DISPERSAL OF SEEDS BY FRUITS

After the seeds have formed they need to be dispersed. Reasons for dispersal are:-

1. To prevent overcrowding, which causes competition.2. To spread the species so new areas can be colonised.

METHODS OF DISPERSAL

Plants employ many methods of seed dispersal. They fall into four main categories:-

1. Dispersal by animals.2. Dispersal by wind.3. Dispersal by mechanical means.4. Dispersal by water (not a common method).

Animal Dispersal

Many fruits are succulent and juicy. The animals eat the fruit, the seeds are either discarded, e.g. cherry, plum, etc, or pass undigested through the body, e.g. gooseberry, tomato, blackberry, etc. Some succulent fruits have only one seed surrounded by a woody ‘stone’. These are called drupes, e.g. cherry, plum, etc. Others are many seeded and not surrounded by a woody ‘stone’. These are called berries, e.g. tomato, grape, cucumber, etc. Some succulent fruits have a juicy receptacle, the ovary wall does not form the pericarp (fruit) tissue, e.g. strawberry, apple, etc. These are called false fruits.

Another type of animal dispersed fruit is the nut. These are edible, but are not succulent. The seed is surrounded by a woody shell derived from the ovary wall. The woody shell is covered by modified leaves. Examples include hazel nut, acorn, walnut, etc.

Dispersal is achieved when rodents collect the nutsto store as food. Sometimes they drop a fruit, thusachieving dispersal. Sometimes, after the nuts havebeen buried, the seed inside germinates before the animal has had time to eat it.

A different method of animal dispersal, which is achieved passively, involves the use of hooks. Goosegrass, herb bennet, etc, produce dry fruits which have hooks. When an animal passes through the undergrowth, the hooks catch into the fur and the fruit is pulled away. Later, when the animal cleans itself, it scratches out the fruits, thus bringing about dispersal.

Wind Dispersal

Some plants produce small, light, fruits, with downy hairs which act as a parachute, e.g. dandelion, thistle, etc. When the wind blows the fruits are blown from the plant and are carried away.

Other plants, e.g. sycamore, ash, elm, etc, have larger fruits with a wing, When the fruit falls from the plant, the wing makes the fruit spin and it glides away from the plant.

Mechanical Dispersal

This is achieved by plants which produce fruits in pods. After the fruit and seeds have formed, the fruit dries out. As it does so, tension builds up in the pod, until the pod bursts at a weak spot and the seeds are shot out violently. Examples include wallflower, pea, bean, gorse, lupin, etc.

Water Dispersal

The coconut fruit drifts away in the sea and is washed up on another island, where the seed can germinate and grow into a new plant.

A different kind of mechanical mechanism is shown by the poppy. Its fruit is called a capsule. When the fruit is ripe, holes form round the top of the fruit. When the wind blows the capsule is rocked and the seeds are shaken out of the fruit like pepper out of a pepper pot.

SEED GERMINATION

WHAT ARE SEEDS?

Seeds are formed from ovules after the egg inside has been fertilised. A seed contains an embryo plant and a food reserve, surrounded by a seed coat, the testa. The embryo plant comprises a plumule (shoot), radicle (root) and one or two embryonic leaves, cotyledons. The food may be stored either in tissue around the embryo, endosperm, i.e. endospermous seeds such as maize, onion etc, or in the cotyledons, i.e. non-endospermous seeds such as the broad-bean or pea. The testa develops from the integuments of the ovule. When seeds are released they are dormant to conserve energy, and contain little water.

Seeds of various species vary in the following ways:-

1. Size and shape (related to the method of dispersal).2. The number of cotyledons (some seeds have one, others two).3. The size of the cotyledon(s) (depending on where food is stored)4. The presence or absence of endosperm.

The diagrams below show a broad-bean seed (non-endospermous) and a wheat seed (endospermous).

Broad-bean seed Broad-bean seed Wheat seed (External view) (Testa removed) (Section down middle)

SEED GERMINATION

Germination is the sprouting of a seed to produce a new plant, the seedling. Germination involves the following events:-

1. Dormancy must be broken: How this occurs varies (some seeds need to be chilled, others kept in darkness, others in the light).

2. Water absorption: The dry seed imbibes water through the micropyle and expands.3. The seed coat ruptures: This allows the root to grow out.4. Root growth: The young root emerges and grows downwards, attracted by gravity.5. Stem growth: The plumule emerges and grows upwards away from gravity.

So that germination can proceed, starch stored in the endosperm or cotyledons breaks down to sugar, and moves to the growing points. Stored protein breaks down to amino-acids, which are moved to growing points. Root hairs develop to increase surface area for absorption of water and mineral ions. When the shoot emerges form the soil, it develops green leaves and starts to photosynthesise. As stored food is used, the cotyledons shrink and wither away.

STRUCTURE OF SEEDLINGS

In the broad-bean and its relatives, the shoot is bent back as it pushes up through the soil. This protects the plumule. In grasses, wheat, maize, etc, the plumule is protected by a sheath called the coleoptile. Coleoptyles are used for growth experiments. In some seedlings, e.g. runner-beans, the first leaves used for photosynthesis are the cotyledons. In others, e.g. broad-bean, the cotyledons remain below ground and wither away as the food reserve is used.The diagram below shows a young broad-bean seedling. The plumule is the youngest part of the shoot where growth takes place. The root tip is the youngest part of the root, where growth takes place. The arrows indicate transport of food from the cotyledons to the growing points.

CONDITIONS NECESSARY FOR GERMINATION

Even if dormancy is broken, germination will not occur unless certain conditions are present.

1. Water: This is needed for swelling and bursting of the seed, movement of food reserves, and for growth of the shoot and root.

2. Oxygen: This is needed for respiration (energy release) by the growing seedling.3. Warmth: This is needed for the efficient functioning of the enzymes in the embryo.

Light is not needed for germination, but it may be needed to break dormancy in some species of seed, and it is needed for the formation of chlorophyll, and for photosynthesis by the seedling.