2 — Two Ways Of Classifying Seeds Seed → classified on two independent bases: (a) number of cotyledons → monocotyledonous (one cotyledon; maize) or dicotyledonous (two cotyledons; bean) ; (b) presence of endosperm at maturity → albuminous (endosperm present; maize) or exalbuminous (endosperm absent; bean)
6.2.4 Structure Of The Bean Seed
Beans come in many kinds — broad bean, lima bean, french bean — but their seeds are built alike. Most are kidney-shaped, with one curved-out convex side and one curved-in concave side.
The bean seed is dicotyledonous and exalbuminous, so its two cotyledons are thick with stored food. Its parts are shown in Fig. 6.2-1.
Diagram
Fig. 6.2-1 — Structure Of The Bean Seed Line drawing in four panels. A — whole seed seen from the convex side; label testa, convex side. B — whole seed seen from the concave side; label concave side, hilum, micropyle, testa. C — seed with the testa removed, showing the two cotyledons in place with the radicle visible at one end; label cotyledon, radicle. D — seed cut open with one cotyledon removed to expose the embryo; label testa, tegmen, cotyledon, plumule with two rudimentary leaves, epicotyl, hypocotyl, radicle. Labels on straight, non-crossing lead lines.
6.2.4.1 The Seed Coat, Hilum And Micropyle
The seed coat is made of two layers. The testa is the outer one — hard, brownish and unyielding. It shields the delicate parts within from injury and from attack by bacteria, fungi and insects.
Lying just inside the testa is the tegmen, a thin inner layer. It too is protective, and is easiest to see once a soaked seed has been cut open.
On the concave side of the seed is a distinct whitish oval scar, the hilum. This marks the spot where the ovule, now the seed, was attached to the ovary wall by a short stalk. It is the seed’s equivalent of a navel.
Close beside the hilum lies a tiny pore, the micropyle. It marks the opening through which the pollen tube entered the ovule before fertilisation. Being a genuine hole through the coat, it goes on serving the seed long after fertilisation is over.
The micropyle has two uses:
- When the seed is soaked, water is absorbed mainly through the micropyle, and so reaches the embryo.
- It allows respiratory gases to diffuse in and out for the growing embryo.
Both uses matter because the testa itself is nearly waterproof and nearly airtight. Without a pore, the very coat that protects the embryo would also seal it away from the water and oxygen it needs.
6.2.4.2 The Cotyledons And The Embryo
Directly below the seed coat lie the two thick cotyledons. They hold the food for the embryo, and their bulk also cushions it.
Separate the two cotyledons carefully and the embryo can be seen, attached to one of them. Its radicle points towards the micropyle end; its plumule consists of a very short stem bearing a pair of tiny leaves, with a growing point between them.
Two regions of the embryo’s axis are named for their position relative to the cotyledons. The epicotyl is the region between the point where the cotyledons are attached and the plumule. The hypocotyl is the region of the axis below the cotyledons.
These two names are worth learning now, because which of them elongates decides what a germinating seed looks like — a point we return to in Section 6.5.
6.2.5 Structure Of The Maize Grain
The maize grain is not a seed at all. It is a single-seeded fruit, in which the fruit wall and the seed coat are fused into one protective layer — which is why such a fruit is called a grain. Rice, wheat and oat are grains of the same kind.
On one flat face of the grain there is a small, light-coloured oval area. This marks the position of the embryo inside. The rest of the grain — much the larger part — is filled with endosperm rich in starch. Maize is therefore monocotyledonous and albuminous.
A thin layer of cells, the epithelium, separates the endosperm from the embryo. Food dissolved out of the endosperm must pass through this layer to reach the embryo. The outermost layer of the endosperm, just beneath the fused coat, is rich in protein and is called the aleurone layer.
The parts are shown in Fig. 6.2-2.
Diagram
Fig. 6.2-2 — Structure Of The Maize Grain Line drawing in two panels. A — external features of a whole grain seen from the flat face; label fused pericarp and testa, remains of style, position of plumule, position of radicle. B — longitudinal section of the same grain; label fused pericarp and testa, aleurone layer, endosperm, epithelium, scutellum (cotyledon), coleoptile, plumule, radicle, coleorhiza, and bracket the last five as the embryo. Labels on straight, non-crossing lead lines.
The Embryo Of The Grain
The embryo of maize carries a single cotyledon, which has a name of its own — the scutellum. It is shield-shaped and pressed flat against the endosperm, and it absorbs food from it.
The radicle lies towards the pointed end of the grain, wrapped in a protective sheath called the coleorhiza. The plumule lies towards the broader upper side, wrapped in a similar sheath called the coleoptile.
A bean seedling pushes an arch of soft stem through the soil; a maize seedling has no such arch. Instead, each growing tip is capped by its own sheath, and the sheath, not the tip, takes the wear of forcing a way through the soil.
6.2.6 Bean Seed Compared With Maize Grain
The two structures you have just examined differ at almost every point. Table 6.3 gathers those differences.
