School Science Lessons
Biology experiments
Updated: 2008-09-04
Biology names
Table of contents
9.6.0 Germination of seeds
9.7.0 Tropisms and nastic movements
9.8.0 Photosynthesis
9.10.0 Respiration in plants
9.11.0 Food tests
9.6.0 Germination of seeds
6.3.1 Parts of a seed, morphology of the seed
6.3.2 Drinking glass garden
6.3.3 Test viability of seed before planting
6.3.5 Physical breakdown of starch during germination
6.3.6 Enzyme activity during germination
6.3.9 Conditions necessary for germination
6.3.13 Development of plant embryos
6.3.14 Germination from seed to plant
6.3.15 Function of cotyledons
6.3.16 Natural growth inhibitors
6.3.17 Germination and air, germination and the need for oxygen
6.3.18 Germination and light, watercress, onions, love-in-a-mist
6.3.19 Germination and water
6.3.20 Germination and temperature
6.5.0 Light is necessary for photosynthesis
6.5.0.1 Bean plants
6.5.0.2 Tree leaf
6.5.0.3 Elodea leaf
6.5.0.4. Nasturtium leaf
6.5.1 Carbon dioxide in the air is necessary for photosynthesis, Nasturtium
6.5.2 Plants need water, daisy, potted plants
6.5.3 Plants need salts, maize
6.5.13 Bromothymol blue test for carbon dioxide
6.5.14 Release of oxygen during photosynthesis
6.5.15 Measure the rate of photosynthesis, Elodea .
6.5.16 Measure the effects of factors on photosynthesis - light intensity, sodium hydrogen carbonate conc., light quality, light intensity
6.5.17 Plants growing in the dark
6.5.18 Plants need salts, hydroponics, Knop's solution
9.109 Endospermic and non-endospermic seed
9.110 Hypogeal germination, e.g. broad bean, pea, wheat
9.111 Epigeal germination, e.g. castor bean, common bean, lettuce, tomato, marrow, onion
9.112 Physical breakdown of starch during germination
9.113 Swelling of seeds, imbibition, during germination
9.114 Development of plant embryos
9.115 Germination from seed to plant
9.116 Function of cotyledons
9.117 Natural growth inhibitors
9.118 Germination and air, germination and the need for oxygen
9.119 Germination and light
9.120 Germination and water
9.121 Germination and temperature.
9.122 Viability of seed before planting
6.5.2 Plants need water, daisy, potted plants
6.5.3 Plants need salts, maize
6.5.18 Plants need salts, hydroponics, Knop's solution
1.27 Drink-can garden (Primary)
1.28 Grow plants from seed (Primary)
2.33 Grow plants from seeds (Primary)
5.27 Germinate bean seeds (Primary)
5.28 Depth of seeds (Primary)
5.29 Germinate maize grain (Primary)
9.7.0 Tropisms and nastic movements
6.4.1 Geotropism, clinostat
6.4.2 Geotropic responses of soaked seeds
6.4.3 Geotropic responses in shoots, e.g. broad bean, Pelargonium
6.4.4 Gravity affects the growth of stems and roots
9.125 Phototropism
6.4.6 Phototropic responses of seedlings, e.g. mustard, wheat, barley
6.4.7 Phototropic response, Pelargonium
6.4.8 Light affects stems
6.4.9 Sprouting potato
9.126 Thermonasty
6.4.11 Thermonastic responses in flowers, crocus, tulip.
6.4.13 Swelling movements for dispersal of pine seed
9.123 Chemotropism, germinating pollen, pollen tubes
9.8.0 Photosynthesis
3.36 Photosynthesis equation, carbon dioxide and photosynthesis
9.145 Light is necessary for photosynthesis
6.5.17 Plants growing in the dark
9.152 Waterweed in the light and in the dark
9.146 Carbon dioxide in the air is necessary for photosynthesis
6.5.4 Chloroplasts
6.5.5 Chlorophyll is necessary for photosynthesis, variegated leaf
6.5.6 Chloroplasts in cells of waterweeds and algae
6.5.7 Extract chlorophyll from green leaves, potato, onion, Tropaeolum, Fuchsia, hyacinth, lilac
6.5.8 Separate chlorophyll pigments with paper chromatography
6.5.5 Chlorophyll is necessary for photosynthesis, variegated leaf
6.5.9 Fluorescence of chlorophyll
6.5.11 Tests for starch
6.5.12 Tests for sugars
9.154 Limewater test for carbon dioxide
6.5.13 Bromothymol blue test for carbon dioxide
6.5.14 Release of oxygen during photosynthesis
6.5.15 Measure the rate of photosynthesis, Elodea
6.5.16 Measure the effects of factors on photosynthesis - light intensity, sodium hydrogen carbonate conc., light quality, light intensity
1.30 Plants need sunlight (Primary)
1.31 Plants need water (Primary)
9.11.0 Food tests, different foods
9.11.1 Enzymes
9.11.2 Carbohydrates
9.11.3 Fats and oils
9.11.4 Proteins
9.11.0 Food tests
3.98 Elements in foods
9.128 Heat different foods
9.3.15 Moisture content of plant organs and ash content of plant dry matter
19.4.0 Food chemistry
19.2.0 Composition of food
19.3.6 Food preservation
19.4.4 Shopping chemistry, food additives, EEC Code numbers
2.34 Three kinds of food (Primary)
9.11.1 Enzymes
9.3.10 Activity of diastase
9.3.11 Tests for oxidase and peroxidase in plant tissues
9.3.12 Tests for zymase and catalase in yeast
9.3.14 Action of lipase in castor oil seeds
9.11.2 Carbohydrates
9.129 Hydrolysis of starch by dilute hydrochloric acid
9.130 Hydrolysis of starch by salivary amylase (ptyalin)
9.131 Hydrolysis of sucrose by dilute acids
9.132 Tests for starch, iodine test for starch
9.134 Solubility of carbohydrates in water
9.135 Tests for cellulose, iodine test for cellulose
9.136 Tests for cellulose, solubility test for cellulose
9.140 Tests for simple sugars, reducing sugars, Fehling's test
9.141 Tests for reducing sugars, Benedict's test
9.142 Tests for starch, Fehling's test for starch
9.143 Tests for vitamin C (L-ascorbic acid)
9.144 Tests for wood
12.18.5.1a Dehydration of sugar by sulfuric acid
16.3.1.1 Carbohydrates
16.3.1.3 Monosaccharides
16.3.1.3.1 Left-handed and right-handed structural forms, D and L sugars
16.3.1.4 Disaccharides
16.3.1.5 Starches, amylum, glycogen
16.3.1.6 Cellulose, hemicellulose, lignin, test for wood
16.3.1.7 Chitin
16.3.1.8 Pectin
9.11.3 Fats and oils
9.137 Tests for fats and oils
9.11.4 Proteins
9.138 Tests for nitrogen compounds in food, soda lime test
9.139 Tests for proteins, biuret reaction, Millon's reagent, xanthoproteic reaction
16.6.1 Heat test for proteins
16.6.2 Burning test for proteins
16.6.3 Prepare protein solutions
16.6.4 Tests for albumin and gelatine
16.6.5 Tests for proteins, Biuret test
16.6.6 Tests for proteins, xanthoproteic test
16.6.7 Tests for proteins, Millon's test
16.6.8 Tests for proteins, Albustix test strips
16.6.9 Nitrogen in an organic compound, Kjeldahl method
16.6.10 Tests for proteins, Sakaguchi's arginine test
16.6.11 Tests for sulfur in proteins
16.6.12 Proteins are amphoteric
16.6.13 Urea forms biuret
6.3.1 Parts of a seed, morphology of the seed
See diagram 9.72.3
A seed is a megasporangium containing an embryo and food. It is the post fertilization transformation of an ovule. The embryo is the new diploid generation. The endosperm, the female gametophyte, and the nucellus and seed coats are the parent diploid generation.
1. In an open 100 mm diameter flat glass dish put five bean seeds for swelling. Next day pick out a seed that has swollen nicely for close observation. The first thing that strikes you is that the bean seed is enclosed in a relatively tough skin. Because of the swelling this skin nearly always bursts and can be easily removed with a pair of forceps. The skin is called the seed pod. The seed pod encloses two whitish, thick, fleshy formations. Their outline is kidney shape. These are the two seed leaves, cotyledons. With the forceps take one seed leaf, without crushing it, and bend the second a little to one side with a dissecting needle. By doing this you can see that both seed leaves hang together atone end. At this spot there are two whitish, small leaflets. They lie folded in between the two seed leaves. They are the first leaves of the subsequent bean plant. They rest on a very short, delicate stalk which merges at its other end into a pointed peg, the root. The seed contains a complete miniature model plant, called the embryo. State the parts that go to make up the bean seed. Do a simple drawing showing the parts and their position in relation to each other. How many seed leaves has the bean seed? In seed plants you can distinguish between one seed leaf, monocotyledon, and two seed leaves, dicotyledon, plants. To which group does the bean plant belong?
