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why do plants have seeds

Seed-bearing plants

Plants are living:

  • They grow and die.
  • They produce new individuals.
  • They are made of cells.
  • They need energy, nutrients, air and water.
  • They respond to their environment.

Plants are different to animals partly because they use the energy from sunlight through a process called photosynthesis (although there are a few plants that don’t). Plant cells have much in common with animal cells, but they have some different structures.

There are many different kinds of plants. Just take a walk in the garden or bush to see the amazing variety. Botanists organise the plant kingdom into groups based on features found in different plants.

Plants are divided into two big groups, based on how they reproduce:

  • Plants that produce seeds (flowering plants and cone plants).
  • Plants that produce spores (ferns, mosses, liverworts and green algae).

Seed plants have special structures on them (flowers or cones) where special male and female cells join through a process called fertilisation. After fertilisation, a tiny plant called an embryo is formed inside a seed. The seed protects the embryo and stores food for it. The seed is dispersed away from the parent plant, and when conditions are right, the embryo germinates and grows into a new plant.

There are two main groups of seed plants:

  • Gymnosperms – plants with cones.
  • Angiosperms – plants with flowers.

Gymnosperms

These seed plants do not have flowers or fruit – their seeds are held in cones. Next time you pick up a pine cone, look for loose seeds inside. Male cones make pollen, which is carried to female cones by the wind. After the female gametes are fertilised by male gametes from the pollen, the female cones produce seeds. These are then scattered away from the plant by wind or animals.

Most gymnosperms are trees. There are about 20 native gymnosperms in New Zealand, including our tallest tree, the kahikatea (Dacrycarpus dacrydioides, white pine). Others include mataī (Prumnopitys taxifolia, black pine), tōtara (Podocarpus totara), rimu (Dacrydium cupressinum, red pine) and kauri (Agathis australis). The main tree that makes up New Zealand’s plantation forests is the exotic gymnosperm Pinus radiata.

Angiosperms

Angiosperms produce flowers, which are special structures for reproduction. They contain male parts that make pollen and female parts that contain ovules. Some plants have these male and female parts in different flowers. Pollen is carried from a male part to a female part by wind or animals (a process called pollination), where it releases male gametes that fertilise the female gametes in the ovules. The ovules develop into seeds, from which new plants will grow. In most angiosperms , part of the flower develops into fruit, which protects the seeds inside them. Fruit can be soft like oranges or hard like nuts.

Flowering plants form the biggest group of seed plants, with about 300,000 species around the world – that’s 90% of the whole plant kingdom. New Zealand has about 2,000 native angiosperms , and an amazing 25,000 introduced species found mainly in gardens, farms and orchards.

Flowering plants are all around us, even if sometimes we don’t recognise them as having flowers. We all know the showy flowers of native kōwhai, flax and pōhutakawa and all those lovely coloured flowers in our gardens, but the tall toetoe and the grasses in our lawns are also flowering plants.

Plants are living:

DK Science: Seed Plants

Most plants grow from seeds. These seed plants fall into two groups, angiosperms and gymnosperms. Angiosperms are the flowering plants. Their seeds develop inside a female reproductive part of the flower, called the ovary, which usually ripens into a protective FRUIT. Gymnosperms (conifers, Ginkgo, and cycads) do not have flowers or ovaries. Their seeds mature inside cones. Seeds may be carried away from the parent plant by wind, water, or animals.

Dandelion seeds have feathery parachutes to help them fly far from their parent plant. A dandelion flower is actually made up of many small flowers, called florets. Each develops a single fruit. The fruits form inside the closed-up seed head, after the yellow petals have withered away. When the weather is dry, the seed head opens, revealing a ball of parachutes. The slightest breeze lifts the parachutes into the air.

INSIDE A SEED

A seed is the first stage in the life cycle of a plant. Protected inside the tough seed coat, or testa, is the baby plant, called an embryo. Food, which fuels germination and growth, is either packed around the embryo or stored in special seed leaves, called cotyledons.

SPREADING WITHOUT SEEDS

Seeds are not the only means of reproduction. Some plants create offshoots of themselves – in the form of bulbs, tubers, corms, or rhizomes – that can grow into new plants. This type of reproduction is called vegetative reproduction. As only one parent plant is needed, the offspring is a clone of its parent.

A bulb is an underground bud with swollen leaf bases. Its food store allows flowers and leaves to grow quickly. New bulbs develop around the old one.

Tuber

A tuber is a swollen stem or root with buds on its surface. When conditions are right, the tuber’s food store allows the buds to grow.

A corm is a swollen underground stem that provides energy for a growing bud. After the food in the old corm is used up, a new corm forms above it.

Rhizome

A rhizome is a horizontal stem that grows underground or on the surface. It divides and produces new buds and shoots along its branches.

GERMINATION OF A RUNNER BEAN

Most seeds require damp, warm conditions in order to sprout. During germination, the seed absorbs water and the embryo starts to use its food store. A young root, or radicle, begins to grow downward. Then a young shoot, or plumule, grows upward. This develops into the stem and produces leaves. The first leaves, called seed leaves or cotyledons, fuel the early growth until the plant’s true leaves appear.

FRUITS

A flower’s ovary usually develops into a fruit to protect the seeds and help disperse them. A fruit may be succulent (fleshy) or dry. Fruit is often tasty and colourful to attract fruit-eating animals. Its seeds can pass through an animal unharmed, falling to the ground in droppings. Seeds may also be dispersed on animals’ coats, by the wind, or by the fruit bursting open.

DRY FRUITS

The seeds of dry fruits are dispersed in various ways. Peapods are dry fruits that split and shoot out their seeds by force. The hogweed fruit forms a papery wing around the seed, helping it to float on the breeze. The strawberry is a false fruit, but it is covered by tiny dry fruits, each with a seed.

SUCCULENT FRUITS

Fleshy, brightly coloured, and often scented, succulent fruits are designed to attract the animals that eat and disperse them. Fleshy fruits such as apricots and cherries have a woody stone or pip that protects the seed. Called drupes, these fruits form from a single ovary. Many drupes, formed from many ovaries, may cluster to form a compound fruit, such as a raspberry.

Most plants grow from seeds. These seed plants fall into two groups, angiosperms and gymnosperms. Angiosperms are the flowering plants. Their seeds de