Notice that the first two rows explain most of the rest. Because the bean stores its food in two fleshy cotyledons, its embryo is bulky and its parts are easy to see. Because maize stores its food in an endosperm, its embryo is small and tucked into one corner of the grain.
6.3 Germination
A seed lying in a paper packet on a shelf does nothing for months. Sown in warm, damp soil, the same seed sends out a root within days.
Nothing was added to the seed except its surroundings. Everything that grew was already inside it. This section is about that switch — what the seed is doing while it waits, and what it must be given before it starts.
6.3.1 Dormancy And The Meaning Of Germination
Inside a dry seed the embryo is alive but inactive. This resting state is called dormancy, a period during which the embryo does not grow even though it remains living.
Outwardly a dormant seed appears lifeless. In fact the chemical activities of life are still going on inside it — only very slowly, and using very little of the stored food.
The evidence is measurable. Dry seeds take in oxygen and give out carbon dioxide, both in extremely minute quantities, and they release a small amount of heat as they do so. A stone does none of these things.
From Dormant Embryo To Seedling
When the seed is placed in proper conditions, the dormant embryo becomes active. It begins to grow, and it grows into a seedling — a young plant with a root and a shoot.
All the changes that lead from the resting embryo to that seedling are collectively called germination.
The main steps are set out in FC 6.3. Follow them in order, and notice that each step makes the next one possible.
Flowchart
FC 6.3 — The Sequence Of Events In Germination Dry seed → absorbs water, mainly through the micropyle → seed swells → seed coat ruptures → enzymes become active → stored food is dissolved into a diffusible form → dissolved food reaches the embryo → food is oxidised in respiration, releasing energy → cells divide and enlarge → radicle emerges and grows downwards → plumule grows upwards → seedling
There is one further point. A freshly harvested seed usually will not germinate at once, even in perfect conditions. It must first pass through a period of dormancy, during which it completes its physiological maturation — the internal changes that leave it ready to grow.
6.3.2 Conditions Necessary For Germination
Three things must reach a seed before germination will begin: water, a suitable temperature, and air, which supplies oxygen.
6.3.2.1 Water
In natural conditions the seed draws its water from the soil around it. Water is taken in over the whole surface of the seed, but mainly through the micropyle.
Water has two main uses in germination.
- The seed swells as it takes in water. The swelling ruptures the seed coat, and the elongating radicle can then push out and begin forming the root system.
- Water is needed for the chemical reactions of germination, and for the enzymes to act on the food stored in the cotyledons or endosperm. Their action converts that food into a diffusible, dissolved form which can travel to the growing embryo and be used by it.
The second point is easy to miss but is the more important of the two. Stored starch is solid and cannot move. Until water is present, the food supply might as well not be there.
6.3.2.2 Suitable Temperature
Both very low and very high temperatures are unsuitable for germination, and they fail for different reasons.
A very low temperature inhibits the growth of the embryo. The chemical reactions of germination slow almost to a halt, so nothing happens, although the seed is unharmed and will still grow if warmed.
A very high temperature is worse, because it destroys the delicate tissues of the embryo. That damage cannot be undone by cooling the seed afterwards.
A moderately warm temperature, usually between 25 °C and 35 °C, is favourable for germination. The temperature at which a seed germinates best is called its optimum temperature.
The optimum is not the same for every plant. Seeds of tropical plants generally need a higher temperature to germinate than seeds of plants from temperate regions — a match between where a plant lives and the warmth its seeds expect.
The general pattern is drawn in Fig. 6.3-1.
Diagram
Fig. 6.3-1 — How Germination Depends On Temperature Schematic line graph, no numerical values on the vertical axis. Horizontal axis labelled temperature in degrees Celsius, marked from low to high with a shaded band labelled 25 °C to 35 °C. Vertical axis labelled percentage of seeds that germinate, low to high. Draw a single smooth curve that stays near zero at low temperatures, rises steeply as temperature increases, reaches a broad peak over the shaded band, then falls steeply back to zero at high temperatures. Label the peak region ‘optimum temperature’. Add a note on the left of the curve reading ‘growth inhibited’ and on the right reading ‘tissues destroyed’.
6.3.2.3 Oxygen
During germination there is rapid cell division and cell growth. Both processes need energy, and the seed must release that energy from its own food store.
Energy is made available by respiration, the oxidation of food inside living cells. Oxidation of food requires oxygen, and so the germinating seed requires a supply of air.
This is why the third condition is often written as ‘air’ rather than ‘oxygen’. It is the oxygen in the air that the seed actually needs.
6.3.3 Why Deeply Sown Seeds Fail To Germinate
Sowing depth matters, and seeds sown very deep in the soil usually fail to germinate. There are two main reasons.
- There is no proper supply of oxygen deep in the soil, so the seed cannot respire fast enough to release the energy that growth demands.
- The embryonic parts — the hypocotyl or the epicotyl — do not have enough pushing force to break through so great a thickness of soil above them.
