Green_Plants_Concept_2
Flowering Plants
Flowering Plants
Although the flower heads of different plants vary, most have recognisably similar parts. The first part of the flower to grow on the end of a specialised flower stem is the receptacle, from which ail of the other flower parts eventually grow. A covering called the sepal, which splits to let the flower out when it is ready: protects the flower bud. The flower petals are designed to attract insects or other small animals by their colour or smell.
Working inwards from the sepal (see Figure 3), there are the petals, the stamen (male organ) and the pistil (female organ). Those flowers that most closely resemble their ancient ancestors, such as the buttercup, have this basic format, whereas flowers that have developed more recently can be very different. The daisy, for example, is a compact cluster of many minute flowers crammed together on one head Some plants produce separate male and female flowers (such as grapevines and oak trees); some even have separate male and female plants (such as willow trees).
The pistil (also called the carpel in some books) has a sticky opening at the top, called the stigma, to which pollen grains become attached (see Figure 4). Below the stigma is the style, which contains the pollen tube that the sperm (usually two per pollen grain) travel along to reach the ovary, where the ovule that contains the eggs can be found.
The main part of the flower consists of a thin stalk called a filament, topped by an anther in which the pollen grows until it is released on reaching maturity.
Some flowers are self-pollinating, which means that the pollen from the anther can fertilise eggs in the ovary of the same flower However; most flowers are cross-pollinating and rely on the wind water or an animal to carry the pollen produced in one flower to the stigma of another flower of the same species, Flowering plants evolved long after animals appeared on land, and many have developed a relationship of mutual dependence with a particular species of animal.
When it reaches the stigma, the pollen grain will break to release hid the sperm, one of which will fertilise the eg contained in the ovary. An ovary may contain just one or many eggs, depending on the type of plant.
Once the egg is fertilised, the ovule surrounding it will harden to provide a protective seed shell. The ovary might expand to form a fleshy fruit (apple, cherry), a dry fruit (almond hazelnut) or a grain (wheat, rice).
To further the species, plants must disperse their fertilised seeds in order to colonise new habitats and give each seed the best chance of survival, By covering them in a fleshy fruit, the plant encourages larger animals to eat them and then, later, to excrete the seeds further away. Dandelion and sycamore seeds, with their 'parachute' and 'wing' structures, are excellent examples of dispersal by wind. The coconut, which grows by the sea, has a large seed that will float to another location to grow. The teasel has a 'fruit' that is covered in hooks and will become securely attached to mammal fur (or human clothing).
None of these dispersal systems are particularly effective, and plants have to produce many seeds to make it likely that some of them will find somewhere suitable to grow.
On finding a place where the conditions are appropriate (sufficient amounts of water, warmth and oxygen — light may also be a factor), a seed will germinate and begin to grow into a new plant. This might be after a considerable period of dormancy — but as time goes by, the viability of the seed decreases.
In germination, water is absorbed by the seed, causing the embryo inside to expand and split the casing. The embryo now has access to oxygen, which allows the energy stored within the seed to be utilised for growth. The radicle (root) is the first part of the seedling to emerge, followed by the hypocotyl (shoot), to which the plumule (rudimentary leaves) are attached.
Once the hypocotyl has reached the surface of the soil and is receiving light, photosynthesis can begin and the seedling no longer has to rely on the store of energy in the seed. If the seed is buried too deep or is in too dark a place to begin photosynthesising before its store of energy is depleted, it will die.
Some flowering plants (particularly those in desert regions) have irregular flowering cycles based on the availability of water. After lying dormant for months or even years, these plants will spring into action when it finally rains and go through their flowering cycle before it becomes dry once more, Other flowering plants in tropical conditions, where there are no seasonal variations and the growing conditions are ideal the whole year round may be constantly in flower or may only flower once in every few years.
Most flowering plants progress through a life cycle whose stages are broadly similar to those described above. In order to understand gardening and horticulture, the children need to know about the plant's sexual organs and the parts that they play in the reproductive cycle, and how to identify them in various species,
Dormant — when growth is suspended at a very low level of metabolism.
Fertilisation — the point at which the sperm from the pollen meets the egg in the ovary.
Germination — the point at which the dormant embryo begins to grow into a seedling.
Ovule — the part of the ovary that contains the egg.
Pistil — the female part of the flower, containing the stigma, style and ovary,
Pollen — the granule that delivers the male genetic material to the female seed
Pollination — the process by which the pollen reaches the stigma.
Stamen — the male part of the flower, comprising the anther and filament.
The 'stinking corpse lily' produces a flower (no prizes for guessing the scent) that is 1m across, with each petal growing up to 45cm in length. The largest natural British bloom, the white water lily, is only 15cm wide.
The smallest flower (l mm in diameter) belongs to an orchid.
The 'double coconut' tree produces a fruit weighing 1 8kg.
A flowering herb found in the Bolivian Andes flowers only once in its lifetime, after 80 to 150 years of growth.
Old seeds won't grow,
It depends on how they have been stored. Older seeds tend to be less viable (a smaller proportion will germinate), but some will still grow if they have been stored correctly (in dry, cool, dark conditions). An Arctic lupin seed found frozen in Canada was thawed out and grown — and that was approximately i 5,000 years old! There have been several report of seeds found in Egyptian pyramids germinating successfully after thousands of years,
Why don't flowers just pollinate themselves?
Most flowers are able to self-pollinate, but cross-pollination results in the sharing of genetic material, which means that the new plant will be slightly different from its parents. This increases variation and so gives the plant species a greater chance of long-term survival
Flower dissection (observing, recording)
Children can work in small groups to observe different flowers, noting similarities and differences. They can cut open the flower heads with sharp knives (on cutting boards, with careful supervision) to reveal the ovary and other parts, then fix them to a sheet of paper with double-sided sticky tape and label them,
Flower walk (observing, sorting)
Take a group of children for a walk through a flower garden or wild flower meadow (or even a garden centre). Ask them to observe and draw some of the different flower heads, then attempt to sort them into their own different groups. (NB Be aware of any children who are allergic to pollen and take action accordingly.)
Bean dissection (observing, recording)
Soften runner or broad beans by soaking them in water. Ask the children to draw what they think might be inside; discuss their ideas. Cut open some beans (using a sharp knife) and ask the children to draw what they observe. Discuss how what they can see compares with their original ideas.
Germination (observing, recording)
A group of children can soak some beans and place them on a sheet of damp kitchen paper, with a further damp sheet over themThey should keep the beans somewhere dark and return to observe them on a daily basis, recording changes. Remind them to keep the beans damp — but not to drown them!
Grow a plant (observing, planning)
Working in groups, children can follow the instructions on seed packets and grow plants from seeds.They should keep a diary of the changes, (NB Choose the species carefully to ensure that the seeds are of a manageable size and beware of seeds covered in chemicals.)