6.3.2 Drinking glass garden
See diagram 9.125.2
Put rolled paper towels into the drinking glass. Fill the centre with peat moss, cotton, sawdust, or wood shavings. Put a piece of graph paper with water insoluble ink, cut to size, between the glass and the paper. Plant bean seeds between the paper and the glass. Water the centre of the glass. Make frequent observations of growth, using the graph paper lines as reference points. Results may be presented by constructing graphs which portray time on the horizontal axis and the root and stem elongation in millimetres along the vertical axis.
6.3.3 Test viability of seed before planting
See diagram 9.122
1. Do a germination test on your seeds before planting. Soak seeds in water until they are swollen then put them seeds on wet absorbent paper or newspaper in a closed container. Each day, record the number of seeds that have germinated and calculate the percentage germination, i.e. the number of germinated seeds / the number of seeds planted X 100. Note the diversity of time taken to germinate among seeds that look the same. Also, germinate seeds in rolled absorbent paper or paper towels in a drinking glass.
2. Place a piece of absorbent paper or cotton wool in a saucer. Thoroughly moisten it with water. On this absorbent paper, sow a hundred wheat grains. Do the same with other seeds, preferably garden seeds, cress (garden cress), radish, onion, beet, carrot, etc. Each day record the number which have germinated and calculate the percentage. More water may be added to keep the absorbent paper moist. Note the diversity of time taken to germinate.
3. Fold 1 m2 muslin twice in the same direction. Near one end mark out with a pencil 8 or 10 squares 5 X 5 cm. Number the squares, and on each square place ten seeds from a particular packet. Note the arrangement of the different seeds. Fold the opposite end of the muslin over the seeds. Roll up the tester and tie it loosely with string. Saturate the rest with water. Keep it moist and in a warm place for several days. The unroll it and see how many of each kind of seeds have germinated. Report the viability as a percentage or by constructing graphs.
6.3.5 Physical breakdown of starch during germination
1. Use needles to take some starch from the following:
1.1 a cotyledon of a bean seedling, with a root 2 cm long
1.2 the endosperm near the embryo of a wheat seedling with coleoptile 3 to 5 mm long.
1.3 As a control, similarly take starch from an ungerminated bean seed and a wheat grain.
Examine the starch samples under high power. Only the starch grains from the germinated plants show clear corrosion patterns.
2. The starch stored in the cotyledons or in the endosperm is converted to sugar during germination by amylases which becomes available to the developing seedling as a building material and fuel. The corrosion of the starch grains can be clearly seen if a specimen is examined under the microscope. Exercise using a dissecting needle take some starch from one of the cotyledons of a bean seedling, a root 1 to 2 cm long, and mount them on a microscope slide in a drop of water. Make another specimen in the same way using starch taken from the endosperm in the vicinity of the embryo from a wheat seedling with a coleoptile 3 to 5 mm long. For control purposes, mount two more starch specimens from an ungerminated bean seed and wheat grain respectively and examine all the specimens under a microscope, magnification 600 X. compared with the starch from the ungerminated dry seeds, the grains from the germinated plants show clear corrosion patterns.
6.3.6 Enzyme activity during germination
At the start of germination, the substances stored in the seeds must be converted into a form which can be utilized by the seedling which, at first, is not nutritionally and physiologically independent. To do this it is necessary to activate the enzymes which cause these conversions. About 10 bean seeds and 10 maize seeds are placed in flat glass dishes half filled with water and left to swell for 2 days. The contents of four tubes containing starch agar are liquefied by warming the water bath and poured out into four half flat glass dishes. The swollen seeds are halved longitudinally and placed, separately according to species, with the cut surface downwards into two half flat glass dishes containing starch agar. For control purposes halved unswollen bean and maize seeds are similarly placed in the two remaining dishes containing starch agar. All the dishes are placed in the glass tank, it is filled to half the height of the dishes with water and then covered with the glass disk, forming a moist chamber. In order to inhibit, as far as possible, the development of bacteria or mildew, the test assembly is left at a temperature of 15oC. After 2 days, dilute iodine potassium iodide solution is dropped onto the starch agar in all the dishes. The starch agar turns blue violet. However a fairly bright halo is to be seen around the swollen seeds, which is not present around the unswollen seeds. On swelling, and with it the incipient germination, the amylase contained in the seeds is activated. It transforms starch into sugar, which becomes available to the seedling as a building.
6.3.9 Conditions necessary for germination
See diagram 9.112: Epigeal and hypogeal germination
1. A seed can germinate only if the following conditions are present:
1.1. presence of water,
1.2. presence of oxygen,
1.3. presence of adequate temperature.
2. To show that water is necessary for the germination of seeds, fill two flat glass dishes with dry garden soil to a depth of 1 cm. Ensure that the soil is dry by spreading it out on a sheet of paper and letting it dry until it can be crumbled into dust. Then put it in the dishes. In each of two other flat glass dishes put three circular filters and smooth them out on the bottom of the dishes. In each of the four flat glass dishes put 10 dry bean or pea seeds. In the dishes containing garden soil, the seeds should just peep out of the soil. Into one of the two dishes with the circular filters pour just enough water to cover the seeds. Into one of the two dishes containing soil pour just enough water for the soil to be well and evenly moistened. Put the lids on the dishes so that the water does not evaporate too quickly from the dishes which have been kept moist and so that air can enter. After 24 hours the seeds which have been lying in water in the flat glass dish without soil have soaked up most of the water. Keep on pouring more water into this dish, but only enough for the filters on which the seeds are resting to be very moist. Note in which dishes the seeds germinate.
6.3.13 Development of plant embryos
Put seeds in a flat dish, add water, and wait for the seeds to swell. To study embryos in an early stage of development, put the swollen seeds on moist absorbent paper and leave them until the desired stage has been reached. To study seeds in a later stage of development, put the swollen seeds in a flowerpot filled with damp sawdust or sand. When the plants have reached the desired stage of growth, remove them from the sawdust or sand and rinse under water. Open the carpels of embryo plants at different ages, e.g. shepherd's purse. Remove the ovules and put them in a drop of 5% potassium hydroxide solution on a microscope slide. Put a coverslip over them and press down on the coverslip with a thumb wrapped in absorbent paper or a scalpel handle to squeeze the embryos out of the ovules. Be careful not to break the coverslip because the broken pieces are very sharp. Check with the microscope after each time you press down on the coverslip. Examine the preparation to find embryos in different stages of development depending on the age of the plants.
6.3.14 Germination from seed to plant
Sow seeds of pea, broad bean, common dwarf bean or climbing bean, at the same distances apart and at equal depths, almost touching the sides of a glass or plastic container, so you can see them through the walls. Fix black paper or aluminium foil around the container to keep the seeds in the dark. Water the seeds regularly and note daily how they develop. Replace the black paper or aluminium foil after every observation. Measure the daily growth of a bean plant. Mark the height of a bean plant on the graph paper every day at the same time or attach the top of a bean plant to a clinostat.
6.3.15 Function of cotyledons
Place six similar bean seedlings on which sprouting primary leaves are just emerging, in test-tubes so that the roots are immersed completely in water. Hold the seedlings in place with cotton wool plugs. The water must not contact the cotton wool. Keep two seedlings with both cotyledons as a control. Remove one cotyledon from each of two other seedlings. Remove both cotyledons from the last two seedlings. After two weeks compare the growth of the bean plants. The plants with both cotyledons have developed best, and those without cotyledons have developed worst. Note that the cotyledons which were not cut off have shrivelled. When you remove a cotyledon the seedling gets less nutrients and it may starve.
6.3.16 Natural growth inhibitors
The formation of growth inhibitors in the immediate vicinity of the embryos, e.g. in the endosperm, in the seed coats, in the pulp, may prevent the premature germination of seeds. Put seeds in a dish with water for ten minutes and allow them to swell, e.g. garden cress, Lepidium sativum. Take four dishes and put two pieces of absorbent paper in each dish and moisten with water. Put a thin slice of apple in dish 1, a thin slice of orange in dish 2, and a thin slice of tomato in dish 3. Put ten of the swollen garden cress seeds on each of the slices and also on the absorbent paper in the dish 4 as a control. Put lids on the dishes and leave at room temperature. After 48 hours the seeds laid on the slices of fruit have hardly altered. However, the seeds on the absorbent paper in dish 4 have germinated. They may have grown a small root and the first tiny leaves. The flesh of apple, tomato and orange all contain growth inhibitory substances. To ensure germination, you must separate seed from the old surrounding fruit tissue.