Both reasons follow from what you have just read. The first is the oxygen condition failing; the second is a purely mechanical limit on a very small, very soft structure.
This also explains a rule gardeners follow without thinking about it. Large seeds, with large food stores and strong embryos, are sown deeper; tiny seeds are scattered on the surface and barely covered.
6.4 Some Experiments On Germination
Water, warmth and oxygen were stated as conditions in the previous section. Stating them is not the same as showing them, and in Biology a claim of this kind is settled by experiment.
Every experiment that follows is built the same way. Two sets of seeds are treated identically in every respect but one, and only that one factor is allowed to differ. If the two sets then behave differently, the difference can be pinned on that factor and on nothing else.
Set-up A in each case is the one that has everything a seed needs. It is the standard against which set-up B is judged.
6.4.1 Water Is Necessary For Germination
Take two beakers and mark them A and B.
In beaker A, place some seeds of green gram, or of chick pea, on wet cotton wool. In beaker B, place similar seeds on dry cotton wool. Keep both beakers in an ordinary room, so that both receive the same air and the same temperature. The set-up is shown in Fig. 6.4-1.
In a day or two, the seeds in beaker A germinate. Those in beaker B do not. The only difference between the two was the water, so water is necessary for germination.
Diagram
Fig. 6.4-1 — Experiment To Show That Water Is Necessary For Germination Line drawing of two glass beakers side by side, drawn to the same size. Beaker A on the left contains a layer of cotton wool with several gram seeds resting on it, two of them showing a short emerging radicle; shade the cotton wool to show it is damp. Beaker B on the right contains the same arrangement with unchanged, ungerminated seeds; leave the cotton wool unshaded. Label beaker A, wet cotton wool, germinating seeds, beaker B, dry cotton wool, seeds unchanged.
6.4.2 A Suitable Temperature Is Necessary For Germination
Take two beakers and name them A and B. Place some green gram seeds on wet cotton wool in each of them, so that both sets have water.
Keep beaker A in an ordinary room. Keep beaker B in a refrigerator. Fig. 6.4-2 shows the arrangement.
In a day or two, the seeds in beaker A germinate. The seeds in beaker B may show no sign of germination at all, or they may germinate after several days, but never to the extent of those in beaker A. A suitable temperature is therefore necessary for germination.
Notice the careful wording of that result. The cold seeds are not destroyed, only held back, which is why some of them may sprout late.
Diagram
Fig. 6.4-2 — Experiment To Show That A Suitable Temperature Is Necessary For Germination Line drawing of two beakers, each holding damp cotton wool with green gram seeds. Beaker A stands on an ordinary table, its seeds showing emerging radicles. Beaker B stands on a shelf inside an open refrigerator, its seeds unchanged. Label beaker A, ordinary room, seeds germinate, beaker B, inside refrigerator, seeds do not germinate.
6.4.3 Oxygen Is Necessary For Germination
Showing that oxygen is needed is harder, because oxygen cannot simply be left out of a room. It must be removed from the air around the seeds, and something must be used that will take it up.
That something is alkaline pyrogallic acid, which absorbs oxygen from the air enclosed with it. The steps of the set-up are listed in FC 6.4, and the finished apparatus is shown in Fig. 6.4-3.
Flowchart
FC 6.4 — Setting Up The Oxygen Experiment Take two conical flasks, A and B → spread wet cotton wool in each → place soaked gram seeds on the cotton wool in each → lower a small test tube of alkaline pyrogallic acid into flask B on a thread, allowing no drop to fall on the seeds or the cotton wool → lower a small test tube of plain water into flask A in the same way → fix a cork on each flask, with the thread held by the cork so that the tube hangs above the seeds → keep both flasks in an ordinary room
Flask A is the one with plain water in its test tube. Water absorbs no oxygen, so flask A differs from flask B in one respect only: the air inside A still contains its oxygen, while the air inside B is steadily stripped of it.
The seeds in flask A germinate. The seeds in flask B do not. At most there may be very slight germination, using the small amount of energy that anaerobic respiration — respiration without oxygen — can release. This shows the importance of oxygen for germination.
Diagram
Fig. 6.4-3 — Experiment To Show That Oxygen Is Necessary For Germination Line drawing of two corked conical flasks side by side. In each flask, a layer of wet cotton wool with soaked gram seeds lies on the bottom, and a small test tube hangs from a thread fixed under the cork, well clear of the seeds. Flask A on the left has a test tube of plain water and seeds showing emerging radicles; flask B on the right has a test tube of alkaline pyrogallic acid and unchanged seeds. Label flask A, plain water, soaked gram seeds, wet cotton wool, seeds germinate; flask B, alkaline pyrogallic acid, seeds do not germinate. Caption panels A and B.
6.4.4 The Three-Bean Seeds Experiment
The three experiments so far each test one factor. This last one tests two at once, in a single container, and it is worth studying closely for what it does and does not prove.
