V. PLANT GROWTH AND NUTRITION. CAUSES OF GROWTH
Problem.—What causes a young plant to grow?
(a) The relation of the young plant to its food supply.
(b) The outside conditions necessary for germination.
(c) What the young plant does with its food supply.
(d) How a plant or animal is able to use its food supply.
(e) How a plant or animal prepares food to use in various parts of the body.
Laboratory Suggestions
Laboratory exercise.—Examination of bean in pod. Examination and identification of parts of bean seed.
Laboratory demonstration.—Tests for the nutrients: starch, fats or oils, protein.
Laboratory demonstration.—Proof that such foods exist in bean.
Home work.—Test of various common foods for nutrients. Tabulate results.
Extra home work by selected pupils.—Factors necessary for germination of bean. Demonstration of experiments to class.
Demonstration.—Oxidation of candle in closed jar. Test with lime water for products of oxidation.
Demonstration.—Proof that materials are oxidized within the human body.
Demonstration.—Oxidation takes place in growing seeds. Test for oxidation products. Oxygen necessary for germination.
Laboratory exercise.—Examination of corn on cob, the corn grain, longitudinal sections of corn grain stained with iodine to show that embryo is distinct from food supply.
Demonstration.—Test for grape sugar.
Demonstration.—Grape sugar present in growing corn grain.
Demonstration.—The action of diastase on starch. Conditions necessary for action of diastase.
What makes a Seed Grow.—The general problem of the pages that follow will be to explain how the baby plant, or embryo, formed in the seed as the result of the fertilization of the egg cell, is able to grow into an adult plant. Two sets of factors are necessary for its growth: first, the presence of food to give the young plant a start; second, certain stimulating factors outside the young plant, such as water and heat.
Three views of a kidney bean, the lower one having one cotyledon removed to show the hypocotyl and plumule.
If we open a bean pod, we find the seeds lying along one edge of the pod, each attached by a little stalk to the inner wall of the ovary. If we pull a single bean from its attachment, we find that the stalk leaves a scar on the coat of the bean; this scar is called the hilum. The tiny hole near the hilum is called the micropyle. Turn back to the figure (page [54]) showing the ovule in the ovary. Find there the little hole through which the pollen tube reached the embryo sac. This hole is identical with the micropyle in the seed. The thick outer coat (the testa) is easily removed from a soaked bean, the delicate coat under it easily escaping notice. The seed separates into two parts; these are called the cotyledons. If you pull apart the cotyledons very carefully, you find certain other structures between them. The rodlike part is called the hypocotyl (meaning under the cotyledons). This will later form the root (and part of the stem) of the young bean plant. The first true leaves, very tiny structures, are folded together between the cotyledons. That part of the plant above the cotyledons is known as the plumule or epicotyl (meaning above the cotyledons). All the parts of the seed within the seed coats together form the embryo or young plant. A bean seed contains, then, a tiny plant protected by a tough coat.
Food in the Cotyledons.—The problem now before us is to find out how the embryo of the bean is adapted to grow into an adult plant. Up to this stage of its existence it has had the advantage of food and protection from the parent plant. Now it must begin the battle of life alone. We shall find in all our work with plants and animals that the problem of food supply is always the most important problem to be solved by the growing organism. Let us see if the embryo is able to get a start in life (which many animals get in the egg) from food provided for it within its own body.
Organic Nutrients.—Organic foods (those which come from living sources) are made up of two kinds of substances, the nutrients or food substances and wastes or refuse. An egg, for example, contains the white and the yolk, composed of nutrients, and the shell, which is waste. The organic nutrients are classed in three groups.
Starch grains in the cells of a potato tuber.
Carbohydrates, foods which contain carbon, hydrogen, and oxygen in a certain fixed proportion (C6H10O5 is an example). They are the simplest of these very complex chemical compounds we call organic nutrients. Starch and sugar are common examples of carbohydrates.
Fats and Oils.—These foods are also composed of carbon, hydrogen, and oxygen in a proportion which enables them to unite readily with oxygen.
Proteins.—A third group of organic foods, proteins, are the most complex of all in their composition, and have, besides carbon, oxygen, and hydrogen, the element nitrogen and minute quantities of other elements.
Test for Starch.—If we boil water with a piece of laundry starch in a test tube, then cool it and add to the mixture two or three drops of iodine solution,[3] we find that the mixture in the test tube turns purple or deep blue. It has been discovered by experiment that starch, and no other known substance, will be turned purple or dark blue by iodine. Therefore, iodine solution has come to be used as a test for the presence of starch.
Test for Starch.