6.3.17 Germination and air, germination and the need for oxygen
See diagram 9.122
1. Put absorbent paper in the bottom of two dishes. Add 10 dried beans in each dish. Add tap water to the first until the beans are only just covered. Fill the second dish with water to deeply cover the beans. By the next day the beans in the first dish have absorbed most of the water, have swollen and are now lying exposed to the air on the damp absorbent paper. The beans in the second dish have also swollen but remain covered with water. After two days, most of the beans in the first dish have germinated. The beans in the second dish are still immersed in water and have swollen with no further change. Eventually they will die and decompose.
2. Take two pieces of foam rubber which must be small enough to float freely in a preparation glass, 150 mm tall, d 80 mm. Using scissors make a hole in both pieces of foam rubber and pull a piece of thin string through each hole. Attach each piece of foam rubber by means of the string and a strip of sticking plaster to the lower side of the lids of the preparation glass. With the lid in place the pieces of foam rubber should hang halfway down in the glasses. Put 100 mL water into one of the preparation glasses and 50 mL 20% pyrogallol and 20% sodium hydroxide solution in the other, to absorb the oxygen, the pieces of foam rubber are well soaked with water, on each put 15 unswollen garden cress, seeds, smear the rim of the preparation glasses with glycerine to aid sealing and then put the lids in place with the foam rubber pieces hanging down. The pieces of foam rubber with the cress seeds should not come into contact with the fluids underneath them. Leave the preparation glasses to stand at room temperature. Within 2 days the cress seeds in the preparation glass with water have germinated, they have developed into small plants. The cress seeds in the preparation glass with the pyrogallol sodium hydroxide solution, however, have not germinated. The pea seeds which were completely covered with water, experiment 1, could not come into direct contact with the air. They suffered from lack of oxygen, and germination could not take place. Experiment 2 shows that oxygen is the determining factor.
3. Investigate whether germinating seeds need air (oxygen) and water. Explain why the seed does not sprout in flooded fields and may not germinate in very wet garden beds. Put absorbent paper in the bottom of two dishes, dish A and dish 2. Put 10 dried beans in each dish. Add tap water to dish A until the beans are just covered and to dish B up to the rim. Label each dish and leave them in the classroom. Cover the dishes to make exchange of air possible. Keep adding water to dish B to ensure that the surface of the water is always a few millimetres above the beans. By the following day, the beans in dish A, which was not filled to the rim, have absorbed most of the water. These beans have swollen and are now lying exposed to the air on the damp filter paper. The beans in dish B have also swollen but remain covered with water. After two days, most of the beans in dish A still lying on the damp paper and exposed to the air have germinated. The beans which are in dish B are still completely immersed in water and have are still only swollen with no further change. Seeds need air (oxygen) and water for germination. However the beans in dish B that were completely covered with water had no direct contact with the air. They did not obtain enough oxygen to germinate.
4. Take two wide tubes of equal length, about 10 cm. Holding them vertically, fix a wad of moist cotton wool in each, about 2 inches from the top end. Drop in 6 pea seeds or 50 wheat grains so that they lie on the wool. Insert a stopper in each. Now immerse the open lower end of one of the tubes in a beaker of water and the other in a beaker of pyrogallic acid and support in a vertical position. Record the number of grains which have germinated each day, and, realizing that pyrogallic acid absorbs the oxygen from the atmosphere, deduce the role of oxygen in germination.
6.3.18 Germination and light, watercress, onions, love-in-a-mist
1. Test the effect of light on the germination of different seeds, e.g. watercress, onions, love-in-a-mist. Note in which seeds germination is promoted by light, hindered or is indifferent to light. Such seeds are called light germinators, dark germinators and seeds unaffected by light.
The germination of many plant seeds is not affected by light. Some seeds, however, can only germinate if they are exposed in their swollen condition for some length of time to light, while the germination of others is prevented by exposure to light. So plants may be light germinators or dark germinators.
2. To examine the effect of light on seeds, set up saucers containing seeds but having duplicate sets. Place one set in the light and another set in a dark room or box. Do the same with examples of light hard and light-sensitive seeds. Record the results as percentages, and from them discuss the effect of light on germination.
3. Examine the effect of light on the germination of seeds of watercress, onions, and love-in-a-mist. In each of six flat glass dishes of 100 mm diameter put two circular filters of 90 mm diameter and smooth them out on the bottom of the dishes. In each of two of the dishes put 20 watercress seeds, in each of another two 20 onion seeds, and in each of the remaining two 20 seeds of love-in-a-mist. Pour in just enough water to cover the seeds in all the dishes, close each dish with its lid. Keep one of each of the dishes with the same seeds in the light, and put the other immediately next to it, both with the same temperature under a light proof cover, pasteboard carton. After a few days compare the extent of germination of the seeds in the dishes kept in the light and in the dark respectively. Note in which seeds germination is promoted by light and in which is it hindered by light, i.e. which plants are light germinators and which dark germinators. Note in which seeds germination is unaffected by light.
6.3.19 Germination and water
See diagram 9.122: Germination test
1. Use a dish with absorbent paper in the bottom and another dish containing dry, finely divided garden soil. Put 10 dry beans in each dish. Fill the first dish with tap water until the beans are just covered. Leave the dishes to stand at room temperature. Within 24 hours only the beans covered with tap water have swollen considerably and have absorbed much water. Most beans germinate after two days. The beans in the other dish have not changed.
2. Seeds absorb water through the surface of the seed coats and through their micropyle. Weigh 10 dry bean seeds, then block their micropyles with collodion or rubber solution, e.g. the solution used to stick soles on shoes. Be careful! Rubber solution can be toxic when inhaled so work in a fume cupboard, fume hood. Leave the solution to dry then put the seeds in water. Compare the weight and volume of the treated seeds with untreated seeds.
3. Half fill measuring cylinders and add equal volumes of seeds, e.g. pea, common green bean and broad bean. Observe the swelling seeds and calculate how much water they absorb. At first, water is absorbed by imbibition through the seed coats that swell and wrinkle. Then water passes through the micropyle causing the embryo to swell inside the seed coats and make them smooth again. Starch food reserves are hydrolysed to glucose sugar to enable the embryo to grow first by cell enlargement then by cell division. Later growth occurs mainly at the two meristems,. the dividing cells in the embryo shoot, plumule, and in the embryo root.
4. Seeds can only germinate when they absorb water and swell. Take two flat glass dishes and put a circular filter into each, a third dish is filled almost up to the edge with completely dry, finely divided garden soil. 15 to 20 dry pea seeds are placed in each of the dishes. In the dish filled with soil they should project only a little above the soil, one of the other dishes is filled with water until the pea seeds are just covered. All three dishes are then left to stand at room temperature with the lid placed on them obliquely, to allow exchange of air. Within 24 hours the pea seeds covered with water have swollen considerably. They have absorbed a lot of the water. After a further 1 to 2 days they have almost all germinated, the pea seeds in the other two flat glass dishes on dry filter paper and in dry garden soil have not changed. Seeds only germinate after absorbing water and after the resultant swelling. Dry seeds cannot germinate whether they are in open air or in dry soil.
6.3.20 Germination and temperature
Put moist absorbent paper in two flat dishes. Add dry seeds to each dish, e.g. garden cress, Lepidium sativum. Close the lids on the dishes and leave one dish at room temperature and the other dish at 12oC. Within 24 hours most of the cress seeds at room temperature have germinated but the cress seeds at 12oC have not developed. Like every other living process, germination is greatly affected by temperature, and the heat needs of different species of seeds can vary greatly. Each species has an optimum temperature for germination.
6.4.1 Geotropism, clinostat
See diagram 9.124: Geotropism
Tropisms take the form of movement, or turning, in relation to the direction from which the stimulus comes. Geotropism is the tropism that occurs in response to the stimulus caused by the earth's gravitational field.
1. Leave seeds to swell in water. The next day sow them in a flowerpot filled with sawdust. Fix a wire gauze square over the flowerpot. Invert the flowerpot and fix it over a container of water with the water touching the rim of the flowerpot. Keep the sawdust dump by watering through the hole in the bottom of the flower pot. After a few days roots grow through the wire gauze down towards the damp air in the container of water. After 10 days remove the sawdust from the flower pot and note that the shoots have grown upwards from the seed into the sawdust.