Take three mature, air-dried bean seeds and tie them to a glass slide at three different positions. Stand the slide in a beaker of water so that the top seed is well above the water, the middle seed is just at the water level, and the bottom seed is completely immersed. Leave the set-up in a warm place for a few days. Fig. 6.4-4 shows the arrangement.
Diagram
Fig. 6.4-4 — The Three-Bean Seeds Experiment Line drawing of a beaker partly filled with water, with a glass slide standing at a slant inside it. Three bean seeds are tied to the slide at three levels: one well above the water surface, one exactly at the water surface, and one fully immersed near the bottom. Show the middle seed with a well-developed shoot and root, the top seed unchanged, and the bottom seed with only a very short radicle. Label top seed, middle seed, bottom seed, water level, glass slide.
The three seeds behave in three different ways.
- The middle seed germinates. It is the only one that gets both water, from the surface it touches, and oxygen, from the air just above it.
- The top seed does not germinate at all. It has plenty of oxygen but no water.
- The bottom seed does not germinate, or else puts out a small radicle and then stops. It has water in plenty but very little oxygen, since it must rely on the small amount of air dissolved in the water.
Table 6.4 sets the three side by side.
What The Experiment Does And Does Not Prove
The comparison between the top seed and the middle seed is clean. They differ in water alone, and only the one with water germinated. The experiment therefore proves conclusively that water is essential for germination.
The comparison between the bottom seed and the middle seed is less clean. Water dissolves a little air, so the bottom seed is not entirely without oxygen, and it may begin to germinate before stopping. The oxygen requirement is thus suggested but not fully demonstrated, and the flask experiment of Section 6.4.3 remains the better test of it.
6.5 Types Of Germination
Dig up a young pea plant and its two seed halves are still below the soil. Do the same with a young bean plant and its seed halves are sitting in the air, on top of a green stalk.
Both seeds germinated normally. The difference lies in which part of the embryo’s axis grew longer — and that single difference splits germination into two types.
The rule is simple. The epicotyl and the hypocotyl never elongate together. In any germinating seed, it is either one or the other.
6.5.1 Hypogeal Germination
Suppose the epicotyl elongates. The epicotyl is the region above the cotyledons, so lengthening it pushes only the plumule upwards. The cotyledons are left where they were, below the soil.
This is hypogeal germination — from hypo, meaning below, and geo, meaning earth. Pea and gram germinate in this way.
If the seed happens to be lying on the surface rather than buried, the cotyledons simply stay on the ground instead of under it. The defining point is that they are not lifted.
6.5.2 Epigeal Germination
Now suppose the hypocotyl elongates instead. The hypocotyl is the region below the cotyledons, so lengthening it lifts everything above it — cotyledons included — clear of the soil.
This is epigeal germination — from epi, meaning above. Castor and bean germinate in this way.
The two routes are set out in FC 6.4.
Flowchart
FC 6.4 — Which Type Of Germination A Seed Shows Germinating seed → radicle emerges and grows downwards → one region of the axis begins to elongate → branches into two: (a) epicotyl elongates → only the plumule is pushed up → cotyledons remain underground → hypogeal germination (pea, gram) ; (b) hypocotyl elongates → the region below the cotyledons lengthens → cotyledons are carried above the ground → epigeal germination (bean, castor)
The difference is drawn in Fig. 6.5-1. Look at where the soil line falls in each half of the figure.
Diagram
Fig. 6.5-1 — Hypogeal And Epigeal Germination Compared Simple schematic line drawing in two halves, sharing one horizontal soil line drawn across the width. Left half labelled hypogeal: a seedling with both cotyledons shown below the soil line, an elongated region above them carrying the plumule up into the air, and a root below; label cotyledons (underground), epicotyl (elongated), plumule, radicle. Right half labelled epigeal: a seedling with an elongated region below the cotyledons that has carried both cotyledons above the soil line into the air, with young leaves above them and a root below; label cotyledons (above ground), hypocotyl (elongated), first leaves, radicle. Keep both halves at the same scale.
6.5.3 Hypogeal Germination Compared With Epigeal Germination
Table 6.5 gathers the differences.
Read the third row as a tendency rather than a rule. Pea and gram are dicotyledonous seeds, yet both germinate hypogeally. The reliable test is never the number of cotyledons; it is which region of the axis elongates.
6.5.4 A Method To Observe The Stages Of Germination
Germination happens under the soil, where it cannot be watched. The following arrangement brings it into view.
Take a glass vessel and line filter paper, or ordinary white paper, against the inside of its wall. Fill the vessel with sterilized sand, pressing the paper flat against the glass.
Place the seeds in between the paper and the glass, so that each seed is held against the transparent wall where you can see it.
Now add water to the sand in the vessel. The paper draws the water across to the seeds, which absorb it and germinate in a few days.
The point of the method is that nothing is disturbed. Because the seeds are pressed against the glass, you can follow one seed through every stage — swelling, rupture of the coat, emergence of the radicle, and the rise of the plumule — without digging it up and stopping its growth.
The sand is sterilized for a reason. Damp, warm sand is as good for moulds as it is for seeds, and a seedling attacked by mould will rot before the later stages can be observed.
