Chapter III: Part 3

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 deg. 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.

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.

VI. THE ORGANS OF NUTRITION IN PLANTS--THE SOIL AND ITS RELATION TO THE ROOTS

_Problem.--What a plant takes from the soil and how it gets it._
_(a) What determines the direction of growth of roots?_
_(b) How is the root built?_
_(d) What is in the soil that a root might take out?_
_(e) Why is nitrogen necessary, and how is it obtained?_

LABORATORY SUGGESTIONS

_Demonstration_.--Roots of bean or pea.
_Demonstration or home experiment_.--Response of root to
gravity and to water. What part of root is most responsive?
_Laboratory work_.--Root hairs, radish or corn, position on
root, gross structure only. Drawing.
_Demonstration._--Root hair under compound microscope.
_Demonstration._--Apparatus illustrating osmosis.
_Demonstration or a home experiment._--Organic matter
present in soil.
_Demonstration._--Root tubercles of legume.
_Demonstration._--Nutrients present in some roots.

Uses of the Root.--If one of the seedlings of the bean spoken of in the last chapter is allowed to grow in sawdust and is given light, air, and water, sooner or later it will die. Soil is part of its natural environment, and the roots which come in contact with the soil are very important. It is the purpose of this chapter to find out just how the young plant is fitted to get what it needs from this part of its environment; namely, the soil.

The development of a bean seedling has shown us that the root grows first. _One of the most important functions of the root to a young seed plant is that of a holdfast, an anchor to fasten it in the place where it is to develop._ It has many other uses, as the taking in of water with the mineral and organic matter dissolved therein, the storage of food, climbing, etc. All functions other than the first one stated arise after the young plant has begun to develop.

Root System.--If you dig up a young bean seedling and carefully wash the dirt from the roots, you will see that a long root is developed as a continuation of the hypocotyl. This root is called the _primary_ root. Other smaller roots which grow from the primary root are called _secondary_, or _tertiary_, depending on their relation to the first root developed.

Downward Growth of Root. Influence of Gravity.--Most of the roots examined take a more or less downward direction. We are all familiar with the fact that the force we call gravity influences life upon this earth to a great degree. Does gravity act on the growing root? This question may be answered by a simple experiment.

Plant mustard or radish seeds in a pocket garden, place it on one edge and allow the seeds to germinate until the root has grown to a length of about half an inch. Then turn it at right angles to the first position and allow it to remain for one day undisturbed. The roots now will be found to have turned in response to the change in position, that part of the root near the growing point being the most sensitive to the change. This experiment seems to indicate that the roots are influenced to grow downward by the force of gravity.

Experiments to determine the Influence of Moisture on a Growing Root.--The objection might well be interposed that possibly the roots in the pocket garden[8] grew downward after water. That moisture has an influence on the growing root is easily proved.

Footnote 8: _The Pocket Garden._--A very convenient form of
pocket germinator may be made as follows. Obtain two cleaned
four by five negatives (window glass will do); place one
flat on the table and place on this half a dozen pieces of
colored blotting paper cut to a size a little less than the
glass. Now cut four thin strips of wood to fit on the glass
just outside of the paper. Next moisten the blotter, place
on it some well-soaked radish, mustard seeds or barley
grains, and cover with the other glass. The whole box thus
made should be bound together with bicycle tape. Seeds will
germinate in this box and with care may live for two weeks
or more.

Plant bird seed, mustard or radish seed in the underside of a sponge, which should be kept wet, and may be suspended by a string under a bell jar in the schoolroom window. Note whether the roots leave the sponge to grow downward, or if the moisture in the sponge is sufficient to counterbalance the force of gravity.

Water a Factor which determines the Course taken by Roots.--_Water, as well as the force of gravity, has much to do with the direction taken by roots._ Water is always found below the surface of the ground, but sometimes at a great depth. Most trees, and all grasses, have a greater area of surface exposed by the roots than by the branches. The roots of alfalfa, a cloverlike plant used for hay in the Western states, often penetrate the soil after water for a distance of ten to twenty feet below the surface of the ground.

Fine Structure of a Root.[9]--When we examine a delicate root in thin longitudinal section under the compound microscope, we find the entire root to be made up of cells, the walls of which are uniformly rather thin. Over the lower end of the root is found a collection of cells, most of which are dead, loosely arranged so as to form a cap over the growing tip. This is evidently an adaptation which protects the young and actively growing cells just under the root cap. In the body of the root a central cylinder can easily be distinguished from the surrounding cells. In a longitudinal section a series of tubelike structures may be found within the central cylinder. These structures are cells which have grown together at the small end, the long axis of the cells running the length of the main root. In their development the cells mentioned have grown together in such a manner as to lose their small ends, and now form continuous hollow tubes with rather strong walls. Other cells have come to develop greatly thickened walls; these cells give mechanical support to the tubelike cells. Collections of such tubes and supporting woody cells together make up what are known as _fibrovascular bundles_.

