FUNCTIONS OF A FLOWER

• Flowers are the organs of sexual reproduction in plants

PARTS OF A FLOWERS

LEAVES

Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf.

Cross section of a leaf, showing the anatomical features important to the study of photosynthesis: stoma, guard cell, mesophyll cells, and vein. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.Water enters the root and is transported up to the leaves through specialized plant cells known as xylem (pronounces zigh-lem). Land plants must guard against drying out (desiccation) and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata; Greek for hole) flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost. Cottonwood trees, for example, will lose 100 gallons of water per hour during hot desert days. Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures.

Pea Leaf Stoma

outer layer of the stem

tubes that carry sap

new parts of the stem

woody part of the stem

central part of the stem

protective covering of the stem

Different types of roots: axis of the plant which grows in the opposite direction from the stem, maintain the plant in place and absorbs nutrients.Fibrous: root formed in bundles where it is not possible to

determine the primary root.Cauline: roots that shoot from the stem.Tubercular: root in the form of a tubercle.Taproot: root that grows vertically into the earth.

REPRODUCTIVE ORGANS IN PLANTS1. Sepals

- protect the unopened to flower bud

2. petals- may be brightly coloured to attract insects

3. stamens - the male parts of the flower consisting of the anther held up on the filament

4. Anthers- produce male sex cells (pollen grains)

5. Stigma- the top of the female part of the flower which collects pollen

grains6. Ovary

- produces the female sex cells (ovules)

7. Nectaries- produce sugary nectar which attracts insects

Asexual reproduction in plantsAsexual reproduction in plants can take a number of forms:

Vegetative propagation: Many plants develop underground food storage organs which overwinter and develop into the following year's plant. Examples are bulbs, tubers (eg potatoes) and rhizomes

Daffodil bulb

Plantlets: These can take the form of runners (eg strawberries) or side branches (busy lizzy).

Cuttings: We can make cuttings or grafts, which in the right conditions will develop roots and grow into a new plant.

Jade plant cutting growing new roots

Tissue culture: We can take a few cells from a plant and grow them into a complete specimen. Tissue culture is a type of cloning; for more on this topic see the Revision bite on Cloning and genetic engineering)

As only one parent is involved in asexual reproduction, all the offspring have exactly the same genes as their parent. The offspring are identical and they are called clones. Because of this, any genetic problems there may be will always be passed on to the new generation.

Sexual reproduction in plantsMany plants reproduce sexually. The advantage to the plant is that its offspring have a selection of genes from two parents, so each individual's genes are different. The offspring are not identical, and there is variety in the species.A flowering plant's sexual organs consist of:•the stamen, or male sex structure, consisting of a filament and a pollen-bearing anther at the tip •the pistil or female sex structure, consisting of ovary and ovule, style, and stigma at the tip. (The pistil is also sometimes called the carpel.) Here's how it works: 1.An insect or the wind carries pollen grains from the anther of another flower. 2.The pollen grains land on the stigma and a pollen tube grows down through the style to the ovary. 3.The nucleus of the pollen grain passes down the tube. It fertilises the egg cell inside the ovule. 4.The fertilised egg cell develops into an embryo. The ovary becomes the fruit and the ovule becomes a seed - from which (once dispersed) the offspring plant will grow.

FERTILISATION

• When pollen grains land on the stigma of a flower of the correct species they germinate. A pollen tube grows through the tissues of the flower until it reaches an ovule inside the ovary. The nucleus of the pollen grain (the male gamete) then passes along the pollen tube and joins with the nucleus of the ovule (the female gamete). This process is called fertilisation.

• After fertilisation the female parts of the flower develop into a fruit. The ovules become seeds and the ovary wall becomes the rest of the fruit.

POLLINATION

DIFFERENCE BETWEEN INSECT AND WIND POLLINATED INSECT POLLINATED• large, brightly coloured petals - to

attract insects• often sweetly scented - to attract

insects• usually contain nectar - to attract

insects• moderate quantity of pollen - less

wastage than with wind pollination• pollen often sticky or spiky - to

stick to insects • anthers firm and inside flower - to

brush against insects • stigma inside the flower - so that

the insect brushes against it• stigma has sticky coating - pollen

sticks to it

WIND POLLINATED• small petals, often brown or dull green -

no need to attract insects• no scent - no need to attract insects• no nectar - no need to attract insects• pollen produced in great quantities -

because most does not reach another flower

• pollen very light and smooth - so it can be blown in the wind

• anthers loosely attached and dangle out - to release pollen into the wind

• stigma hangs outside the flower - to catch the drifting pollen

• stigma feathery or net like - to catch the drifting pollen

Seed dispersal

• Seeds are dispersed away from each other and from the parent plant so that there is less competition. The commonest methods of seed dispersal are:

1. wind e.g. dandelion, sycamore fruits are light and have extensions which act as parachutes or wings to catch the wind

2. animal internal e.g. tomato, plum, raspberry, grape have brightly coloured and succulent fruits which contain seeds with indigestible coats which allow the seeds to pass through the animal undamaged

3. animal external e.g. goose grass, burdock, the fruits have hooks which attach them to the fur of passing animals.

ASEXUAL REPRODUCTION

Advantages for the plant of asexual and sexual reproduction

• Asexual reproduction only one parent plant

is required young plants are

identical to the parent, so that good features will always be passed on

• Sexual reproduction characteristics are

inherited from two parents - this produces variation in the offspring;

this gives a good chance of at least a few surviving diseases, changes of climate, etc.