6.6 Germination In Some Common Seeds
You now know what a seed contains, what it needs, and what decides whether its cotyledons rise or stay. This section follows three familiar seeds right through, from the dry seed to the young plant.
Watch for one thing common to all three. In every case the radicle comes out first, before any part of the shoot.
6.6.1 Germination Of The Pea Seed
The pea seed absorbs water and swells considerably. The testa softens and then bursts under the pressure of the swollen contents.
The radicle emerges through the split, turns downwards, and grows into the root system. Only afterwards does the plumule begin to grow upwards to form the shoot of the seedling.
In the early stages the plumule is arched, not straight. The bend goes up through the soil first, so the delicate growing tip and its young leaves are dragged behind it rather than pushed against the soil.
The cotyledons stay where they were, below the soil, supplying food until the seedling can support itself. Once that food is spent, they wither and shrivel up.
Because the cotyledons remain underground, germination in pea is hypogeal. The stages are shown in Fig. 6.6-1.
Diagram
Fig. 6.6-1 — Germination Of The Pea Seed Line drawing in three panels sharing a horizontal soil line where relevant. A — whole pea seed seen from outside; label testa, micropyle, hilum. B — the same seed with the radicle just emerging through the burst testa; label radicle, testa. C — four successive stages of a germinating pea drawn left to right below and above the soil line, showing the radicle lengthening into a tap root, the arched plumule rising, and finally a leafy shoot with the cotyledons still underground; label cotyledons (remaining underground), plumule, leaves, stem, tap root, testa. Mark panel C ‘note that the cotyledons remain underground (hypogeal)’.
6.6.2 Germination Of The Bean Seed
The bean seed absorbs water and swells. As in pea, the radicle emerges first and grows downwards to form the root system.
What happens next is different. The hypocotyl — the region below the cotyledons — grows into an arch that loops above the soil surface.
The arch then straightens. As it does so, it carries the cotyledons up with it, above the soil. Germination in bean is therefore epigeal.
Out in the light, the cotyledons turn green and act as the seedling’s first leaves. They fall off soon after the true foliage leaves have grown and taken over the work of making food. Fig. 6.6-2 shows the sequence.
Diagram
Fig. 6.6-2 — Germination Of The Bean Seed Line drawing of three successive stages left to right, sharing one horizontal soil line. Stage 1: seed below the soil with the radicle growing down and the hypocotyl beginning to arch upwards. Stage 2: the hypocotyl arch breaking the surface, still bent, with the cotyledons being drawn up behind it. Stage 3: the hypocotyl straightened, the two cotyledons held above the soil, and young foliage leaves opening above them. Label hypocotyl, cotyledons, epicotyl, foliage leaves, root system, soil line.
6.6.3 Germination Of The Maize Grain
The maize grain imbibes water and swells considerably. The radicle then grows, piercing first through its protective sheath, the coleorhiza, and then through the fruit wall.
It grows downwards and forms the first root system — but this root dies off soon afterwards. New roots then develop from the base of the stem. Roots that arise from any part of the plant other than the radicle are called adventitious roots, and in maize they form the fibrous root system of the mature plant.
The plumule grows straight upwards, piercing through its own protective sheath, the coleoptile. Both sheaths can still be seen for a time as a thin membranous covering on the axis of the young seedling.
Meanwhile the scutellum absorbs food from the endosperm and passes it to the growing embryo, until the endosperm is exhausted.
The hypocotyl does not elongate at all. The grain therefore stays where it was sown, and germination in maize is hypogeal. The stages are shown in Fig. 6.6-3.
Diagram
Fig. 6.6-3 — Germination Of The Maize Grain Line drawing of four successive stages left to right, sharing one horizontal soil line, with the grain remaining at the same depth throughout. Stage 1: grain with the radicle emerging downwards through the coleorhiza. Stage 2: the pointed coleoptile pushing upwards through the soil while the first root lengthens. Stage 3: the first leaf breaking out through the split coleoptile above the soil. Stage 4: a young maize plant with several leaves and a fibrous cluster of roots from the base of the stem. Label grain, radicle, coleorhiza, coleoptile, leaf, fibrous roots, soil line.
The Three Compared
Table 6.6 places the three germinations side by side. The last row is the one to hold on to.
6.6.4 Viviparous Germination
Ordinarily a seed leaves the parent plant, lands somewhere, and germinates there. In a few plants that order is reversed.
Mangrove plants such as Rhizophora and Sonneratia, which grow in coastal mud, show a special mode of germination called vivipary, in which the seed germinates inside the fruit while it is still attached to the parent plant.
The seedling grows there, hanging from the branch, until it is a long, heavy structure. The parent plant then drops it, point downwards, into the mud below, where it develops roots and fixes itself. The word viviparous means producing live young ones. Fig. 6.6-4 shows both stages.