Footnote 9: Sections of tradescantia roots are excellent for
demonstration of these structures.

Root Hairs.--Careful examination of the root of one of the seedlings of mustard, radish, or barley grown in the pocket germinator shows a covering of tiny fuzzy structures. These structures are very minute, at most 3 to 4 millimeters in length. They vary in length according to their position on the root, the most and the longest root hairs being found near the point marked _R. H._ in the figure. These structures are outgrowths of the outer layer of the root (the _epidermis_), and are of very great importance to the living plant.

Structure of a Root Hair.--A single root hair examined under a compound microscope will be found to be a long, round structure, almost colorless in appearance. The wall, which is very flexible and thin, is made up of cellulose, a substance somewhat like wood in chemical composition, through which fluids may easily pass. Clinging close to the cell wall is the protoplasm of the cell. The interior of the root hair is more or less filled with a fluid called _cell sap_. Forming a part of the living protoplasm of the root hair, sometimes in the hairlike prolongation and sometimes in that part of the cell which forms the epidermis, is found a _nucleus_. The protoplasm and nucleus are alive; the cell wall formed by the living matter in the cell is dead. _The root hair is a living plant cell_ with a wall so delicate that water and mineral substances from the soil can pass through it into the interior of the root.

How the Root absorbs Water.--The process by which the root hair takes up soil water can better be understood if we make an artificial root hair large enough to be easily seen. An egg with part of the outer shell removed so as to expose the soft skinlike membrane underneath is an example. Better, an artificial root hair may be _made_ in the following way. Pour some soft celloidin into a test tube; carefully revolve the test tube so that an even film of celloidin dries on the inside. This membrane is removed, filled with white of egg, and tied over the end of a rubber cork in which a glass tube has previously been inserted. When placed in water, it gives a very accurate picture of the root hair at work. After a short time water begins to rise in the tube, having passed through the film of celloidin. If grape sugar, salt, or some other substance which will dissolve in water were placed in the water outside the artificial root hair, it could soon be proved by test to pass through the wall and into the liquid inside.

Osmosis.--To explain this process we must remember that gases and liquids of different densities, when separated by a membrane, tend to flow toward each other and mingle, the greater flow always being in the direction of the denser medium. _The process by which two gases or fluids, separated by a membrane, tend to pass through the membrane and mingle with each other, is called osmosis._ The method by which the root hairs take up soil water is exactly the same process. It is by osmosis. The white of the egg is the best possible substitute for living matter; the celloidin membrane separating the egg from the water is much like the delicate membrane-like wall which separates the protoplasm of the root hair from the water in the soil surrounding it. The fluid in the root hair is denser than the soil water; hence the greater flow is toward the interior of the root hair.[10]

Footnote 10: For an excellent elementary discussion of
osmosis see Moore, _Physiology of Man and Other Animals_.
Henry Holt and Company.

Passage of Soil Water within the Root.--We have already seen that in an exchange of fluids by osmosis the greater flow is always toward the denser fluid. Thus it is that the root hairs take in more fluid than they give up. The cell sap, which partly fills the interior of the root hair, is a fluid of greater density than the water outside in the soil. When the root hairs become filled with water, the density of the cell sap is lessened, and the cells of the epidermis are thus in a position to pass along their supply of water to the cells next to them and nearer to the center of the root. These cells, in turn, become less dense than their inside neighbors, and so the transfer of water goes on until the water at last reaches the central cylinder. Here it is passed over to the tubes of the woody bundles and started up the stem. The pressure created by this process of osmosis is sufficient to send water up the stem to a distance, in some plants, of 25 to 30 feet. Cases are on record of water having been raised in the birch a distance of 85 feet.

Physiological Importance of Osmosis.--It is not an exaggeration to say that osmosis is a process not only of great importance to a plant, but to an animal as well. Foods are digested in the food tube of an animal; that is, they are changed into a soluble form so that they may pass through the walls of the food tube and become part of the blood. The inner lining of part of the food tube is thrown into millions of little fingerlike projections which look somewhat, in size at least, like root hairs. These fingerlike processes are (unlike a root hair) made up of many cells. But they serve the same purpose as the root hairs, for they absorb liquid food into the blood. This process of absorption is largely by osmosis. Without the process of osmosis we should be unable to use much of the food we eat.

Composition of Soil.--If we examine a mass of ordinary loam carefully, we find that it is composed of numerous particles of varying size and weight. Between these particles, if the soil is not caked and hard packed, we can find tiny spaces. In well-tilled soil these spaces are constantly being formed and enlarged. They allow air and water to penetrate the soil. If we examine soil under the microscope, we find considerable water clinging to the soil particles and forming a delicate film around each particle. In this manner most of the water is held in the soil.