Water transport in celeryAim To observe a movement of water in

the xylem of celery

Equipment1. Celery stick with leaves, 2. 2 beakers, 3. Razor blade, 4. Dye

Method1. Arrange the apparatus as shown.2. 2. Leave it overnight and then observe

the celery stalk closely.3. Cut the celery stick lengthways and

across the stalk, and note the presence of any

dye.

Questions1. Describe the directions in which the dye travelled.2. Construct diagrams of the horizontal slice and of the vertical slice. In each diagram show where the dye travelled.3. Explain why one half of the celery stalk was left in water with no dye.

A product of photosynthesisAim To investigate the products of photosynthesis

Equipment1. 2 x 600 mL beakers, 2. 2 glass funnels, 3. 2 test tubes, 4. Sodium hydrogen carbonate solution (0.5 per cent), 5. 2 pieces of actively growing Elodea (Canadian pond weed),6. light source,7. wooden splint, 8. safety goggles

Method1. Half-fill each beaker with

sodium hydrogen carbonate solution.

2. Place a piece of plant in each beaker and cover the plant with a

funnel.3. Invert a test tube full of water

over the stem of each funnel.4. Place one beaker in the dark,

the other in continuous light for

several days.

Green leaves and photosynthesis

Aim To examine where the products of photosynthesis are stored in leaves

WARNING: Ethanol is highly flammable. At no stage should the

test tube containing ethanol be placed near any flame.

Equipment1. Potted plant with variegated leaves, 2. potted plant of the same species with completely green

leaves 3. 3 beakers of boiling water 4. 2 large test tubes containing ethanol or methylated spirits, 5. iodine solution, 6. forceps, 7. scissors,8. 2 watch-glasses or 2 glass Petri dishes, 9. safety goggles

Method1. Cut a leaf from each plant. Cut a small nick in the edge of the

variegated leaf so it can be identified later.2. Sketch two outlines of the variegated leaf side by side. Do the same

for the green leaf.3. Drop both leaves into one beaker of boiling water for a few minutes.

This kills the leaf cells so that no further reactions can occur.4. Using the forceps, remove the leaves and place one in each test

tube of ethanol.5. Stand both test tubes in the second beaker of boiling water. The

ethanol will start to boil, and green colour will be dissolved from the leaves. After around 10 minutes the leaves should look quite pale.

6. Using the forceps, remove the leaf from one test tube and dip it into the third beaker of boiling water for a few seconds. This removes the ethanol and softens the leaf. Place the leaf on a watch-glass or Petri dish. Repeat this step for the other leaf.

7. Add iodine solution to each leaf. Allow it to stand for a minute.8. Dispose of all solutions as instructed by your teacher.9. On the outlines prepared in step 2, draw and colour in the areas stained blue-black on each leaf.

Extracting chlorophyll from leavesAim: To extract chlorophyll from leaves and separate different types of chlorophyll by paper chromatography.

Materials:1. Soft leaves2. 250 ml beaker3. 50 ml beaker4. 100 ml measuring cylinder5. Bunsen burner6. Heat mat7. Gauze mat8. Metal tongs9. Matches10. Methylated spirits11. A test tube12. Chromatography paper

Method:

A product of respirationAim To investigate the products of respiration

Equipment1. Flasks and glassware as shown2. Filter pump,3. sodium hydroxide solution, 4. limewater, 5. Potted plant, several insects or earthworms

Method1 Set up the apparatus as shown.2 Slowly draw air through the apparatus by means of the filter pump.3 Record any changes in the colour of the limewater in flasks B and D.

1. Sodium hydroxide absorbs carbon dioxide from the air. Carbon dioxide dissolves in limewater to form a milky solution. Explain the purpose of flasks A and B.

2. Explain the purpose of flask D.

3. Justify the use of the:

a. plastic bag

b. b black paper.

4. Explain any changes observed in the limewater during the experiment.

5a.Modify the experiment, using an animal (eg earthworm) in place of the potted plant in flask C.

b. Compare and contrast the results of the two experiments.

Stomata and chloroplasts

Aim To examine stomata and chloroplasts in leaves

Equipment1. Compound microscope, 2. microscope slides and cover slips,3. dropper, 4. tweezers, 5. razor blade, 6. stain such as methylene blue or iodine, 7. leaves from various plants such as rhubarb and agapanthus, elodea (a

water plant)

MethodPart A—Stomata1. Set up the microscope.

2. Peel the lower epidermis (outer layer) from the bottom of a leaf. Using tweezers may help.

3. Place the epidermis flat on the microscope slide.

4. Add a drop of water and carefully lower the cover slip on top. Be careful not to trap any air bubbles under the slip.

5. Add a drop of stain at one edge of the cover slip and hold a piece of paper towel at the opposite edge to draw the stain under the cover slip and across the leaf sample.

6. View the slide under the microscope, identify and draw the stomata.

7. Try looking at the stomata of other plant leaves in the same way.

8. Choose another leaf and try to find stomata on the upper epidermis.

Part B—Chloroplasts1. Take a leaf of elodea.2. Use a razor blade to cut a very thin slice off the leaf.3. Place the leaf slice on a microscope slide and add a drop of water and a cover slip.4. View the slide under the microscope. Identify and draw the cells containing green chloroplasts.

1. Outline the purpose of stomata.

2. Stomata are mainly found on the underside of leaves. Explain why.

3. Outline the function of a guard cell.

4. Describe the role of chloroplasts.