2. Plant seeds in a two flowerpots, e.g. oats, radish, or mustard. When the seedlings emerge, attach the first flowerpot horizontally to the vertical disc of a clinostat turntable. Switch on the clinostat so that it turns slowly. Lay the second flowerpot on its side. After a few days the plants in the flowerpot on its side have become curved with their shoots pointing up. The plants rotating on the clinostat continue to grow forward horizontally. The gravitational pull affects the plants attached to the clinostat evenly, so no geotropic curvature occurs. Shoots show negative geotropism. They grow against the pull of the gravitational field. Roots show positive geotropism. They grow towards the direction of the gravitational pull. Leaves show diageotropism, movement at right angles to the vertical.
3. Soak broad bean seeds in water overnight then sow them in flower pots filled with damp sawdust. Make a lid for a wide-necked jar with cardboard or cork and insert long pins through the lid. Put water in the jar. When the radicles of the broad bean seeds are one centimetre long, fix seedlings to the pins inside the jar, with the radicles pointing in different directions. Select seedlings with straight radicles and fix them clear of the water in the jar. Observe positive geotropic curvatures in the radicles.
4. Fill a flowerpot and a culture pot of a clinostat with sawdust and put 10 bean sprouts with 1 cm roots in each pot. When the seedlings break through, put the clinostat on a window sill so that its disc is vertical. Attach the culture pot with the bean plants to the disc and switch on the clinostat. Put the flowerpot with the bean plants in the wooden holder on its side next to the clinostat. After a few days the bean plants in the flowerpot have become curved while growing and their shoots point upwards. The plants rotating on the clinostat continue to grow forward horizontally. The shoots are negatively geotropic. They grow against the pull of the gravitational field. However, the roots behave in a positively geotropic manner. They grow towards the direction of the gravitational pull. This behaviour is independent of the orientation of the air and the soil. The gravitational pull affects the plants attached to the clinostat evenly, so no geotropic curvature can occur.
5. Use a clinostat to show that geotropic responses are no longer shown if roots and shoots are placed horizontally and slowly revolved about their long axes.
6.4.2 Geotropic responses of soaked seeds
Soak broad bean seeds in water overnight and sow with the hilum downwards in sand or sawdust. Take a wide necked glass jar or a rectangular museum jar, half fill with water, and close the top by means of a cork or a piece of stout cardboard. Insert several long blanket pins through the latter. When the radicles have attained a length of about half an inch, fix several seedlings to the pins inside the jar, with the radicles pointing in various directions. Seedlings with straight radicles should be selected, and they should be fixed clear of the water. Place the jar at a temperature of 15oC to 25oC and inspect from time to time for positive geotropic curvatures in the radicles.
6.4.3 Geotropic responses in shoots, broad bean, Pelargonium
Cut shoots from herbaceous plants and fix in a large test-tube of water with a rubber stopper. Seal the hole in the stopper with wax. Fix the specimen so that the shoot is horizontal, preferably in a fairly warm place where uniform light is provided. Examine periodically for negative geotropic curvature in the stem and diageotropic responses in the leaves.
6.4.4 Gravity affects the growth of stems and roots
1. Sprout seeds and select one that is straight. Pierce the seed with a long pin or needle and stick this into a cork. Put damp cotton or absorbent paper in a bottle. Put the cork and seedling in the bottle. Put the bottle in dark cupboard and look at it every four hours.
2. Put seeds that grow rapidly, oats, radish, or mustard seeds on moist absorbent paper between two glass plates secured with rubber bands. After germination, turn the apparatus vertically through 90o and allow to remain undisturbed. Repeat the turning at intervals and observe the effect on the roots.
9.125 Phototropism
See diagram 9.125: Phototropism
1. Cover seedlings growing in pots or a sprouting potato with a black box with a hole in one side. Observe the change in direction of growth as the plants turn towards the light from one side. Repeat the experiment by removing the growing tip from some plants. These plants do not turn towards the light. Repeat the experiment with the hole in the box covered with red, then yellow then blue cellophane. Phototropism is caused by increased concentration of the growth hormone auxin on the dark side of the growing tip of the shoot. So the plant cells grow more on the shaded side. Plants growing in the dark grow faster than plants growing in the light, but they become etiolated, pale yellow, due to lack of sunlight. Phototropism is more responsive to blue light than any other colour.
2. Find a plant that grows in the shade and put it in a flowerpot. Turn the flowerpot on its side and observe the resulting direction of growth of the shoot and the leaves. The shoot turns towards the light and the leaves turn to be at right angles to the source of light. Leaves are diaphototropic, orientated at right angles to the vertical in response to light.
3. Observe plants that turn towards the sun, e.g. sunflower.
4. Tropism as a reaction to light stimuli, phototropism, is shown by the shoots and roots of higher plants. Put a few erect bean seedlings with roots 5 cm long on a cork disc with seven holes. Insert the roots through the holes. Put the disc on the liquid surface of a glass container filled with nutrient solution. Put the apparatus in a window box and check the growth of the seedlings daily. The shoots bend towards the light but the roots turn away from the light. The shoots and roots react in positive and negative phototropic manner respectively. In this way shoots can adapt to the light conditions necessary for the plant and roots grow into the soil to obtain nutrients.
5. Plant ten bean seedlings, each having roots 1 cm long in two flowerpots containing garden soil. Put the pots, each sitting in one half of a dish, in a window box. Put one of them on a clinostat and set it in motion. Regularly water and check the growth of the plants. The plants in the static pot still bend towards the light. The plants rotated on the clinostat grow straight up. The plants on the rotating clinostat all receive an equal amount of light so do not bend to one side.
6.4.6 Phototropic responses of seedlings, mustard, wheat, barley
These seedlings may be grown in pots, under conditions of uniform illumination. When they have grown to a height of 5 cm, place the pots under a hood made from a box or a piece of black cloth arranged so that light now reaches the shoots from one side. Alternatively place the plants in a darkroom about 60 cm to the side of a 40 watt lamp. Note the time taken for positive phototropic curvature to appear. Instances of diaphototropic responses in leaves can be found in plants grown in living rooms or against the wall.
6.4.7 Phototropic response, Pelargonium
Grow the plant in uniform light partly screened with a black cloth so that light now reaches it from the side. In a few hours time the leaves begin to adjust their position. A positive response will also develop in the stem.
6.4.8 Light affects stems
Plant seeds that grow rapidly such as oats, radish, bean or mustard seeds in two flowerpots. when the seedlings are 2.5 cm high, cover one pot with a box that has a hole cut near the top. From time to time lift the box and observe the direction of growth. Turn the box so that light comes from a different direction and observe again after a few days.
6.4.9 Sprouting potato
See diagram 9.125
1. Put two light baffles in a long, narrow box and cut a hole in the end. Plant a sprouting potato in a small pot that will fit in the box. Put the pot behind the baffle farthest from the hole. Cover the box and put near a window. Observe the direction of growth from time to time.
2. Plant some fast growing seeds in four flowerpots. Keep the pots in a darkened room until the seedlings are 2.5 cm high. Put one pot near a sunny window and observe the effect. Turn the plants away from the light and observe. Leave the pot in a place away from direct light for a few days and observe the results.
3. Put each of the three remaining pots of seedlings in a different box. Cut a window in each box and cover each window with a different colour of cellophane, red, yellow and blue. Put the three boxes containing the pots of seedlings in good light with the window facing the light. Observe any difference in the effect produced by different coloured light on the growth of stems.
9.126 Thermonasty
See diagram 9.126
1. A nastic movement is a response not affected by the direction of origin of the stimulus. Transplant two flowering plants with many petals, e.g. daisy, to two tubes containing water. Put one tube in a beaker of cold water and the other tube in a beaker of warm water. After 30 minutes the flower in the beaker containing warm water has opened further but the flower in the beaker containing cold water has closed. The In addition to temperature and light, the diurnal rhythm of the plant and other autonomic stimuli control the opening and closing of flowers. The four o'clock plant, Mirabilis jalapa, does not close its flowers because the time is four o'clock, but because of drop in temperature.
2. Movements made in response to a stimulus, tropic movements, where the direction of the movement is not controlled by the stimulus are called nastic movements. These stimuli often consist only of a general change in some external factors, temperature, light. Nastic movements caused by temperature changes can cause the opening and closing of flowers and is called thermonasty. Half fill two beakers with water. Heat the water in one beaker to 30oC. Fill two collecting tubes to a height of 2 cm with water. Put one daisy with half opened petals in each collecting tube and put the tubes in the beakers containing warm and cold water. The tubes must float so use glass beads to balance the tubes. Observe the behaviour of the petals After 30 minutes the flower in the beaker containing warm water has opened further but the flower in the beaker containing cold water has closed. The different temperatures cause a differing degree of expansion in the upper and lower side of the petals which causes thermonastic movements. In addition to temperature and light, the diurnal rhythm of the plant and other autonomic stimuli play a role in opening and closing flowers.