How Water is held in Soil.--To understand what comes in with the soil water, it will be necessary to find out a little more about soil. Scientists who have made the subject of the composition of the earth a study, tell us that once upon a time at least a part of the earth was molten. Later, it cooled into solid rock. Soil making began when the ice and frost, working alternately with the heat, chipped off pieces of rock. These pieces in time became ground into fragments by action of ice, glaciers, running water, or the atmosphere. This process is called weathering. Weathering is aided by oxidation. A glance at almost any crumbling stones will convince you of this, because of the yellow oxide of iron (rust) disclosed. So by slow degrees this earth became covered with a coating of what we call inorganic soil. Later, generation after generation of tiny plants and animals which lived in the soil died, and their remains formed the first organic materials of the soil.

You are all familiar with the difference between the so-called rich soil and poor soil. The dark soil contains more dead plant and animal matter, which forms the portion called _humus_.

Humus contains Organic Matter.--It is an easy matter to prove that black soil contains organic matter, for if an equal weight of carefully dried humus and soil from a sandy road is heated red-hot for some time and then reweighed, the humus will be found to have lost considerably in weight, and the sandy soil to have lost very little. The material left after heating is inorganic material, the organic matter having been burned out.

Soil containing organic materials holds water much more readily than inorganic soil, as a glance at the accompanying figure shows. If we fill each of the vessels with a given weight (say 100 grams each) of gravel, sand, barren soil, rich loam, leaf mold, and 25 grams of dry, pulverized leaves, then pour equal amounts of water (100 c.c.) on each and measure all that runs through, the water that has been retained will represent the water supply that plants could draw on from such soil.

The Root Hairs take more than Water out of the Soil.--If a root containing a fringe of root hairs is washed carefully, it will be found to have little particles of soil still clinging to it. Examined under the microscope, these particles of soil seem to be cemented to the sticky surface of the root hair. The soil contains, besides a number of chemical compounds of various mineral substances,--lime, potash, iron, silica, and many others,--a considerable amount of organic material. Acids of various kinds are present in the soil. These acids so act upon certain of the mineral substances that they become dissolved in the water which is absorbed by the root hairs. Root hairs also give off small amounts of acid. An interesting experiment may be shown (see Figure on page 80) to prove this. A solution of _phenolphthalein_ loses its color when an acid is added to it. If a growing pea be placed in a tube containing some of this solution the latter will quickly change from a rose pink to a colorless solution.

A Plant needs Mineral Matter to Make Living Matter.--Living matter (protoplasm), besides containing the chemical elements carbon, hydrogen, oxygen, and nitrogen, contains a very minute proportion of various elements which make up the basis of certain minerals. These are calcium (lime), sulphur, iron, potassium, magnesium, phosphorus, sodium, and chlorine.

That plants will not grow well without certain of these mineral substances can be proved by the growth of seedlings in a so-called nutrient solution.[11] Such a solution contains all the mineral matter that a plant uses for food. If certain ingredients are left out of this solution, the plants placed in it will not live.

Footnote 11: See Hunter's _Laboratory Problems in Civic
Biology_ for list of ingredients.

Nitrogen in a Usable Form necessary for Growth of Plants.--A chemical element needed by the plant to make protoplasm is nitrogen. The air can be proven by experiment to be made up of about four fifths nitrogen, but this element cannot be taken from either soil, water, or air in a pure state, but is usually obtained from the organic matter in the soil, where it exists with other substances in the form of _nitrates_. Ammonia and other organic compounds which contain nitrogen are changed by two groups of little plants called _bacteria_, first into nitrites and then nitrates.[12]

Footnote 12: It has recently been discovered that under some
conditions these bacteria are preyed upon by tiny one-celled
animals (_protozoa_) living in the soil and are so reduced
in numbers that they cannot do their work effectively. If,
then, the soil is heated artificially or treated with
antiseptics so as to kill the protozoa, the bacteria which
escape multiply so rapidly as to make the land much richer
than before.

Relation of Bacteria to Free Nitrogen.--It has been known since the time of the Romans that the growth of clover, peas, beans, and other legumes in soil causes it to become more favorable for growth of other plants. The reason for this has been discovered in late years. On the roots of the plants mentioned are found little swellings or nodules; in the nodules exist millions of bacteria, which take nitrogen from the atmosphere and fix it so that it can be used by the plant; that is, they assist in forming nitrates for the plants to use. Only these bacteria, of all the living plants, have the power to take the free nitrogen from the air and make it over into a form that can be used by the roots. As all the compounds of nitrogen are used over and over again, first by plants, then as food for animals, eventually returning to the soil again, or in part being turned into free nitrogen, it is evident that any _new_ supply of usable nitrogen must come by means of these nitrogen-fixing bacteria.

Rotation of Crops.--The facts mentioned above are made use of by careful farmers who wish to make as much as possible from a given area of ground in a given time. Such plants as are hosts for the nitrogen-fixing bacteria are planted early in the season. Later these plants are plowed in and a second crop is planted. The latter grows quickly and luxuriantly because of the nitrates left in the soil by the bacteria which lived with the first crop. For this reason, clover is often grown on land in which it is proposed to plant corn, the nitrogen left in the soil thus giving nourishment to the young corn plants. In scientifically managed farms, different crops are planted in a given field on different years so that one crop may replace some of the elements taken from the soil by the previous crop. This is known as rotation of crops.[13] The annual yield of the average farm may thus be greatly increased.

