Seeds equipped for survival

Sunlight and showers draw out young shoots from seeds that have provided all the plant needs to survive and flourish

the air is warm and the earth fresh after a sprinkling of rain. Beneath the ground, unnoticed, a tiny seed bursts into life, a small green shoot sprouting from its ruptured case and pushing through the rich soil. In a matter of days, it will emerge into the sunlight, one of countless fragile seedlings heralding the start of spring. The seed may have waited a long time to begin its journey. With no need for water, seeds can lie dormant in the soil for many weeks or even years. This is one reason why seed-producing plants have become so successful, growing in almost every ecological niche on earth. “Seeds can remain viable in dry or cold conditions that the mother plant can’t survive in,” says Professor Richard Ellis, dean of life sciences at the University of Reading. “They can await the right conditions for a new plant to thrive before they germinate. They are also often mobile. Unlike the mother plant that is literally rooted to a spot, seeds can travel, distributing the species to new places.”

Fruit and nuts

Seed-producing plants fall into two classes: the angiosperms, or flowering plants, and

gymnosperms, which include conifers. Encompassing some 400,000 species, from the tiniest duckweed to the greatest oak, angiosperms are by far the biggest group. In angiosperms, the female reproductive cells, or ovules, lie within an ovary in the centre of a flower. If pollen, the male reproductive cell, should fall on the flower, it will germinate, growing down into the ovary to fertilise an ovule and create a seed. The ovary then enlarges to become a fruit containing one or, if many ovules have been fertilised, several seeds. In plants such as the buttercup and bluebell, the fruit is dry and thin, but in others, such as the gooseberry or tomato, it becomes succulent and plump. In some plants, the ovary becomes very hard, resulting in a nut.

Growing room

The seed is protected from damage by a hard outer shell called the testa, which also stops harmful bacteria and fungi making their way inside. Lying within is the embryo; a young root, or radicle, and shoot, or plumule, connected by a band of tissue which will become the stem. A large part of the seed is taken up by a food store, which the embryo will use to grow once germination begins. “The vast majority of seed-producing species are dicots, in which two seed leaves, or cotyledons, attached to the embryo provide energy,” explains Richard. “In monocots, on the other hand, there is only one, much thinner, cotyledon and the embryo draws on energy-rich tissue called endosperm, which lies adjacent to it.” Grasses, as well as lilies, orchids, onions and tulips, are monocots. Carrots, potatoes, roses and daisies, along with most broad-leaved trees, are dicots. “Endosperms and cotyledons are rich in starch and protein,” says Richard. “This is why seeds are so important in the human diet. In the case of cereals, which have a particularly large endosperm, they are literally our daily bread. Maize, wheat and rice seeds are staple foods for many of our 7.5 billion, and rising, population.”

Seed dispersal

Having spent time and effort creating seeds, the plant must now make sure it loses them. Were all the seeds released to stay where they fell, the whole area would be littered with seedlings. All would be competing with each other, and their parent, for nutrients and light. To thrive, the seeds must find new places to grow. The seeds produced by many plants are light enough to drift on the wind. Foxgloves and poppies depend on this means of dispersal, their seeds usually travelling up to 10ft (3m) before they come to rest. Some plants have evolved special adaptations to help their seeds travel further. A prime example is the dandelion, whose seeds are attached to lift-creating hairs and parachute through

the air. Under ideal conditions, a dandelion seed may float six miles or more. Even more impressive is the black poplar, whose seeds are surrounded by puffs of white hair that can carry them 18 miles from the parent tree. Sycamore seeds are propelled by two thick, flat wings, spinning as the wind pushes each arm in a different direction, while a cluster of lime fruits glides on a wing-like leaf. Unsurprisingly, many plants that live in water use it to disperse their seeds. The simplest method is to produce seeds with water-resistant coats, enabling them to stay on the surface as they bob further downstream. The seeds of the white water lily float, thanks to a layer of trapped air, and other seeds have air-filled tissues for buoyancy. Trees and flowers living along the riverbank, such as the alder, often harness both wind and water. Alder seeds contain air bubbles and have a spongy coating which allows them to float, yet are light enough to be borne away on the wind. These seeds may even germinate on water, taking root when the seedling eventually washes onto land. Many plants get animals to do their work for them. “Fruits, nuts and seeds are eaten by animals and carried away by them, perhaps many miles, especially in the case of birds,” says Richard. “They are then deposited in the droppings, their own handy supply of nutrients. The seed’s passage through the animal’s gut often weakens or scratches the outer covering, which helps promote germination.” Birds have very good colour vision and will be tempted by bright red berries, vivid against a backdrop of green foliage. Small