6.4.11 Thermonastic responses in flowers, crocus, tulip.
If you bring flowers in the closed condition into the warm laboratory, opening usually commences promptly even if the flowers are placed in a dark cupboard. The movements are reversed if you put the specimens into a refrigerator. Use daisy or dandelion flowers to illustrate the photonastic properties of these flowers. They may be performed outdoors or daisy plants can be transplanted into pots in the greenhouse. If certain plants are covered over with a box in the afternoon, examination next day will show that at a time when exposed plants have opened their flowers, the flowers in darkness are still closed. The sleep movements of leaves should be studied in suitable examples in the garden and field.
6.4.13 Swelling movements for dispersal of pine seed
See diagram 9.50.1
The swelling of biological material is caused by molecules of water penetrating between the carbohydrate chains, cellulose, starch, or molecules of protein, forming aqueous films. As a result of this the molecules or groups of molecules, the micelles, which are stored without much space between them before swelling, are forced apart. Infiltration of water, and with it swelling, takes place predominantly in a vertical direction to the longitudinal axis of the micelle. If the structure of a micelle, consisting of strongly bound layers of a membrane, differs, then the direction of the greatest swelling between these layers is different. As soon as the swelling changes in shape, curvature, etc. must occur. Swelling movements of this kind play a role in the dispersal of seeds and spores.
1. Cut strips 15 cm long and 3 cm wide from a sheet of writing paper. Using a soluble adhesive each of the strips is smeared over its whole surface and they are then bonded together as follows: strip a on strip b, strip c on strip d and strip e on strip f, the double strips obtained in this way are dried, using forceps, at a suitable height over a Bunsen burner flame. The double strip e / f turns into a corkscrew. Double strip c / d bends into a circle. Double strip a / b remains unaltered. When manufacturing paper, most of the cellulose fibres end up mainly facing in one direction after the milling process to make large pieces. Because a soluble adhesive is used to bond the paper strips together, water penetrates between the fibres and leads to swelling, mainly vertically to the direction in which the fibres run, double strips c / d and e / f, where the direction of the fibres in the individual strips differs, both resemble a biological membrane consisting of two layers of different micellar structure, on drying, shrinkage, these strips must warp. Plants have special mechanisms for the widest possible dissemination of their seeds and these mechanisms for seed dissemination have adapted to different conditions. Put an open dry pine cone in a 250 mL beaker, filled with water By next morning the cone has closed. If it is taken out of the water, the scales reopen again when it dries. Put the closed cone in a warm place, in the sun or on the radiator As it dries, the cone opens again. The scales that cover the cones are composed of two layers that swell at different rates when exposed to moisture. In this way, "swelling movements" occur. In dry weather, when conditions are favourable for widespread dissemination of the seeds, the cones open. The seeds can fall out and be carried away by the wind. During wet or damp weather the cones close because the released seeds would stick to the moist soil close to the street and could not be disseminated by the wind. Pull a seed out of an open pine or spruce cone with sharp forceps. Each seed has a large, thin, membrane-like appendage, called a "wing". It increases the capacity for flight of the seed.
9.123 Chemotropism, germinating pollen
See diagram 9.123: Lilium pollen grain
1. A tropism is a response affected by the direction of the origin of the stimulus. For example, geotropism is a response to gravity and in the direction of gravity. Shake fresh pollen from several kinds of flowers on absorbent paper soaked in a 10% sugar solution. Put a cover over the pollen and leave it for 12 hours. Use a magnifying glass to see little tubes growing from the pollen grains, the pollen tubes. Shake fresh pollen from flowers onto stigmas of the same kind of flower and look for germinating pollen on the stigmas. Observe pollen stained with methylene blue under low power.
2. Dissolve 2.5 g of sucrose and 1 g of gelatine in 50 mL of demineralized water in a beaker. Put one drop of the solution to a microscope slide. Cut a piece of stigma or style from a female marrow flower and squash it in the drop on the microscope slide. Pick some mature stamens and cut the anthers over the drop so that the pollen falls near the squashed piece of stigma or style. Keep the microscope slide in a damp place and examine it the next day. Note the pollen grains that have germinated to form pollen tubes. Most of the pollen tubes are growing towards the squashed stigma or style. The pollen tubes respond to a chemical that diffuses into the sugar gelatine from the female parts of the flower. In a similar way, the pollen tube would normally grow down the style to reach the ovary and fertilize the ovule. Be careful! Do not break the coverslip because the broken pieces are very sharp and dangerous.
6.5.0 Light is necessary for photosynthesis
See diagram 9.145
Fix a band over a leaf. Fold "silver paper" from a chocolate packet, or aluminium cooking foil, in a band around a large leaf on a tree growing in the sunlight. After three days, remove the band and drop the leaf in boiling water to kill the cells. Put the leaf in methylated spirits to remove the chlorophyll. Do the iodine test for starch on the whole leaf. The part of the leaf not covered by the band paper turns a blue-black colour. The part of the leaf covered by the band does not turn blue-black because of lack of sunlight for photosynthesis to make starch.
All food comes originally from green leaves in the sunlight. A process is a change in something, or a way in which something is made. So you can say that photosynthesis is the process by which green plants make food. Show the students the prepared demonstrations. Where does all your food come from? [Green plants in the sunlight.] In sunlight green, plants use carbon dioxide gas from the air, water, and plant nutrients from the soil to make food. This process is called photosynthesis. ("Photo" means light, "synthesis" means putting together). Plant nutrients are chemicals in the soil that plants need, plant nutrients are needed to make the green colour that absorbs the sunlight energy. This energy remains stored in the food. The first food made in green plants is the simple sugar, glucose. Photosynthesis equation
Carbon dioxide gas + water + sunlight energy --> simple sugar (food) + oxygen gas. The oxygen gas produced by photosynthesis goes out into the air. All your food comes originally from photosynthesis in a green plant because your food is either plants, or animals that have eaten plants. Animals breathe in oxygen gas and breathe out carbon dioxide gas that is a waste. Plants use the carbon dioxide gas during photosynthesis and give out oxygen gas. To make food crops grow well they need: sunlight. Seedlings need some shade and water, but soil must not be waterlogged (filled with water). The plant nutrients must be the kinds of chemicals the plants need. The leaves must not be damaged by insects or disease and they must not turn brown or yellow.
During the light reactions of photosynthesis plants make ATP in their chloroplasts where electrons come from excited light activated chlorophyll molecules instead of from breakdown of glucose. Plants use the ATP they have made to synthesize glucose.
6.5.0.1 Bean plants
See diagram 9.145.1 Sunlight is needed for photosynthesis
Compare the growth of the bean plants growing in sunlight and growing kept in the dark. The plants kept in the dark have a pale yellow colour due to absence of chlorophyll, small leaves and long thin stems because of abnormal lengthening of the internodes and small leaves. These plants are describes as being etiolated. Plants ultimately die if kept too long in the dark.
6.5.0.2 Tree leaf
Three days before the lesson fold a piece of silver paper from a chocolate packet to form a band around a leaf on a tree growing in the sunlight. After three days, pick the leaf off the tree and remove the silver paper. Drop the leaf in boiling water to kill it. Put the leaf in methylated spirits to remove the green substance called chlorophyll. Place the leaf in an iodine solution. The part of the leaf not covered by the silver paper turns blue black colour. The part of the leaf covered by the silver paper does not turn blue black because there was no sunlight for photosynthesis to make starch. Students could form their own initials out of silver paper and "write" them on a leaf. The simple sugars produced by photosynthesis are soon changed to starch in the leaf. When iodine solution is added to starch it changes from a white colour to a blue black colour. A leaf with a silver paper band around it is left on a tree for three days. The leaf is then picked and tested for starch. The part of the leaf shaded by the silver paper shows no starch present. The part of the leaf not shaded shows starch.
6.5.0.3 Elodea leaf
See diagram 9.149.1
Choose two small corks equal in size and use two pins to fix them opposite each other on the surfaces of a leaf of the water-weed Elodea, without removing the leaf from the plant. Do not crush the tissues of the leaf. After 24 hours and towards the end of the day, test the leaf for starch with the iodine test.