Footnote 13: That crop rotation is not primarily a process
to conserve the fertility of the soil, but is a sanitary
measure to prevent infection of the soil, is the latest
belief of the scientist.

Five of the elements necessary to the life of the plant which may be taken out of the soil by constant use are calcium, nitrogen, phosphorus, potassium, and sulphur. Several methods are used by the farmer to prevent the exhaustion of these and other raw food materials from the soil. One method known as _fallowing_ is to allow the soil to remain idle until bacteria and oxidation have renewed the chemical materials used by the plants. This is an expensive method, if land is dear. The most common method of enriching soil is by means of fertilizing material rich in plant food. Manure is most frequently used, but many artificial fertilizers, most of which contain nitrogen in the form of some nitrate, are used, because they can be more easily transported and sold. Such are ground bone, guano (bird manure), nitrate of soda, and many others. These also contain other important raw food materials for plants, especially potash and phosphoric acid. Both of these substances are made soluble so as to be taken into the roots by the action of the carbon dioxide in the soil.

The Indirect Relation of this to the City Dweller.--All of us living in the city are aware of the importance of fresh vegetables, brought in from the neighboring market gardens. But we sometimes forget that our great staple crops, wheat and other cereals, potatoes, fruits of all kinds, our cotton crop, and all plants we make use of grow directly in proportion to the amount of raw food materials they take in through the roots. When we also remember that many industries within the cities, as mills, bakeries, and the like, as well as the earnings of our railways and steamship lines, are largely dependent on the abundance of the crops, we may recognize the importance of what we have read in this chapter.

Food Storage in Roots of Commercial Importance.--Some plants, as the parsnip, carrot, and radish, produce no seed until the second year, storing food in the roots the first year and using it to get an early start the following spring, so as to be better able to produce seeds when the time comes. This food storage in roots is of much practical value to mankind. Many of our commonest garden vegetables, as those mentioned above, and the beet, turnip, oyster plant, sweet potato and many others, are of value because of the food stored. The sugar beet has, in Europe especially, become the basis of a great industry.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Bigelow, _Applied Biology_. The Macmillan Company.
Coulter, _Plant Life and Plant Uses_, Chaps. III, IV.
American Book Company.
Mayne and Hatch, _High School Agriculture_. American Book
Company.
Moore, _The Physiology of Man and Other Animals_. Henry Holt
and Company.
Sharpe, _Laboratory Manual in Biology_, pp. 73-87. American
Book Company.

ADVANCED

Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part
II. Amer. Book Co.
Duggar, _Plant Physiology_. The Macmillan Company.
Goodale, _Physiological Botany_. American Book Company.
Green, _Vegetable Physiology_, Chaps. V, VI. J. and A.
Churchill.
Kerner-Oliver, _Natural History of Plants_. Henry Holt and
Company.
MacDougal, _Plant Physiology_. Longmans, Green, and Company.

VII. PLANT GROWTH AND NUTRITION--PLANTS MAKE FOOD

_Problem.--Where, when, and how do green plants make food?_
_(a) How and why is moisture given off from leaves?_
_(b) What is the reaction of leaves to light?_
_(c) What is made in green leaves in the sunlight?_
_(d) What by-products are given off in the above process?_
_(e) Other functions of leaves._

LABORATORY SUGGESTIONS

_Demonstration._--Water given off by plant in sunlight. Loss
of weight due to transpiration measured.
_Laboratory exercise._--
(_a_) Gross structure of a leaf.
(_b_) Study of stoma and lower epidermis under microscope.
(_c_) Study of cross section to show cells and air spaces.
_Demonstration._--Reaction of leaves to light.
_Demonstration._--Light necessary to starch making.
_Demonstration._--Air necessary to starch making.
_Demonstration._--Oxygen a by-product of starch making.

What becomes of the Water taken in by the Roots?--We have seen that more than pure water has been absorbed through the root hairs into the roots. What becomes of this water and the other substances that have been absorbed? This question may be partly answered by the following experiments.

Passage of Fluids up the Stem.--If any young growing shoots (young seedlings of corn or pea, or the older stems of garden balsam, touch-me-not, or sunflower) are placed in red ink (eosin), and left in the sun for a few hours, the red ink will be found to have passed up the stem. If such stems were examined carefully, it would be seen that the colored fluid is confined to collections of woody tubes immediately under the inner bark. Water evidently rises in that part of the stem we call the wood.

