Seed Dispersal & Germination

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Dispersal Strategies

Only a few seeds successfully grow into mature plants. Most seeds actually never get to germinate (start growing). So plants must produce large numbers of seeds to ensure some survive to maturity.

Seed dispersal involves the spreading of seeds over a distance and is essential to reduce competition between the seed and parent plant, other seedlings and mature plants for nutrients, water and sunlight. If seeds are not dispersed away from the parent plant, they will suffer from competition and not thrive.

As plants cannot move, most rely on dispersal agents such as water, wind, insects and other animals to carry their seeds away from the parent plant. Some plants can eject their seeds by exploding them out of pods!

Animals spread seeds in several ways:
- - Eaten and passed out in droppings - Plants that develop succulent (juicy) fruits (e.g. apples and plums) usually have their seeds spread by birds or mammals. Animals are attracted by the colour or smell, and eat the fruits which are rich in sugars. The seeds pass through the digestive system of the animal and are deposited elsewhere in droppings. E.g. strawberries and tomatoes.
  - Carried on fur or feathers - these seeds may be sticky or have hooks e.g. cocklebur and bidi-bidis (NZ burr)
  - Collected and carried away for food - Some seeds develop hard resistant coats and become nuts. The nuts are removed by animals to other locations where they may get a chance to germinate. E.g. acorns

Wind dispersal: Some plants have very tiny and light seeds that are easily carried by wind - e.g. poppie and orchids. These seeds are tiny and lightweight because they have little stored food. Some plants have structures to help them to be carried over long distances in the wind - e.g. dandelions (parachutes), sycamore trees (wings), pine trees.

Exploding pods: Many plants spread their own seeds by forming pods which eventually dry up and split, thus explosively releasing the seeds. E.g. as gorse pods dry, tension builds, which causes pods to split down two lines of weakness. The pods then curl back and release the tension like a spring. This violently ejects seeds away from the parent plant.
Buoyant seeds: Some seeds are found in pods that can float in water. This helps spread seeds along the banks of streams. Examples include the coconut, which floats on seawater, and the kowhai, which produces pods that float down rivers and streams.

These features of fruits, pods and seeds are called adaptations because they assist the plant to reproduce and help ensure the survival of the species.

Structure

A seed consists of an embryo plant (early stage of life) and cotyledons (food stores) within a protective testa (seed coat).

Inside a seed there are either one or two cotyledons attached to the embryo. The cotyledons contain energy-rich starch, which is changed into glucose to supply the growing plant with energy.

Plants with two cotyledons (e.g. beans and peas) are called dicotyledons (dicots), while those with only one cotyledon (e.g. corn and flax plants) are called monocotyledons (monocots).

The embryo has a radicle that develops into the main root, and a plumule that grows into the main shoot and first leaves.

The presence of starch in the cotyledons of seeds can be confirmed using the iodine test for starch.

Germination

Seeds remain dormant until the right environmental factors are present. For germination to occur, three requirements must be met:

Water for chemical reactions that occur inside the plant cells, allowing the seed to swell and rupture the testa.
Warmth for the enzymes controlling chemical reactions to function effectively and quickly enough to keep the seedling alive.
Oxygen for cellular respiration, to convert glucose into ATP energy.

While soil is not initially necessary for germination, it will eventually provide essential minerals for the growing plant.

Firstly, water is absorbed through the micropyle, a tiny opening in the seed coat. This water uptake causes the seed to swell and the seed coat to rupture.

Then the embryo needs energy to start the life process of growth. This energy is released from glucose stored in the cotyledons through the process of cellular respiration. During this process, glucose reacts with oxygen from the air to release energy in the form of ATP.

Glucose + oxygen → carbon dioxide + water + energy (ATP)

As the embryo grows, the radicle (embryonic root) expands and emerges through the seed coat. The radicle develops into the main root system, anchoring the plant and absorbing water and minerals from the soil. Behind the root tip, root hairs grow out into the soil, significantly increasing the surface area for absorbing water and minerals. As the root grows down into the soil, it develops side roots which help anchor the plant.

Next, the plumule (embryonic shoot) emerges from the seed coat and grows upwards towards the light to become the main shoot. As the plumule develops, tiny leaves rapidly unfold and expand, turning green as they are exposed to sunlight. The green leaves are able to make food through the process of photosynthesis. As more leaves develop, the plant becomes self-sufficient, no longer relying on the stored food reserves within the cotyledons (which by now have run out of starch).

The emergence of the first true leaves marks the end of germination.

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