6.5.0.4. Nasturtium leaf
See diagram 2.1.7: Starch grains | 4.3.4 Iodine test for starch | 16.3.2.0.1 Fehling's test for reducing sugars and aldehydes in solution, glucose and fructose
Use pins to fix cork discs in pairs on leaves of a potted nasturtium plant so that each leaf has two discs exactly opposite each another on the upper and lower surface. In this way, you exclude light from part of the leaves. Do this in the evening. The next morning, place the plant in strong sunlight. After six hours, cut these leaves from the plant and kill the cells by putting the leaves in boiling water for one minute. Then put them in 96% ethanol. After one hour, the leaves will have become almost colourless. Rinse off the ethanol with water and do the starch test with iodine solution. Draw the leaves and record where you can detect starch.
6.5.1 Carbon dioxide in the air is necessary for photosynthesis, Nasturtium
1. See diagram 9.149.1
Choose a green leaf but do not remove it from the plant. Smear petroleum jelly over some of its area. Do this on both sides of the leaf making the areas coincide with each other. Allow the leaf to remain in such a condition on the plant for two days. Remove the leaf from the plant and scrape off the petroleum jelly. Test the whole leaf for starch using the iodine test. Note where the leaf was smeared with petroleum jelly.
2. See diagram 9.149.1
Insert the split rubber stopper firmly. Smear petroleum jelly over the surface of the stopper to prevent any air entering the vessel either between the stopper and the neck or between the stopper and the plant stem. Place the apparatus in sunlight. Leave for two days, then do the iodine test for starch on a leaf from that part of the twig inside the vessel and also a leaf outside the vessel. The potassium hydroxide has absorbed the carbon dioxide in the enclosed air.
3. See diagram 9.146.1
Carbon dioxide is necessary for photosynthesis in green plants. Terrestrial plants extract it from the air. With the aid of light energy they form carbohydrates from carbon dioxide and water and are thus primary producers in the cycle of natural products.
To show the need for carbon dioxide for photosynthesis, put a well developed nasturtium in a plant pot in a dark place in the evening or place it in artificial darkness by covering it with a light proof cardboard box. On the following morning, fill a glass container one third full with 20% potassium hydroxide solution. Take the plant out of darkness. Put it beside the glass container. Bend a leaf stem without breaking it and insert one leaf into the upper air filled section of the glass container. Cover the jar with a slotted cover to let the leaf stem to pass through the slot. Seal the slot around the stem with adhesive tape. Completely seal the glass container with adhesive tape Potassium hydroxide absorbs carbon dioxide so the leaf sealed inside the glass container is in an atmosphere without carbon dioxide. Put the plant and culture jar in direct sunlight so that the leaf inside the glass container is exposed to the light source. After three hours, detach the leaf inside the glass container from the plant. Also detach some other leaves which have had equal exposure to light. Kill the cells in the leaves by dropping them into boiling water. After one minute, remove them with tweezers and place them in 96% ethanol. After 2 hours the leaves become almost colourless. Rinse the leaves in water and pour potassium iodide solution over them. Note which leaf is coloured blue violet by the iodine potassium iodide solution.
See diagram 9.146: Nasturtium
4. Show that plants need air for photosynthesis. Smear a band of petroleum jelly over both sides of leaf on a growing plant. After two days, remove the leaf from the plant and scrape off the petroleum jelly. Do the iodine test for starch on the whole leaf. See the pattern of the band where the petroleum jelly was on the leaf.
5. Show that plants need carbon dioxide for photosynthesis. Put a flowerpot containing a soft leaf plant, e.g. nasturtium, in the dark. The next day put the flowerpot next to a 20% potassium hydroxide solution in a container with a slotted cover. Bend a stem and insert one leaf into the upper section of the container. Seal the slot around the stem with petroleum jelly or adhesive tape. Potassium hydroxide absorbs carbon dioxide so the leaf sealed inside the container is in an atmosphere without carbon dioxide. Put the plant and container in sunlight. After three hours, detach the leaf inside the container and detach other leaves which had equal exposure to light. Drop the leaves in boiling water to kill the cells. Put the leaves in methylated spirit then remove them when they are almost colourless. Rinse the leaves in water then do the iodine test for starch. Compare the blue-black colour of the leaves.
6.5.2 Plants need water, daisy, potted plants
Plants wither and die if not supplied with water to absorb. Plants need water for dissolving and transporting nutritive substances, and maintaining turgor excess pressure in plant cells. Use dyes to show the conduction of water to all parts of the plant.
1. Put a fresh daisy in each of two test-tubes. Fill one test-tube two thirds full with tap water and put no water in the other test-tube. The next day, the daisy standing in water has remained fresh, but the other one is limp and faded.
2. Put a daisy flower in a test-tube two thirds full of acid fuchsine solution. Within 15 minutes, the originally white petals of the daisy have turned a reddish colour. Also, you can also see the colour change in other parts of the plant.
3. To show that water is necessary for photosynthesis, Place a potted plant in a dark room for 48 hours, so that at the end of that time there is no starch present in the leaves. Then remove two leaves. Stand one leaf in water and place in the light, put the other leaf in the light also, but do not supply it with water. After about eight hours, test both leaves for starch.
6.5.3 Plants need salts, maize
1. Plants need nutritive salts from the soil to develop normally.
Prepare three glass containers using large beakers or buckets or small aquaria as follows:
1.1 Put good garden soil and deionized water in the glass container to 2 cm below the brim. Stir o contents of the vessel and allow to settle.
1.2 Fill a third glass container with aerated water and add two measures of calcium nitrate, one measure each of potassium nitrate, monobasic potassium phosphate and magnesium sulfate, and a trace of iron(II) sulfate.
1.3 Fill a glass container with deionized water to 2 cm below the rim.
2. Plant 50 maize grains in a large pot containing old sawdust. Water them regularly. When the young maize plants are 3 cm high, select 21 of equal size, pull them out of the sawdust and wash their roots under the tap. Punch seven holes in each of three cork discs. Insert seven maize plants in each of holes so that their roots hang down below the holes. Put a cork disc with attached plants in each of the glass containers. After two weeks note any difference in development of the maize plants in the different glass containers. The plants in the glass container 3. do not grow well, but the plants in glass containers 1. and 3. grow well. Compare the growth of plants in glass containers 1.1 and 1.3.
9.145 Light is necessary for photosynthesis
See diagram 9.145: Light is necessary for photosynthesis
Fix a band over a leaf. Fold "silver paper" from a chocolate packet, or aluminium cooking foil, in a band around a large leaf on a tree growing in the sunlight. After three days, remove the band and drop the leaf in boiling water to kill the cells. Put the leaf in methylated spirits to remove the chlorophyll. Do the iodine test for starch on the whole leaf. The part of the leaf not covered by the band paper turns a blue-black colour. The part of the leaf covered by the band does not turn blue-black because of lack of sunlight for photosynthesis to make starch.
9.146 Carbon dioxide in the air is necessary for photosynthesis
See diagram 9.146: Nasturtium
1. Show that plants need air for photosynthesis. Smear a band of petroleum jelly over both sides of leaf on a growing plant. After two days, remove the leaf from the plant and scrape off the petroleum jelly. Do the iodine test for starch on the whole leaf. See the pattern of the band where the petroleum jelly was on the leaf.
2. Show that plants need carbon dioxide for photosynthesis. Put a flowerpot containing a soft leaf plant, e.g. nasturtium, in the dark. The next day put the flowerpot next to a 20% potassium hydroxide solution in a container with a slotted cover. Bend a stem and insert one leaf into the upper section of the container. Seal the slot around the stem with petroleum jelly or adhesive tape. Potassium hydroxide absorbs carbon dioxide so the leaf sealed inside the container is in an atmosphere without carbon dioxide. Put the plant and container in sunlight. After three hours, detach the leaf inside the container and detach other leaves that had equal exposure to light. Drop the leaves in boiling water to kill the cells. Put the leaves in methylated spirit then remove them when they are almost colourless. Rinse the leaves in water then do the iodine test for starch. Compare the blue-black colour of the leaves.
9.152 Waterweed in the light and in the dark,  , Elodea
See diagram 9.152: Waterweed in the light and in the dark | See diagram 6.5.9
1. Tap water usually contains enough carbon dioxide to support photosynthesis for submerged plants. Put fresh green waterweed in a test-tube full of water. Place the apparatus in bright sunlight or under a 100 watt lamp. Note the small bubbles rising from the cut ends of the waterweed. Collect the gas in the test-tube by simple downward displacement of water. When the test-tube is full of gas, remove it and test the gas with a glowing splint. The gas is oxygen. Repeat the experiment by adding a 1% solution of sodium hydrogen carbonate (sodium bicarbonate) or potassium bicarbonate. The rate of bubbling increases. Put the waterweed in the dark for a few days. Bubbles of oxygen no longer rise from the surface of the leaves.