Water given off by Evaporation from Leaves.--Take some well-watered potted green plant, as a geranium or hydrangea, cover the pot with sheet rubber, fastening the rubber close to the stem of the plant. Next weigh the plant with the pot. Then cover it with a tall bell jar and place the apparatus in the sun. In a few minutes drops of moisture are seen to gather on the _inside_ of the jar. If we now weigh the potted plant, we find it weighs less than before. Obviously the loss comes from the water lost, and evidently this water escapes as vapor from either the stem or leaves.

The Structure of a Leaf.--In the experiment with the red ink mentioned above we will find that the fluid has gone out into the skeleton or framework of the leaf. Let us now examine a leaf more carefully. It shows usually (1) a flat, broad _blade_, which may take almost any conceivable shape; (2) a _stem_ which spreads out in the blade (3) in a number of _veins_.

The Cell Structure of a Leaf.--The under surface of a leaf seen under the microscope usually shows numbers of tiny oval openings. These are called _stomata_ (singular _stoma_). Two cells, usually kidney-shaped, are found, one on each side of the opening. These are the _guard cells_. By change in shape of these cells the opening of the stoma is made larger or smaller. Larger irregular cells form the _epidermis_, or outer covering of the leaf. Study of the leaf in cross section shows that these stomata open directly into air chambers which penetrate between and around the loosely arranged cells composing the underpart of the leaf. The upper surface of leaves sometimes contains stomata, but more often they are lacking. The under surface of an oak leaf of ordinary size contains about 2,000,000 stomata. Under the upper epidermis is a layer of green cells closely packed together (called collectively the _palisade layer_). These cells are more or less columnar in shape. Under these are several rows of rather loosely placed cells just mentioned. These are called collectively the _spongy tissue_. If we happen to have a section cut through a vein, we find this composed of a number of tubes made up of, and strengthened by, thick-walled cells. The veins are evidently a continuation of the tubes of the stem out into the blade of the leaf.

Evaporation of Water.--During the day an enormous amount of water is taken up by the roots and passed out through the leaves. So great is this excess at times that a small grass plant on a summer's day evaporates more than its own weight in water. This would make nearly half a ton of water delivered to the air during twenty-four hours by a grass plot twenty-five by one hundred feet, the size of the average city lot. According to Ward, an oak tree may pass off two hundred and twenty-six times its own weight in water during the season from June to October.

From which Surface of the Leaf is Water Lost?--In order to find out whether water is passed out from any particular part of the leaf, we may remove two leaves of the same size and weight from some large-leaved plant[14]--a mullein was used for the illustrations given below--and cover the upper surface of one leaf and the lower surface of the other with vaseline. The leaf stalks of each should be covered with wax or vaseline, and the two leaves exactly balanced on the pans of a balance which has previously been placed in a warm and sunny place. Within an hour the leaf which has the upper surface covered with vaseline will show a loss of weight. Examination of the surface of a mullein leaf shows us that the _lower surface of the leaf is provided with stomata_. It is through these organs, then, that water is passed out from the tissues of the leaf.

Footnote 14: The "rubber plant" leaf is an easily obtainable
and excellent demonstration.

Factors in Transpiration.--The amount of water lost from a plant varies greatly under different conditions. The humidity of the air, its temperature, and the temperature of the plant all affect the rate of transpiration. The stomata also tend to close under some conditions, thus helping to prevent evaporation. But there seems to be no certain regulation of this water loss. Consequently plants droop or wilt on hot dry days because they cannot obtain water rapidly enough from the soil to make up for the loss through the leaves.

Green Plants Food Makers.--We have previously stated that green plants are the great food makers for themselves and for animals. We are now ready to attack the problem of how green plants _make_ food.

The Sun a Source of Energy.--We all know the sun is a source of most of the energy that is released on this earth in the form of heat or light. Every boy knows the power of a "burning glass." Solar engines have not come into any great use as yet, because fuel is cheaper, but some day we undoubtedly will directly harness the energy of the sun in everyday work. Actual experiments have shown that vast amounts of energy are given to the earth. When the sun is highest in the sky, energy equivalent to one hundred horse power is received by a plot of land twenty-five by one hundred feet, the size of a city lot. Plants receive and use much of this energy by means of their leaves.

Effect of Light on Plants.--In young plants which have been grown in total darkness, no green color is found in either stems or leaves, the latter often being reduced to mere scales. The stems are long and more or less reclining. We can explain the changed condition of the seedling grown in the dark only by assuming that light has some effect on the protoplasm of the seedling and induces the growth of the green part of the plant. If seedlings have been growing on a window sill, or where the light comes in from one side, you have doubtless noticed that the stem and leaves of the seedlings incline in the direction from which the light comes. The experiment pictured shows this effect of light very plainly. A hole was cut in one end of a cigar box and barriers were erected in the interior of the box so that the seeds planted in the sawdust received their light by an indirect course. The young seedling in this case responded to the influence of the stimulus of light so as to grow out finally through the hole in the box into the open air. This growth of the stem to the light is of very great importance to a growing plant, because, as we shall see later, food making depends largely on the amount of sunlight the leaves receive.