Diagram
Fig. 6.6-4 — Viviparous Germination In A Mangrove Line drawing in two panels. A — a twig of Rhizophora bearing leaves, with two long slender seedlings hanging downwards from the fruits still attached to the branch; label fruit, hypocotyl, radicle. B — a dropped seedling standing upright in mud below shallow water, with roots spreading into the mud and leaves opening at the top; label water, mud, hypocotyl, roots. Draw the water surface as a clear line.
6.6.5 The Seedling
Germination in any plant ends with the formation of a seedling.
A seedling is a stage in the growth of a plant from a seed, before the plant has become wholly independent of the food stored in the seed.
That definition names the moment the seedling stage ends. Its roots absorb water and minerals from the soil; its leaves begin to manufacture food. Once the stored food is finished and the leaves are supplying everything, the plant is a seedling no longer.
From there it keeps growing, becomes a mature plant, and produces flowers — and those flowers produce fruits and seeds in their turn. FC 6.5 closes the circle the chapter opened with.
Flowchart
FC 6.5 — From Seed To Seed Seed → germination → seedling, living partly on stored food → young plant with green leaves making its own food → mature plant → flowers → fertilisation → fruits and seeds → back to the start, as each seed can begin the cycle again
6.7 Summary
- A fruit is the ripened ovary of a flower; its wall protects the seeds and helps disperse them.
- A seed is the ripened ovule, containing an embryo and a food store within a protective seed coat.
- A grain is a single-seeded fruit in which the fruit wall and the seed coat are fused into one layer, as in maize and wheat.
- A seed forms only after fertilisation; the embryo develops from the cell formed when a male gamete nucleus fuses with the egg nucleus.
- The embryo consists of the radicle, which forms the root, the plumule, which forms the shoot, and one or two cotyledons.
- Food for the embryo is stored either in the endosperm or in the cotyledons, because the embryo cannot make its own food until its leaves reach the light.
- Seeds vary greatly in size, from poppy seeds to the double coconut, but their basic internal structure is much the same.
- On the basis of endosperm, seeds are albuminous, in which the endosperm persists and the cotyledons stay thin, or exalbuminous, in which the endosperm is absorbed and the cotyledons become thick and fleshy.
- On the basis of cotyledon number, seeds are monocotyledonous, with one cotyledon, or dicotyledonous, with two.
- In the bean seed, the seed coat has an outer testa and an inner tegmen; the hilum is the scar of attachment and the micropyle is a pore beside it.
- The micropyle admits most of the water the seed absorbs and allows respiratory gases to diffuse in and out.
- The epicotyl is the region of the embryo axis above the cotyledons; the hypocotyl is the region below them.
- The maize grain is monocotyledonous and albuminous, with a large starchy endosperm, an outer protein-rich aleurone layer, and an epithelium separating endosperm from embryo.
- The embryo of maize has a single cotyledon called the scutellum, a radicle sheathed by the coleorhiza, and a plumule sheathed by the coleoptile.
- In a dry seed the embryo is dormant — alive and respiring extremely slowly, but not growing.
- Germination is the process by which the dormant embryo becomes active and develops into a seedling.
- A freshly harvested seed will not germinate at once; it must first complete its physiological maturation.
- Water, a suitable temperature and oxygen are the three conditions necessary for germination.
- Water makes the seed swell and rupture its coat, and allows enzymes to convert the stored food into a dissolved, diffusible form.
- A very low temperature inhibits the growth of the embryo, while a very high temperature destroys its tissues; a moderately warm range of about 25 °C to 35 °C is the optimum for most seeds.
- Oxygen is needed because the rapid cell division and growth of germination are powered by respiration, the oxidation of stored food.
- Seeds sown very deep fail to germinate for want of oxygen, and because the hypocotyl or epicotyl cannot push through so much soil.
- The need for water, for a suitable temperature and for oxygen can each be shown by comparing two set-ups that differ in that one factor alone.
- In the three-bean seeds experiment, only the seed at the water level germinates; the set-up proves water essential but does not fully demonstrate the need for oxygen.
- The epicotyl and hypocotyl never elongate together, and which of them elongates decides the type of germination.
- In hypogeal germination the epicotyl elongates and the cotyledons remain underground, as in pea and gram.
- In epigeal germination the hypocotyl elongates and the cotyledons are carried above the ground, as in bean and castor.
- In every seed the radicle emerges first, anchoring the seedling and supplying it with water before the shoot develops.
- In maize the first root soon dies and is replaced by adventitious roots from the base of the stem, while the scutellum draws food from the endosperm.
- In viviparous germination, shown by mangroves such as Rhizophora and Sonneratia, the seed germinates inside the fruit while still attached to the parent plant, and the seedling is later dropped into the mud.
- A seedling is the stage of growth before the young plant becomes wholly independent of the food stored in the seed.