“Many things grow in the garden that were never sown there.” Thomas Fuller, Gnomologia

mammals, with their better sense of smell, are drawn to scented fruit such as apples or quince. Less conspicuous, but no less important, are insects. The tiny ant can transport seeds over long distances, and some plants, including the wood anemone, have evolved a special means of attracting them. They attach globules of fat, protein and sugar, called elaiosomes, to their seeds. Ants carry the seeds to their nests to feed the elaiosomes to their larvae. Rather than using colour or fragrance to attract a lift, some plants produce fruit with barbs or hooks that will latch onto a passing animal. Sea holly, goosegrass and lesser burdock all hitch-hike in this way. The spikes have the added advantage of making the plant almost inedible. A more dramatic means of dispersal has been adopted by flowers such as lupins and geraniums, where a miniature explosion flings their seeds as far away as possible. This usually relies on the fruit drying out in the sun, then splitting open along lines of tension that are formed. As it does so, the pieces spring apart and the seeds are catapulted out. Gorse seed pods rupture so violently that a popping sound can be heard. The dog violet can eject its seeds 3ft (1m) away; the sides of the seed case pinching together as they dry, shooting out the seeds as if between finger and thumb. Heavy fruit, such as apples or horse chestnuts, will fall straight to the ground. If ripe, the fruit may split open, exposing the seeds. Either way, further means of transport, usually an animal, are needed. A remarkable case is the jay, which gathers acorns and buries them to eat later. A single jay may bury up to 5,000 acorns over an autumn and can remember all their hiding places.

Digging deep

Not all seeds can rely on being buried by animals, and many have developed special structures to help them bore into the earth. “The awn of the wild oat helps the seed find its way down cracks in the soil,” says Richard. “This is driven by cycles of wetting and drying; for example, over autumn nights and days. As the awn takes up moisture, it unwinds, and as it re-dries, it twists, driving the seed deeper.” Once underground, ridges and dips in the seed help anchor it firmly in position.

Time out

Lying in the soil, innumerable seeds wait. Rowan seeds are one of many which must experience a period of cold before they germinate; the tree’s way of making sure winter has been and gone. “Gardeners replicate this process when they stratify seeds, cooling them to bring on germination,” comments Richard. “In the wild, seeds may remain dormant for five, 10 or even, in a few exceptional cases, many more years before germinating. Individual seeds from the same plant may not have identical periods of dormancy, with some germinating soon after being released and others taking much longer.” Plants such as heather shed an

enormous number of seeds, very few of which germinate immediately. This creates an underground stockpile, known as a seed bank, which can become so large that there may be more members of a species below ground than above. Plants that live in unstable environments, perhaps vulnerable to flooding, create substantial seedbanks, as do many considered weeds, such as chickweed, that live in ground that is often disturbed. “Seed viability can be increased considerably, to many decades or even centuries in certain species, by reducing the seed’s temperature and moisture content to very low levels,” says Richard. “This has been exploited by gene banks, allowing them to keep stocks for future plant breeding or for ecological restoration or conservation projects.”

Coming to light

Once dormancy is broken, the embryo can begin its transformation into a seedling. To germinate, a seed needs three things: the optimum temperature, a supply of oxygen and sufficient water. Water uptake is determined by the thickness and hardness of the testa. Members of the bean family, with hard, smooth seed coats, are notably resistant to water penetration. Their coats must be scoured, usually by large particles in the soil, to create minute openings through which water can pass. Gardeners often scratch or pierce the surface of such seeds to try to stimulate germination. With the intake of water, the seed begins to swell, splitting the testa and allowing even more water, and oxygen, to enter. This triggers enzymes in the food store to release nutrients to the embryo, and growth begins. “First to emerge is the radicle, which grows downwards in a response to gravity called geotropism,” says Richard. “It anchors the plant-to-be in the ground and begins to absorb water and minerals from the soil.” The plumule then begins its journey to the surface, in this case as a response to light. As it moves, the tip may curve down to protect the young leaves below as they follow. “Any endosperm in the seed is gradually depleted, and in some plants the cotyledons also remain below ground, releasing food to the seedling. In others, they break the surface with the seedling and begin to photosynthesise. The new plant is finally on its way.”

“First the farmer sows the seed, Then he stands and takes his ease, Stamps his foot and claps his hands And turns him round to view the land.” Traditional

After flowering is finished, the dandelion head dries out and its petals drop off. The bracts reflex to reveal a sphere of seeds to be carried off on the breeze.

Gymnosperm literally means naked seed, and in these plants, which include pines, spruces and other conifers, no fruit is formed. Instead, the seeds produced when pollen blows onto female cones lie exposed on the surface of the bracts. In some...

A seed potato is a dicot, or dicotyledon. The flower parts of a dicot number four to five, and their roots are often fibrous and branched (far left). A corn seed is a monocot, with three flower parts and a tap root system (left).

The radicle of a germinating bean seed illustrates geotropism, with the first curling leaves of the plumule visible.

As grass shoots emerge from the soil, they begin to photosynthesise, producing their own energy.

Professor Richard Ellis’s 45 years’ research at the University of Reading has supported the global network of seed gene banks conserving crop genetic resources for future generations.