Effect of Light on Leaf Arrangement.--It is a matter of common knowledge that green leaves turn toward the light. Place growing pea seedlings, oxalis, or any other plants of rapid growth near a window which receives full sunlight. Within a short time the leaves are found to be in positions to receive the most sunlight possible. Careful observation of any plant growing outdoors shows us that in almost every case the leaves are so disposed as to get much sunlight. The ivy climbing up the wall, the morning-glory, the dandelion, and the burdock all show different arrangements of leaves, each presenting a large surface to the light. Leaves are often definitely arranged, fitting in between one another so as to present their upper surface to the sun. Such an arrangement is known as a _leaf mosaic_. In the case of the dandelion, a _rosette_ or whorled cluster of leaves is found. In the horse-chestnut, where the leaves come out opposite each other, the older leaves have longer petioles than the young ones. In the mullein the entire plant forms a cone. The old leaves near the bottom have long stalks, and the little ones near the apex come out close to the main stalk. In every case each leaf receives a large amount of light. Other modifications of these forms may easily be found on any field trip.

Starch made by a Green Leaf.--If we examine the palisade layer of the leaf, we find cells which are almost cylindrical in form. In the protoplasm of such cells are found a number of little green-colored bodies, which are known as _chloroplasts_ or _chlorophyll bodies_. If we place the leaf in wood alcohol, we find that the bodies still remain, but that the color is extracted, going into the alcohol and giving to it a beautiful green color. The chloroplasts are, indeed, simply part of the protoplasm of the cell colored green. These bodies are of the greatest importance directly to plants and indirectly to animals. _The chloroplasts, by means of the energy received from the sun, manufacture starch out of certain raw materials._ These raw materials are soil water, which is passed up through the bundles of tubes into the veins of the leaf from the roots, and carbon dioxide, which is taken in through the stomata or pores, which dot the under surface of the leaf. A plant with variegated leaves, as the coleus, makes starch only in the green part of the leaf, even though these raw materials reach all parts of the leaf.

Light and Air necessary for Starch Making.--If we pin strips of black cloth, such as alpaca, over some of the leaves of a growing hydrangea which has previously been placed in a dark room for a few hours, and then put the plant in direct sunlight for an hour or two, we are ready to test for starch. We then remove some of the covered leaves and extract the chlorophyll with wood alcohol (because the green color of the chlorophyll interferes with the blue color of the starch test). A test then shows that starch is present only in the portions of the leaves exposed to sunlight. From this experiment we infer that the sun has something to do with starch making in a leaf. The necessity of a part of the air (carbon dioxide) for starch making may also easily be proved, for the parts of leaves covered with vaseline will be found to contain no starch, while parts of the leaf without vaseline, but exposed to the sun and air, do contain starch.

Air is necessary for the process of starch making in a leaf, not only because carbon dioxide gas is absorbed (there are from three to four parts in ten thousand present in the atmosphere), but also because the leaf is alive and must have oxygen in order to do work. This oxygen it takes from the air around it.

Comparison of Starch Making and Milling.--The manufacture of starch by the green leaf is not easily understood. The process has been compared to the milling of grain. In this case the mill is the green part of the leaf. The sun furnishes the motive power, the chloroplasts constitute the machinery, and soil water and carbon dioxide are the raw products taken into the mill. The manufactured product is starch, and a certain by-product (corresponding to the waste in a mill) is also given out. This by-product is oxygen. To understand the process fully, we must refer to a small portion of the leaf shown below. Here we find that the cells of the green layer of the leaf, under the upper epidermis, perform most of the work. The carbon dioxide is taken in through the stomata and reaches the green cells by way of the intercellular spaces and by osmosis from cell to cell. Water reaches the green cells through the veins. It then passes into the cells by osmosis, and there becomes part of the cell sap. The light of the sun easily penetrates to the cells of the palisade layer, giving the energy needed to make the starch. This whole process is a very delicate one, and will take place only when external conditions are favorable. For example, too much heat or too little heat stops starch making in the leaf. This building up of food and the release of oxygen by the plant in the presence of sunlight is called _photosynthesis_.

Manufacture of Fats.--Inasmuch as tiny droplets of oil are found _inside_ the chlorophyll bodies in the leaf, we believe that fats, too, are made there, probably by a transformation of the starch already manufactured.

Protein Making and its Relation to the Making of Living Matter.--Protein material is a food which is necessary to form protoplasm. Protein food is present in the leaf, and is found in the stem or root as well. Proteins can apparently be manufactured in any of the cells of green plants, the presence of light not seeming to be a necessary factor. How it is manufactured is a matter of conjecture. The minerals brought up in the soil water form part of its composition, and starch or grape sugar give three elements (C, H, and O). The element nitrogen is taken up by the roots as a nitrate (nitrogen in combination with lime or potash). Proteins are probably not made directly into protoplasm in the leaf, but are stored by the cells of the plant and used when needed, either to form new cells in growth or to repair waste. While plants and animals obtain their food in different ways, they probably make it into living substance (_assimilate_ it) in exactly the same manner.