Flowchart
FC 6.6 — The Chapter In One Line Flower fertilised → ovule becomes a seed containing a dormant embryo and a food store → seed receives water, suitable temperature and oxygen → seed swells, coat ruptures, enzymes dissolve the stored food, respiration releases energy → radicle emerges first, then the plumule → epicotyl elongates (hypogeal) or hypocotyl elongates (epigeal) → seedling → mature plant bearing flowers and new seeds
6.8 Key Terms
- Adventitious roots — roots that arise from any part of the plant other than the radicle, such as the base of a maize stem.
- Albuminous seed — a seed in which the endosperm persists at maturity and the cotyledons remain thin and membranous.
- Aleurone layer — the outermost, protein-rich layer of the endosperm of a grain.
- Anaerobic respiration — the release of energy from food without using oxygen, yielding far less energy than ordinary respiration.
- Coleoptile — the protective sheath enclosing the plumule in a maize grain.
- Coleorhiza — the protective sheath enclosing the radicle in a maize grain.
- Control — the set-up in an experiment that has the condition being tested, kept for comparison with the one from which it has been removed.
- Cotyledon — a seed leaf of the embryo, which in many seeds also stores food.
- Dicotyledonous seed — a seed whose embryo has two cotyledons, as in pea, gram and bean.
- Dormancy — the resting state of a seed, in which the embryo is alive and respiring very slowly but is not growing.
- Embryo — the tiny living plant inside a seed, formed after fertilisation.
- Endosperm — the food-storing tissue found inside many seeds and grains.
- Epicotyl — the region of the embryo axis lying above the point of attachment of the cotyledons.
- Epigeal germination — germination in which the hypocotyl elongates and carries the cotyledons above the ground.
- Exalbuminous seed — a seed with no endosperm at maturity, its food having been transferred to thick, fleshy cotyledons.
- Fruit — the ripened ovary of a flower, whose wall encloses and protects the seeds.
- Germination — the process by which the dormant embryo of a seed becomes active and develops into a seedling.
- Grain — a single-seeded fruit in which the fruit wall and the seed coat are fused into one protective layer.
- Hilum — the whitish oval scar on a seed marking where it was attached to the ovary wall.
- Hypocotyl — the region of the embryo axis lying below the point of attachment of the cotyledons.
- Hypogeal germination — germination in which the epicotyl elongates and the cotyledons remain underground.
- Micropyle — the tiny pore beside the hilum through which water is mainly absorbed and respiratory gases diffuse.
- Monocotyledonous seed — a seed whose embryo has a single cotyledon, as in maize and the grasses.
- Optimum temperature — the temperature at which a seed germinates best, usually about 25 °C to 35 °C.
- Plumule — the part of the embryo that grows into the shoot.
- Radicle — the part of the embryo that grows into the root, and the first part to emerge from a germinating seed.
- Respiration — the oxidation of food within living cells, by which the energy stored in food is released for use.
- Scutellum — the single, shield-shaped cotyledon of a maize grain, which absorbs food from the endosperm.
- Seed — a ripened ovule, containing an embryo and a food store within a protective seed coat.
- Seedling — the stage in a plant’s growth from a seed, before it becomes wholly independent of the food stored in the seed.
- Tegmen — the thin inner layer of the seed coat, lying just within the testa.
- Testa — the hard, brownish outer layer of the seed coat.
- Vivipary — a mode of germination, seen in mangroves such as Rhizophora, in which the seed germinates inside the fruit while still attached to the parent plant.
6.9 Points To Ponder
A seed coat that keeps water out is useful for survival but useless for germination. How does a single structure manage to do both jobs, and what would a seed lose if its coat were slightly more permeable?
Light is not one of the conditions necessary for germination, yet every green plant depends on light. At what exact moment in a seedling’s life does light become essential, and what marks that moment?
Epigeal germination lifts the whole food store up through the soil; hypogeal germination leaves it behind. Each is a cost paid for some advantage. What might each type gain in return?
A dormant seed is alive but almost inactive. If you were handed two seeds of the same kind, one dormant and one dead, what could you measure — without simply sowing them and waiting — that would tell them apart?
Mangrove seeds germinate while still attached to the parent plant. What would a plant lose by giving up the ordinary dormant seed, and why might a tidal mudflat make that loss worth accepting?
Maize keeps its food in an endosperm; bean keeps its food in the cotyledons. Both arrangements work. What difference might it make to the seedling to have its food in a separate tissue rather than inside the organ that will become its first leaves?
6.10 Objective Questions
- The part of a seed that develops into the root is the: (1 mark)
- plumule
- radicle
- scutellum
- tegmen
- A seed in which the cotyledons are thin and membranous and the endosperm persists is described as: (1 mark)
- exalbuminous
- dicotyledonous
- albuminous
- viviparous
- In the flask experiment on germination, alkaline pyrogallic acid is used because it: (1 mark)
- supplies moisture to the seeds
- absorbs oxygen from the air in the flask
- raises the temperature inside the flask
- kills bacteria on the cotton wool
- Germination is hypogeal when: (1 mark)
- the hypocotyl elongates faster than the epicotyl
- the cotyledons turn green above the soil
- the epicotyl elongates and the cotyledons stay underground
- the radicle dies and adventitious roots replace it
- The scutellum of a maize grain is: (1 mark)
- a protective sheath over the plumule
- the protein-rich outer layer of the endosperm
- the single cotyledon of the embryo
- the scar left where the grain was attached
- In each of the following, choose from these options: (1 mark each)
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): A dry seed can be stored for years without spoiling. Reason (R): The embryo of a dry seed is dead and therefore cannot decay.