Starch in the Bean.—If we mash up a little piece of a bean cotyledon which has been previously soaked in water, and test for starch with iodine solution, the characteristic blue-black color appears, showing the presence of the starch. If a little of the stained material is mounted in water on a glass slide under the compound microscope, you will find that the starch is in the form of little ovoid bodies called starch grains. The starch grains and other food products are made use of by the growing plant.
Test for protein.
Test for Oils.—If the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper.
Protein in the Bean.—Another nutrient present in the bean cotyledon is protein. Several tests are used to detect the presence of this nutrient. The following is one of the best known:—
Place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. Pour over it a little strong (60 per cent) nitric acid and heat gently. Note the color that appears—a lemon yellow. If the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present.
If the protein is in a liquid state, its presence may be proved by heating, for when it coagulates or thickens, as does the white of an egg when boiled, protein in the form of an albumin is present.
Another characteristic protein test easily made at home is burning the substance. If it burns with the odor of burning feathers or leather, then protein forms part of its composition.[4]
A test of the cotyledon of a bean for protein food with nitric acid and ammonium hydrate shows us the presence of this food. Beans are found by actual test to contain about 23 per cent of protein, 59 per cent of carbohydrates, and about 2 per cent oils. The young plant within a pea or bean is thus shown to be well supplied with nourishment until it is able to take care of itself. In this respect it is somewhat like a young animal within the egg, a bird or fish, for example.
Beans and Peas as Food for Man.—So much food is stored in legumes (as beans and peas) that man has come to consider them a very valuable and cheap source of food. Study carefully the following table:—
| Nutrients Furnished for Ten Centsin Beans and Peas at Certain Prices per Pound | |||||
|---|---|---|---|---|---|
| Ten Cents will payfor— | |||||
| Food Materials as Purchased | Prices per Pound | Total Food Material | Protein[TN1] | Fat | Carbo- hydrates |
| Cents | Pounds | Pounds | Pounds | Pounds | |
| Kidney beans, dried | 5 | 2.00 | 0.45 | 0.04 | 1.19 |
| Lima beans, fresh, shelled | 8 | 1.25 | .04 | — | .12 |
| Lima beans, dried | 6 | 1.67 | .30 | .03 | 1.10 |
| String beans, fresh, 30 cents per peck | 3 | 3.33 | .07 | .01 | .23 |
| Beans, baked, canned | 5 | 2.00 | .14 | .05 | .39 |
| Lentils, dried | 10 | 1.00 | .26 | .01 | .59 |
| Peas, green, in pod, 30 cents per peck | 3 | 3.33 | .12 | .01 | .33 |
| Peas, dried | 4 | 2.50 | .62 | .03 | 1.55 |
A series of early stages in the germination of the kidney bean.
Germination of the Bean.—If dry seeds are planted in sawdust or earth, they will not grow. A moderate supply of water must be given to them. If seeds were to be kept in a freezing temperature or at a very high temperature, no growth would take place. A moderate temperature and a moderate water supply are most favorable for their development.
Bean seedlings. The older seedlings at the left have used up all of the food supply in the cotyledons.
If some beans were planted so that we might make a record of their growth, we would find the first signs of germination to be the breaking of the testa and the pushing outward of the hypocotyl to form the first root. A little later the hypocotyl begins to curve downward. A later stage shows the hypocotyl lifting the cotyledon upward. In consequence the hypocotyl forms an arch, dragging after it the bulky cotyledons. The stem, as soon as it is released from the ground, straightens out. From between the cotyledons the budlike plumule or epicotyl grows upward, forming the first true leaves and all of the stem above the cotyledons. As growth continues, we notice that the cotyledons become smaller and smaller, until their food contents are completely absorbed into the young plant. The young plant is now able to care for itself and may be said to have passed through the stages of germination.
What makes an Engine Go.—If we examine the sawdust or soil in which the seeds are growing, we find it forced up by the growing seed. Evidently work was done; in other words, energy was released by the seeds. A familiar example of release of energy is seen in an engine. Coal is placed in the firebox and lighted, the lower door of the furnace is then opened so as to make a draft of air which will reach the coal. You know the result. The coal burns, heat is given off, causing the water in the boiler to make steam, the engine wheels to turn, and work to be done. Let us see what happens from the chemical standpoint.
The limewater test. The tube at the right shows the effect of the carbon dioxide.
Coal, Organic Matter.—Coal is made largely from dead plants, long since pressed into its present hard form. It contains a large amount of a chemical element called carbon, the presence of which is characteristic of all organic material.
Oxidation, its Results.—When things containing carbon are lighted, they burn. If we place a lighted candle which contains carbon in a closed glass jar, the candle soon goes out. If we then carefully test the air in the jar with a substance known as limewater,[5] the latter, when shaken up with the air in the jar, turns milky. This test proves the presence in the jar of a gas, known as carbon dioxide. This gas is formed by the carbon of the candle uniting with the oxygen in the air. When the oxygen of the air in the jar was used up, the flame went out, showing that oxygen is necessary to make a thing burn. This uniting of oxygen with some other substance is called oxidation.