2. See diagram 9.1: Photosynthesis
The photosynthesis activity of leaves can be shown by putting water plants, Elodea in a funnel, inverting the funnel in a large beaker of water and putting a test-tube over the small end of the funnel. A fine piece of tubing or plastic electrical insulation is used in the manner of a drinking straw to remove the air from the test-tube, thus filling it with water. Several dabs of putty put between the funnel and the beaker will permit free circulation of the water from a beaker into the funnel. The water plants should not be in contact with a zinc container before putting in the apparatus. Test the gas that bubbles from the plant by collecting in the test-tube and putting in a glowing wood splint and watching for it to flame. Elodea has a hollow stem, so if you punch at the end with a pin it will release oxygen bubbles faster and in a stream that you can count for quantitative results.
3. See diagram 9.145.1 Water-weed in the light and in the dark
Start the demonstration at the beginning of the lesson and allow students to see it again in the next lesson. A suitable water-weed is Elodea. The demonstration works better if you add some sodium bicarbonate (baking soda) to the water. Tests for oxygen gas Light a thin piece of wood then blow out the flame leaving the wood glowing red. If you put this into oxygen, the glowing wood will burst into flame. Some green water-weed is placed inside a test-tube and turned upside down under water. The test-tube contains no air or bubbles in it. This is left in the sunlight. Another piece of water-weed is placed under a similar test-tube. This is left in the dark. After some hours the water-weeds are looked at again. The water-weed in the light has bubbles of oxygen gas coming from it. No bubbles of oxygen gas come from the water-weed in the dark. The gas was oxygen because it made a glowing piece of wood burst into flames.
What does the word "photosynthesis" mean? ["Putting together using light energy"] What are the four things green plants need to make food? [Sunlight Carbon dioxide gas Water Plant nutrients] What are two kinds of food made by green plants? [Simple sugars, Starch] Which gases do green leaves give out? [Oxygen gas] What are two ways in which plants help us? [They make food that you eat. They make oxygen gas that you breathe.] Write the photosynthesis equation: [light energy + carbon dioxide gas + water --> simple sugar + oxygen gas]
6.5.4 Chloroplasts
See diagram 1.1.3: Filamentous algae, Spirogyra
The green leaf pigment chlorophyll is not usually present in plant cells in solution, but in chloroplasts. To study the shape of the chloroplasts in various plant species, using tweezers, detach a few leaves from a stem of moss that has large leaves. Put them in a drop of water on a microscope slide. Mount a coverslip. Examine the slide under low power. Using tweezers, place a few cell filaments of spirogyra sp. in a drop of water on a slide. Mount a coverslip. Examine the slide under high power. Study cell filaments of a stellate algae, Zygnema. Use a pipette to put cells of a desmid, Closterium, on a microscope slide. Mount a coverslip. Examine the preparation under high power. Note the shapes of the chloroplasts and their position inside the cell. Study other species of plants including seed bearing plants, ferns, mosses, and algae to determine the shape of their chloroplasts.
6.5.5 Chlorophyll is necessary for photosynthesis, variegated leaf
See diagram 9.149: Variegated leaf | See diagram 9.149.1: Photosynthesis
Use a thin variegated leaf, that has green or other colour parts and white parts, e.g. Chinese lantern, Abutilon. The white parts contain no pigment necessary for photosynthesis. Leave the plant for days in strong sunlight, then pick a leaf and drop it in boiling water to kill the cells. Put the leaf in methylated spirits to remove the chlorophyll. Do the iodine test for starch on the whole leaf. The green part, or other coloured part, turns a blue-black colour. The white area part of the leaf does not turn blue-black.
6.5.6 Chloroplasts in cells of waterweeds and algae
See diagram 1.1: Filamentous algae, Spirogyra
Use a glass rod to transfer a drop of water from a beaker to a microscope slide. Using tweezers pluck a leaf from a shoot of water-weed, Elodea. Put the leaf in the drop of water. Mount a coverslip. Examine the specimen with 50 X magnification. The leaf is built up from individual cells as was the onion epidermis examined in 3.2.1. However, besides the parts seen in the onion cells the cells of the water-weed contain many green grains. Examine the slide under low power. Draw the shape of the green grains. Examine the slide under high power. Draw the shape of the green grains, called chloroplasts. They contain the green leaf pigment chlorophyll. Chlorophyll is usually found only in the chloroplasts. Note the large spiral chloroplast in the green algae Spirogyra.
6.5.7 Extract chlorophyll from green leaves, potato, onion, Tropaeolum, Fuchsia, hyacinth, lilac
16.3.2.0.1 Fehling's test for reducing sugars and aldehydes in solution, glucose and fructose | 16.7.8 Iodine test for starch
Heat a beaker of water with an electric heater. Do NOT use a Bunsen burner. Use thin leaves and kill them by putting in boiling water for a few minutes. Put half a large test-tube of ethyl alcohol in a beaker containing recently boiled water. Immerse the killed leaves in the alcohol. Note how the leaves gradually lose their colour because the alcohol dissolves the chlorophyll. To show that only green leaves make starch by photosynthesis, choose a leaf and treat it as above until it becomes whitish through loss of colour. Then use the iodine test for starch.
6.5.8 Separate chlorophyll pigments with paper chromatography
1. Cut dark green spinach leaves into small pieces then crush them with a spoon or a mortar and pestle or an electric mixing machine. Put the crushed leaves and juice in a beaker. Cover the crushed leaves and juice with acetone, nail polish remover. Let the pieces of leaves settle down to the bottom of the green liquid. Cut out a rectangle of paper towel or paper coffee filter or paper napkin. Put a pencil across the beaker. Hang the rectangle of paper over the pencil so that one end of the paper is in the green liquid. Leave the beaker to stand for several hours. Note the colours called pigments that have moved up the paper.
Study the pigments in order of separation from top to base:
1.1 Carotenoid pigment (yellow) and some decomposition products move with the solvent front
1.2 Carotenoid pigment (yellow)
1.3 Chlorophyll a (blue-green)
1.4 Chlorophyll b (yellow-green)
2. Dry dark green leaves, silver beet, in an oven at 70oC. Grind the leaves then add 80% acetone to extract the chlorophyll. Add the extract to a separating funnel containing petroleum ether. On the addition of water to dilute the acetone the pigments become less soluble in the dilute acetone and dissolve in the petroleum ether. A complete transfer of pigment occurs from acetone to petroleum ether. Keep the solution in the dark or it will decompose to a brown colour. Dip a strip of filter paper in the chlorophyll solution and let it run up 2 cm evenly. Pull out the filter paper and dry by waving in the air. Repeat two times with the same piece of filter paper. Open a specimen tube containing two organic solvents, petroleum ether and benzol. Fix the filter paper in the specimen tube so that it does not touch the sides then close the tube. Watch the chromatogram develop leaving the specimen tube closed. The solvent moves ahead of most of the pigments. The pigments move at different rates depending on their relative solubility in the solvent and on their relative absorption by the paper.
3. Study the pigments in order of separation from top to base:
3.1 Carotenoid pigment (yellow) and some decomposition products move with the solvent front
3.2 Carotenoid pigment (yellow)
3.3 Chlorophyll a (blue-green)
3.4 Chlorophyll b (yellow green)
6.5.9 Fluorescence of chlorophyll
1. Grind green leaves in acetone with a mortar and pestle then filter through a coarse filter then absorbent paper. The filtrate is an acetone solution of almost pure chlorophyll. Note the red fluorescence as seen by looking through the solution held against a dark background. Direct a bright beam of light at the solution and note the deep red glow. The fluorescent light emitted by chlorophyll is red light at a longer wavelength, lower energy, than the absorbed light. The chlorophyll electrons become excited by the light energy, but the acetone has dissolved the chloroplast membranes so the absorbed energy cannot be used for photosynthesis but instead the chlorophyll electrons lose their excited energy state as a reddish glow. In the normal situation sunlight energy is converted to a chemical form and used in photosynthesis, not emitted as fluorescent light. Chlorophyll strongly absorbs blue and red light. Leaves appear green because wavelengths of light from the red and blue regions of the visible spectrum are necessary to excite the chloroplast electrons, so the unused green light is reflected.
2. Make an extract of chlorophyll in 85% acetone from dried nettle leaf powder. Place some of the powder in a funnel fitted with a filter paper, slowly add the acetone and collect the filtrate in a beaker or test-tube. The filtrate is an acetone solution of almost pure chlorophyll. Note the red fluorescence as seen by looking through the solution held against a dark background. Chlorophyll strongly absorbs blue and red light. The fluorescent light emitted by chlorophyll is red light at a longer wavelength, lower energy, than the absorbed light. The energy converted to chemical form and used in photosynthesis is not emitted as fluorescent light.