- Assertion (A): Seeds sown very deep in the soil usually fail to germinate. Reason (R): Deep soil supplies too little oxygen for the seed to respire rapidly.
- Assertion (A): A germinating seed needs oxygen even though it carries its own food. Reason (R): Energy is released from stored food only when the food is oxidised.
- Match each item in Column A with the correct item in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Tegmen | i. Sheath covering the plumule of maize |
| b. Hilum | ii. Region of the axis below the cotyledons |
| c. Coleoptile | iii. Thin inner layer of the seed coat |
| d. Hypocotyl | iv. Germination inside the fruit on the parent plant |
| e. Vivipary | v. Scar marking the point of attachment of the seed |
- Fill in the blanks. (1 mark each)
- Water enters a seed mainly through the ______.
- The ripened ovary of a flower is called a ______.
- In ______ germination the cotyledons are pushed above the ground.
- The temperature at which a seed germinates best is called its ______ temperature.
- The radicle of a maize grain is enclosed in a protective sheath called the ______.
- Write true or false for each statement. (1 mark each)
- The plumule of a bean seed develops into the root system.
- A grain has its fruit wall and seed coat fused into a single layer.
- A very high temperature only delays germination and does no lasting harm.
- In pea, the cotyledons remain below the soil during germination.
- The aleurone layer is the outermost layer of the endosperm.
- Name the following. (1 mark each)
- The resting state in which the embryo of a seed is alive but not growing.
- The part of a maize plant’s root system that develops from the base of the stem.
- The tissue in which food is stored in an albuminous seed.
- The type of germination shown by a castor seed.
- Two mangrove plants that show viviparous germination.
- In each set, pick the odd one out and give a reason for your choice. (2 marks each)
- Testa, tegmen, hilum, coleoptile
- Gram, pea, maize, bean
- Water, oxygen, light, suitable temperature
6.11 Check Your Concepts
Define germination. (2 marks)
State the two functions of the micropyle. (2 marks)
Give a reason for each of the following. (2 marks each)
- A dry seed can be stored for a long time without germinating.
- Sterilized sand is used when seeds are grown against the glass wall of a vessel to observe germination.
Name the two protective sheaths found in the embryo of a maize grain, and state what each one covers. (2 marks)
Explain why a fruit and a seed are not the same thing, using one example of each. (3 marks)
Describe the two main uses of water in the germination of a seed. (3 marks)
Why does the radicle emerge before the plumule in a germinating seed? (3 marks)
Differentiate between the following, stating the basis of difference in each case. (3 marks each)
- Albuminous and exalbuminous seeds
- Epicotyl and hypocotyl
- Hypogeal and epigeal germination
Describe the structure of a bean seed as seen when it is soaked and cut open. (4 marks)
What would happen in each of the following cases? Give a reason in each case. (4 marks)
- Bean seeds are sown twenty centimetres deep in heavy soil.
- Soaked gram seeds on wet cotton wool are kept in a refrigerator for two days.
Describe an experiment to prove that oxygen is necessary for the germination of seeds. State the result and the conclusion. (5 marks)
With reference to the three-bean seeds experiment, state the result for each of the three seeds and explain why the experiment does not fully prove the need for oxygen. (5 marks)
Describe the germination of a maize grain from the imbibition of water to the formation of a young plant with a fibrous root system. (5 marks)
6.12 Figure-Based Questions
- With reference to Fig. 6.2-1: (4 marks)
- Name the structure shown in panel B and identify the side of the seed on which the hilum lies.
- Name the two layers that together make up the seed coat.
- State one function of the part marked as the micropyle.
- Name the part of the embryo that will form the shoot.
- With reference to Fig. 6.2-2: (4 marks)
- Identify the structure shown, and state whether it is a seed or a fruit, giving a reason.
- Name the layer that lies between the endosperm and the embryo.
- Name the single cotyledon of this embryo and state its function during germination.
- Which end of the structure does the radicle lie towards?
- With reference to Fig. 6.4-2: (3 marks)
- Name the process the two flasks are set up to investigate, and state which flask is the control.
- Name the chemical placed in the test tube of flask B and state what it does.
- Why is a test tube of plain water placed in flask A?
- Study Fig. 6.5-1 and answer the following. (3 marks)
- Name the type of germination shown in each half of the figure.
- Name the region of the axis that has elongated in each case.
- Name one seed that germinates in each of the two ways.
- Draw a neat labelled diagram of a longitudinal section of a maize grain. Label the aleurone layer, endosperm, scutellum, coleoptile, plumule, radicle and coleorhiza, using straight, non-crossing lead lines. (5 marks)