Diagram to show that when a piece of wood is burned it forms water and carbon dioxide.
Oxidation possible without a Flame.—But a flame is not necessary for oxidation. Iron, if left in a damp place, becomes rusty. A union between the oxygen in the water or air and the iron makes what is known as iron oxide or rust. This is an example of slow oxidation.
Oxidation in our Bodies.—If we expel the air from our lungs through a tube into a bottle of limewater, we notice the limewater becomes milky. Evidently carbon dioxide is formed in our own bodies and oxidation takes place there. Is it fair to believe that the heat of our body (for example, 98.6° Fahrenheit under the tongue) is due to oxidation within the body, and that the work we do results from this chemical process. If so, what is oxidized?
Energy comes from Foods.—From the foregoing experiment it is evident that food is oxidized within the human body to release energy for our daily work. Is it not logical to suppose that all living things, both plant and animal, release energy as the result of oxidation of foods within their cells? Let us see if this is true in the case of the pea.
Food oxidized in Germinating Seeds.—If we take equal numbers of soaked peas, placed in two bottles, one tightly stoppered, the other having no stopper, both bottles being exposed to identical conditions of light, temperature, and moisture, we find that the seeds in both bottles start to germinate, but that those in the closed bottle soon stop, while those in the open jar continue to grow almost as well as similar seeds placed in an open dish would.
Experiment that shows the necessity for air in germination.
Why did not the seeds in the covered jar germinate? To answer this question, let us carefully remove the stopper from the stoppered jar and insert a lighted candle. The candle goes out at once. The surer test of limewater shows the presence of carbon dioxide in the jar. The carbon of the foodstuffs of the pea united with the oxygen of the air, forming carbon dioxide. Growth stopped as soon as the oxygen was exhausted. The presence of carbon dioxide in the jar is an indication that a very important process which we associate with animals rather than plants, that of respiration, is taking place. The seed, in order to release the energy locked up in its food supply, must have oxygen, so that the oxidation of the food may take place. Hence a constant supply of fresh air is an important factor in germination. It is important that air should penetrate between the grains of soil around a seed. The frequent stirring of the soil enables the air to reach the seed. Air also acts upon some materials in the soil and puts them in a form that the germinating seed can use. This necessity for oxygen shows us at least one reason why the farmer plows and harrows a field and one important use of the earthworm. Explain.
A grain of corn cut lengthwise. C, cotyledon; E, endosperm; H, hypocotyl; P, plumule.
Structure of a Grain of Corn.—Examination of a well-soaked grain of corn discloses a difference in the two flat sides of the grain. A light-colored area found on one surface marks the position of the embryo; the rest of the grain contains the food supply. The interesting thing to remember here is that the food supply is outside of the embryo.
A grain cut lengthwise perpendicular to the flat side and then dipped in weak iodine shows two distinct parts, an area containing considerable starch, the endosperm, and the embryo or young plant. Careful inspection shows the hypocotyl and plumule (the latter pointing toward the free end of the grain) and a part surrounding them, the single cotyledon (see Figure). Here again we have an example of a fitting for future needs, for in this fruit the one seed has at hand all the food material necessary for rapid growth, although the food is here outside the embryo.
Longitudinal section of young ear of corn. O, the fruits; S, the stigmas; SH, the sheath-like leaves; ST, the flower stalk. (After Sargent.)
Endosperm the Food Supply of Corn.—We find that the one cotyledon of the corn grain does not serve the same purpose to the young plant as do the two cotyledons of the bean. Although we find a little starch in the corn cotyledon, still it is evident from our tests that the endosperm is the chief source of food supply. The study of a thin section of the corn grain under the compound microscope shows us that the starch grains in the endosperm are large and regular in size. When the grain has begun to grow, examination shows that the starch grains near the edge of the cotyledon are much smaller and quite irregular, having large holes in them. We know that the germinating grain has a much sweeter taste than that which is not growing. This is noticed in sprouting barley or malt. We shall later find that, in order to make use of starchy food, a plant or animal must in some manner change it over to sugar. This change is necessary, because starch will not dissolve in water, while sugar will; in this form substances can pass from cell to cell in the plant and thus distribute the food where it is needed.
Test for grape sugar.
A Test for Grape Sugar.—Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. Add to the fluid twice its bulk of Fehling's solution,[6] which has been previously prepared. Heat the mixture, which should now have a blue color, in the test tube. If grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. Smaller amounts will show less decided red. No other substance than sugar will give this reaction. If Benedict's test[7] is used, a colored precipitate will appear in the test tube after boiling.