Foods serve exactly the same purposes in plants and in animals; they either build living matter or they are burned (oxidized) to furnish energy (power to do work). If you doubt that a plant exerts energy, note how the roots of a tree bore their way through the hardest soil, and how stems or roots of trees often split open the hardest rocks, as illustrated in the figure above.

Starch-Making and its Relation to Human Welfare.--Leaves which have been in darkness show starch to be present soon after exposure to light. A corn plant sends 10 to 15 grams of reserve material into the ears in a single day. The formation of fruit, and especially the growth of the grain fields, show the economic importance of this fact. Not only do plants make their own food and store it away, but they make food for animals as well. And the food is stored in such a stable form that it may be sent to all parts of the world in the form of grain or other fruits. Animals, herbivorous and flesh-eating, man himself, all are dependent upon the starch-making processes of the green plant for the ultimate source of their food. When we remember that in 1913 in the United States the total value of all farm crops was over $6,000,000,000, and when we realize that these products came from the air and soil through the energy of the sun, we may begin to realize why as city boys and girls the study of plant biology is of importance to us.

Green Plants give off Oxygen in Sunlight.--In still another way green plants are of direct use to us in the city. During this process of starch-making oxygen is given off as a by-product. This may easily be proven by the following experiment.[15] Place any green water plant in a battery jar partly filled with water, cover the plants with a glass funnel and mount a test tube full of water over the mouth of the funnel. Then place the apparatus in a warm sunny window. Bubbles of gas are seen to rise from the plant. After two or three hours of hot sun, enough of the gas can be obtained by displacement of the water to make the oxygen test.

Footnote 15: Immediate success with this experiment will be
obtained if the water has been previously charged with
carbon dioxide.

That oxygen is given off as a by-product by green plants is a fact of far-reaching importance. City parks are true "breathing spaces." The green covering of the earth is giving to animals an element that they must have, while the animals in their turn are supplying to the plants carbon dioxide, a compound used in food-making. Thus a widespread relation of mutual helpfulness exists between plants and animals.

Respiration by Leaves.--All living things require oxygen. It is by means of the oxidation of food materials within the plant's body that the energy used in growth and movement is released. A plant takes in oxygen largely through the stomata of the leaves, to a less extent through the _lenticels_ or breathing holes in the stem, and through the roots. Thus rapidly growing tissues receive the oxygen necessary for them to perform their work. The products of oxidation in the form of carbon dioxide are also passed off through these same organs. It can be shown by experiment that a plant uses up oxygen in the darkness; in the light the amount of oxygen given off as a by-product in the process of starch-making is, of course, much greater than the amount used by the plant.

Summary.--From the above paragraphs it is seen that a leaf performs the following functions: (1) breathing, or the taking in of oxygen and passing off of carbon dioxide; (2) starch-making, with the incidental passing out of oxygen; (3) formation of proteins, with their digestion and assimilation to form new tissues; and (4) the transpiration of water.

REFERENCE BOOKS

ELEMENTARY

Hunter, _Laboratory Problems in Civic Biology_. American
Book Company.
Andrews, _A Practical Course in Botany_, pages 160-177.
American Book Company.
Coulter, _A Textbook of Botany_, pages 5-40. D. Appleton and
Company.
Coulter, _Plant Life and Plant Uses_. American Book Company.
Dana, _Plants and their Children_, pages 135-185. American
Book Company.
Sharpe, _A Laboratory Manual in Biology_, pages 90-102.
American Book Company.
Stevens, _Introduction to Botany_, pages 81-99. D. C. Heath
and Company.

ADVANCED

Clement, _Plant Physiology and Ecology_. Henry Holt and
Company.
Coulter, Barnes, and Cowles, _A Textbook of Botany_, Part
II, and Vol. II. American Book Company.
Darwin, _Insectivorous Plants_. D. Appleton and Company.
Duggar, _Plant Physiology_. The Macmillan Company.
Goodale, _Physiological Botany_, pages 337-353 and 409-424.
American Book Company.
Green, _Vegetable Physiology_. J. and A. Churchill.
Lubbock, _Flowers, Fruits, and Leaves_, last part. The
Macmillan Company.
MacDougal, _Practical Textbook of Plant Physiology_.
Longmans, Green, and Company.
Report of the Division of Forestry, U.S. Department of
Agriculture, 1899.
Ward, _The Oak_. D. Appleton and Company.

VIII. PLANT GROWTH AND NUTRITION--THE CIRCULATION AND FINAL USES OF FOOD BY PLANTS

_Problem.--How green plants store and use the food they make._
_(a) What are the organs of circulation?_
_(b) How and where does food circulate?_
_(c) How does the plant assimilate its food?_

LABORATORY SUGGESTIONS

_Laboratory exercise._--The structure (cross section) of a
woody stem.
_Demonstration._--To show that food passes downward in the
bark.
_Demonstration._--To show the condition of food passing
through the stem.
_Demonstration._--Plants with special digestive organs.

