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FLOWERS



The functions of flowers

Flowers are parts of plants that contain sex organs, and so they have the functions of producing and bringing together male and female sex cells ( gametes). In this respect they are like sex organs of mammals, but the role of flowers in sexual reproduction is often confused because most flowers contain both male and female organs, so that it is not clear that two different parents are usually involved in the process.
After pollination and fertilisation, flowers develop into fruits, containing seeds - which form the next generation for the plant. Because sexual reproduction is involved, the resulting offspring vary, i.e. they are all slightly different, which is important because it enables evolution to occur, so that plants are well suited to their environment, as well as interesting to look at!

The structure of flowers

i.e. how they are made up

Although flowers vary a great deal, and actually much of their appeal lies in the apparent differences, they have a rather similar structure.

Most flowers consist of of 4 parts, (which are actually modified leaves!), in rings round the enlarged end of a stalk:

Name of parts
- and collective name
Appearance and number Usual Function
> sepals
- calyx
often green, may be hairy
- often 3, 4, or 5
to protect flower in bud stage
> petals
- corolla
often brightly coloured and marked or scented
- often 3, 4, or 5 (frequently same number as sepals)
to attract insects for pollination, and act as landing pad
> stamens
(male organs)
- androecium
like drumsticks
> anther = tip
> filament = support
- usually several (multiple of above number?)
split open to produce
> pollen (contains male gametes)
> carpel(s)
(female organs)
- gynaecium
varying shape, perhaps flask-shaped
> stigma=receptive surface
> style = stalk, similar to filament
> ovary = usually a bulge at base of flower - often only one or a few
to produce
> egg cell(s) (female gametes) and
> support development/form seeds after fertilisation

Variations in flower structure


Certain flowers ring the changes on this theme. They may have:
- different numbers of flower parts depending on whether they are monocots (multiples of 3) or dicots (multiples of 4 or 5)
- structures called nectaries which produce nectar
- rows of hairs or special shape to discourage small insects
- sepals indistinguishable from petals
- sepals and/or petals joined/partly joined into a tube
- no sepals or petals
- insignificant green petals
- larger stamens and stigma, exposed to the wind
- petals of different shapes, so flower is not "round"
- stamens attached to petals, not to stalk
- flowers of separate sexes on different parts of plant, i.e. each has only male parts OR only female parts (unisexual)
- separate male and female plants, each with only one type of flower
- many different small flowers collected together into one larger "head"
- many flowers crowded together for greater visual impact.

The common use of the word "flower" implies a small pretty plant, but it must be remembered that most everyday green and coloured plants, and also most trees have flowers, even though they may not be very distinctive.

Processes occurring in flowers


Pollination is the transfer of pollen from the anther to the stigma, i.e. from the male parts, where it is made, to one of the female parts. This might be compared to mating in animals, in which sex cells are brought nearly together, but with a few vital differences! It is not the same as fertilisation.

What is the difference between self-pollination and cross-pollination?
   > self pollination involves pollen transfer within same flower
   > cross pollination involves pollen transfer between different flowers

The main ways that pollen is transferred are by means of insects, or by wind. These different methods mean that flowers are adapted in various ways. How do these factors vary as a result?

  insect-pollinated flowers wind-pollinated flowers
position of anthers and stigmas > inside flower > outside flower
type of pollen > "large", sticky > "small", light

Fertilisation is a process which occurs some time after pollination.

The pollen grain on the surface of the stigma may be affected by secretions from the tissue there, and either encouraged or discouraged from developing further.

From the pollen grain grows out a pollen tube.

The tube grows down the style towards the egg cell, inside the ovule in the ovary.

A nucleus from inside the tube enters the ovule and fuses/joins with the nucleus of the egg cell. This is the point of fertilisation. The resulting cell is a zygote.

If an ovary contains several ovules, each one may be fertilised separately by a different pollen tube growing from another pollen grain.

The ovule develops into the seed, containing an embryo plant, and the covering of the ovule becomes the seed coat (testa) .

The wall of the ovary grows thicker and develops into the fruit, with a seed or seeds inside it.

Not all fruits are sweet and soft, like apples or oranges, for example, but we shall see the similarities in structure and function of all fruits, later.

What process occurs inside the fertilised egg cell to give rise to an embryo plant?
   > cell division/mitosis

Growth of flowers and fruits depends on a supply of "food".
In what (chemical) form do you think this is delivered to these structures, and by what route, i.e. via which tissue?
   > sugars      > phloem

What do you think happens to the petals after pollination and fertilisation?
   > usually fall off

What is honey mostly composed of?
   > nectar - sugars produced by flowers to attract pollinating insects

Why do you think bee-keepers were provided with extra sugar rations during the war? (Answer is not "to keep them sweet"!)
   > keeping population of bees in order to ensure pollination

Why do hay fever sufferers worry about the weather in summer?
   > more grass pollen is released under some conditions


ACTIVITY: Examining flowers



Look at the flowers provided, using a hand lens or binocular microscope if necessary.

It is a good idea to place the flower on a tile or petri dish.
Remove the various parts, using a needle or scalpel (CARE!).

Count up and describe the component parts, and put the numbers into the table below, along with the name of the flower. Draw them if you can.

Plant name Number and description (colour, etc) of :
  sepals petals stamens carpels
including stigma,
style, and ovary
ovules
1)  

         
2) 

         
3)  

         
etc  

         
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