Starch changed to Grape Sugar in the Corn.—That starch is being changed to grape sugar in the germinating corn grain can easily be shown if we cut lengthwise through the embryos of half a dozen grains of corn that have just begun to germinate, place them in a test tube with some Fehling's solution, and heat almost to the boiling point. They will be found to give a reaction showing the presence of sugar along the edge of the cotyledon and between it and the endosperm.
Digestion.—This change of starch to grape sugar in the corn is a process of digestion. If you chew a bit of unsweetened cracker in the mouth for a little time, it will begin to taste sweet, and if the chewed cracker, which we know contains starch, is tested with Fehling's solution, some of the starch will be found to have changed to grape sugar. Here, again, a process of digestion has taken place. In both the corn and in the mouth, the change is brought about by the action of peculiar substances known as digestive ferments, or enzymes. Such substances have the power under certain conditions to change insoluble foods—solids—into soluble substances—liquids. The result is that substances which before digestion would not dissolve in water now will dissolve.
A germinating corn grain. C, cotyledon; H, growing root (hypocotyl); P, growing stem (plumule); S, endosperm; d.s., digested starch; p.r., primary root; s.r., secondary root; r.h., root hairs.
The Action of Diastase on Starch.—The enzyme found in the cotyledon of the corn, which changes starch to grape sugar, is called diastase. It may be separated from the cotyledon and used in the form of a powder.
To a little starch in half a cup of water we add a very little (1 gram) of diastase and put the vessel containing the mixture in a warm place, where the temperature will remain nearly constant at about 98° Fahrenheit. On testing part of the contents at the end of half an hour, and the remainder the next morning, for starch and for grape sugar, we find from the morning test that the starch has been almost completely changed to grape sugar. Starch and warm water alone under similar conditions will not react to the test for grape sugar.
Digestion has the Same Purpose in Plants and Animals.—In our own bodies we know that solid foods taken into the mouth are broken up by the teeth and moistened by saliva. If we could follow that food, we would find that eventually it became part of the blood. It was made soluble by digestion, and in a liquid form was able to reach the blood. Once a part of the body, the food is used either to release energy or to build up the body.
Summary.—We have seen: 1. That seeds, in order to grow, must possess a food supply either in or around their bodies.
2. That this food supply must be oxidized before energy is released.
3. That in cases where the food is not stored at the point where it is to be oxidized the food must be digested so that it may be transported from one part to another in the same plant.
The life processes of plants and animals, so far, may be considered as alike; they both feed, breathe (oxidize their food), do work, and grow.
[3] Iodine solution is made by simply adding a few crystals of the element iodine to 95 per cent alcohol; or, better, take by weight 1 gram of iodine crystals, 2/3 gram of iodide of potassium, and dilute to a dark brown color in weak alcohol (35 per cent) or distilled water.
[4] Other tests somewhat more reliable, but much more delicate, are the biuret test and test with Millon's reagent.
[5] Limewater can be made by shaking up a piece of quicklime the size of your fist in about two quarts of water. Filter or strain the limewater into bottles and it is ready for use.
[6] and [7] Directions for making these solutions will be found in Hunter's Laboratory Problems in Civic Biology.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Andrews, A Practical Course in Botany, pages 1-21. American Book Company.
Atkinson, First Studies of Plant Life, Chap. XXX. Ginn and Company.
Bailey, Botany, Chaps. XX, XXX. The Macmillan Company.
Beal, Seed Dispersal. Ginn and Company.
Bergen and Davis, Principles of Botany, Chaps. XX, XXX. Ginn and Company.
Coulter, Plant Life and Plant Uses. American Book Company.
Dana, Plants and their Children. American Book Company.
Mayne and Hatch, High School Agriculture. American Book Company.
Lubbock, Flowers, Fruits, and Leaves. The Macmillan Company.
Newell, Reader in Botany, pages 24-49. Ginn and Company.
Sharpe, A Laboratory Manual in Biology, pages 55-65. American Book Company.
advanced
Bailey, The Evolution of our Native Fruits. The Macmillan Company.
Bailey, Plant Breeding. The Macmillan Company.
Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Company.
De Candolle, Origin of Cultivated Plants. D. Appleton and Company.
Duggar, Plant Physiology. The Macmillan Company.
Farmers' Bulletins, Nos. 78, 86, 225, 344. U. S. Department of Agriculture.
Hodge, Nature Study and Life, Chaps. X, XX. Ginn and Company.
Kerner (translated by Oliver), Natural History of Plants. Henry Holt and Company, 4 vols. Vol. II, Part 2.
Sargent, Corn Plants. Houghton, Mifflin, and Company.