6.5.11 Tests for starch
See diagram 2.0: Starch grain in potato cell | 16.7.8 Iodine test for starch
Be Careful! Heat alcohol with an electric heater or use a water bath. Do NOT use a Bunsen burner!
Sugar, the products of photosynthesis, and the large starch molecules formed from many sugar molecules are present in leaves. A simple starch test consists of applying a dilute iodine solution and watching for the typical blue-black colour that shows that starch is present. The iodine solution is prepared by dissolving 10 g of potassium iodine in 100 mL deionized water and adding 5 g of iodine. Tubers, potatoes or a starch paste may be used to show the colour change. When testing leaves softening the leaf cells by boiling in water for a few minutes is necessary. Then the leaf is put in boiling alcohol until the pigments that will mask the reactions are removed from the leaf. Chlorophyll is usually removed in 5-8 minutes but fleshy leaves may take longer or require a change of alcohol for adequate removal of pigments. The iodine solution should react with the starch within 15 minutes.
6.5.12 Tests for sugars
16.3.2.0.1 Fehling's test for reducing sugars and aldehydes in solution, glucose and fructose
Maize, sugar beet and sprouting onion bulbs are suitable for sugar tests because they contain stored simple sugar rather than the large starch molecule. Cut pieces 2 cm long and put in 2 mL sugar test solution in a Pyrex test-tube and boil the mixture. Make the sugar test solution from 173 g of sodium citrate, 200 g of crystalline sodium carbonate, and 17.3 g of crystalline copper (II) sulfate. Dissolve the carbonate and citrate in 100 mL water. These substances will dissolve faster if the water is warmed. Dissolve the copper (II) sulfate in 100 mL water and slowly pour this solution into the carbonate citrate solution. Cool and add water to make 1 litre of test solution. Show the colour change by dissolving a little cane sugar in 10 mL water in a test-tube. Add saliva that will change the cane sugar (sucrose) into a simple sugar (glucose). Add 3 mL of the test solution and boil over a heat source. A yellowish or reddish precipitate forms when simple sugar is present.
6.5.13 Bromothymol blue test for carbon dioxide
Bromothymol blue solution is used to show the presence of carbon dioxide. Fill four test-tubes three quarters full of water. Add 25 drops of bromothymol blue to each tube. Put a sprig of Elodea or other small water plant in two of the tubes. With a drinking straw, blow bubbles into one tube not containing a plant, and then into one with a plant. Note the colour change that shows the presence of carbon dioxide. Put stoppers in the four test-tubes and note the changes within 15 minutes to an hour. Repeat the experiment, but put the tubes in a dark place, a closed desk.
6.5.14 Release of oxygen during photosynthesis
See diagram 9.146.1
In photosynthesis carbohydrates are formed from carbon dioxide and water in the presence of chlorophyll and with the aid of light energy, i.e. organic substances are formed from inorganic ones. During this process water is decomposed into hydrogen and oxygen. The hydrogen is bound immediately reducing the carbon source and oxygen is released. This oxygen replaces that used for respiration by living things. Thus, photosynthesis plays an important part in the recycling of natural products.
1. To detect the release of oxygen during photosynthesis, fill a glass container with water to 2 cm below the top, drop in 8 well developed shoots of a water-weed, e.g. Elodea, and add a little mineral water to enrich the carbon dioxide content. Put an inverted bell jar with its tap open over the water-weed and push it down into the vessel until water reaches up to the tap. Then close the tap, place the slotted lid on the glass container and suspend the inverted bell jar from the lid by its tap, cushioned on a piece of cotton wool. Put the vessel in as bright a place as possible so that it is exposed, at least for some time, to direct sunlight. If this is not possible, expose the vessel to electric light (from a microscope lamp) for several hours. What can you see in the bell jar on the following day? Hold a glowing wood splint immediately over the outlet of the inverted bell jar and open the tap. What happens? What gas has been detected by this method?
6.5.15 Measure the rate of photosynthesis, Elodea
See diagram 9.157
Put pond weed into the barrel of a 50 cm3 syringe. Fill the syringe with a 5% sodium hydrogen carbonate solution. Insert plunger into the barrel of the syringe. Avoid trapping any air inside the barrel. Connect the syringe to a graduated 1 cm3 pipette with a short length of rubber tubing. Expel any air trapped inside the syringe or pipette turning the apparatus with the open end of the pipette pointing upwards and slowly pushing the plunger into the barrel. Fix the apparatus in a vertical position with clamp and stand. Adjust the position of the plunger until the liquid level lies in the upper region of the 1 cm3 pipette. Oxygen produced by the plant collects above the sodium hydrogen carbonate solution, increasing gas pressure to push the meniscus reading level down the pipette. Record the volume of oxygen released every 10 minutes. When the meniscus reaches the lower end of the pipette, it can be moved up again by adjusting the position of the plunger.
6.5.16 Measure the effects of factors on photosynthesis - light intensity, sodium hydrogen carbonate conc., light quality, light intensity
1. Vary the light intensity by changing the distance of the light source from the apparatus, projector light
2. Vary the concentration of sodium hydrogen carbonate solution from 0 to 5% w / v.
3. Vary the light quality by covering the syringe with cellophane of different colours. Include a control identical to the experiment but without a plant.
4. To note the effect of light intensity on photosynthesis of pond Selaginella, use an overhead projector was used to provide a strong and uniform light source, placed at 1. 00 in, 0.90 m, 0.75 in, 0.60 and 0.50 m from the plant. A 5% sodium hydrogen carbonate solution was used to provide an abundant supply of carbon dioxide. The volume of oxygen released was measured at 10 minute intervals, from which the rate of photosynthesis was calculated. For each light intensity, three measurements were taken and the mean value was used for plotting the graph.
6.5.17 Plants growing in the dark
Compare the growth of the bean plants growing in sunlight and growing in the dark. The plants in the dark have a pale yellow colour due to absence of chlorophyll, long thin stems because of abnormal lengthening of the internodes, and small leaves. These plants are describes as being etiolated. Plants ultimately die if kept too long in the dark.
6.5.18 Plants need salts, hydroponics, Knop's solution
1. Ten elements are essential for the growth of a green plant. Carbon 3., Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg). and Iron (Fe). Plants take in carbon dioxide from the air and hydrogen and oxygen from the water in the soil. Plants absorb other elements with the soil water as salts.
2. Make Knop's solution with 0.8 g of calcium nitrate, 0.2 g of magnesium sulfate, 0.2 g of acid potassium phosphate, 0.2 g of potassium nitrate, 3 drops of ferric chloride solution dissolved in one litre of demineralized water.
3. Make the following variations of Knop's solution:
3.1. Knop's solution omitting nitrogen: calcium sulfate instead of calcium nitrate, potassium sulfate instead of potassium nitrate
3.2. Knop's solution omitting phosphorus: omit potassium phosphate
3.3. Knop's solution omitting potassium, sodium phosphate instead of potassium phosphate, sodium nitrate instead of potassium nitrate
3.4. Knop's solution omitting calcium: sodium nitrate instead of calcium nitrate
3.5. Knop's solution omitting magnesium, sodium sulfate instead of magnesium sulfate
3.6. Knop's solution omitting iron: omit the ferric chloride
3.7. Knop's solution thus omitting sulfur: magnesium nitrate instead of magnesium sulfate
3.8. Knop's solution: control
4. Use 1 litre containers fitted with waxed corks bored with holes to take the plants. Fix seedlings into the corks with cotton wool, e.g. barley, wheat, broad bean. Wrap the glass jars with black paper to exclude the light. Label the jars and record the growth and appearance of the plants.
5. Prepare three containers to hold the following solutions:
5.1. aerated deionized water,
5.2. good garden soil and aerated deionized water,
5.3. aerated deionized water and two measures of calcium nitrate, one measure each of potassium nitrate, monobasic potassium phosphate and magnesium sulfate, and a trace of iron(II) sulfate.
6. Plant 50 maize grains in a large pot containing old sawdust. When the young maize plants are 3 cm high, select 21 plants of equal size and wash their roots. Punch seven holes in each of three cork discs. Insert seven maize plants in each of holes so that their roots hang down below the holes. Put a cork disc with attached plants in each of the containers. After two weeks note any difference in development of the maize plants. The plants in 5.1 do not grow well, but the plants in 5.2 and 5.3 grow well. Compare the growth of plants in 5.2 and 5.3.