The Circulation and Final Uses of Foods in Green Plants.--We have seen that cells of green plants make food and that such cells are mostly in the leaves. But _all_ parts of the bodies of plants grow. Roots, stems, leaves, flowers, and fruits grow. Seeds are storehouses of food. We must now examine the stem of some plant in order to see how food is distributed, stored, and finally used in the various parts of the plant.

The Structure of a Woody Stem.--If we cut a cross section through a young willow or apple stem, we find it shows three distinct regions. The center is occupied by the spongy, soft _pith_; surrounding this is found the rather tough _wood_, while the outermost area is _bark_. More careful study of the bark reveals the presence of three layers--an outer layer, a middle green layer, and an inner fibrous layer, the latter usually brown in color. This layer is made up largely of tough fiberlike cells known as _bast_ fibers. The most important parts of this inner bark, so far as the plant is concerned, are many tubelike structures known as _sieve tubes_. These are long rows of living cells, having perforated sievelike ends. Through these cells food materials pass downward from the upper part of the plant, where they are manufactured.

In the wood will be noticed (see Figure) a number of lines radiating outward from the pith toward the bark. These are thin plates of pith which separate the wood into a number of wedge-shaped masses. These masses of wood are composed of many elongated cells, which, placed end to end, form thousands of little tubes connecting the leaves with the roots. In addition to these are many thick-walled cells, which give strength to the mass of wood. The bundles of tubes with their surrounding hard walled cells are the continuation of the bundles of tubes which are found in the root. In sections of wood which have taken several years to grow, we find so-called _annual rings_. The distance between one ring and the next (see Figure) usually represents the amount of growth in one year. Growth takes place from an actively dividing layer of cells, known as the _cambium layer_. This layer forms wood cells from its inner surface and bark from its outer surface. Thus new wood is formed as a distinct ring around the old wood.

Use of the Outer Bark.--The outer bark of a tree is protective. The cells are dead, the heavy woody skeletons serving to keep out cold and dryness, as well as prevent the evaporation of fluids from within. The bark also protects the tree from attack of other plants or animals which might harm it. Most trees are provided with a layer of corky cells. This layer in the cork oak is thick enough to be of commercial importance. The function of the corky layer in preventing evaporation is well seen in the case of the potato, which is a true stem, though found underground. If two potatoes of equal weight are balanced on the scales, the skin having been peeled from one, the peeled potato will be found to lose weight rapidly. This is due to loss of water, which is held in by the skin of the unpeeled potato (see right hand figure below).

There are also small breathing holes known as _lenticels_ scattered through the surface of the bark. These can easily be seen in a young woody stem of apple, beech, or horse-chestnut.

Proof that Food passes down the Stem.--If freshly cut willow twigs are placed in water, roots soon begin to develop from that part of the stem which is under water. If now the stem is girdled by removing the bark in a ring just above where the roots are growing, the latter will eventually die, and new roots will appear above the girdled area. The food material necessary for the outgrowth of roots evidently comes from above, and the passage of food materials takes place in a downward direction just outside the wood in the layer of bark which contains the bast fibers and sieve tubes. This experiment with the willow explains why it is that trees die when girdled so as to cut the sieve tubes of the inner bark. The food supply is cut off from the protoplasm of the cells in the part of the tree below the cut area. Many of the canoe birches of our Adirondack forest are thus killed, girdled by thoughtless visitors. In the same manner mice and other gnawing animals kill fruit trees. Food substances are also conducted to a much less extent in the wood itself, and food passes from the inner bark to the center of the tree by way of the pith plates. This can be proved by testing for starch in the pith plates of young stems. It is found that much starch is stored in this part of the tree trunk.

In what Form does Food pass through the Stem?--We have already seen that materials in solution (those substances which will dissolve in the water) will pass from cell to cell by the process of osmosis. This is shown in the experiment illustrated in the figure. Two thistle tubes are partly filled, one with starch and water, the other with sugar and water, and a piece of parchment paper is tied over the end of each. The lower ends of both tubes are placed in a glass dish under water. After twenty-four hours, the water in the dish is tested for starch, and then for sugar. We find that only the sugar, which has been dissolved by the water, can pass through the membrane.

Digestion.--Much of the food made in the leaves is stored in the form of starch. But starch, being insoluble, cannot be passed from cell to cell in a plant. It must be changed to a soluble form, for otherwise it could not pass through the delicate cell membranes. This is accomplished by the process of _digestion_. We have already seen that starch is changed to grape sugar in the corn by the action of a substance (an enzyme) called _diastase_. This process of digestion seemingly may take place in all living parts of the plant, although most of it is done in the leaves. In the bodies of all animals, including man, starchy foods are changed in a similar manner, but by other enzymes, into soluble grape sugar.

The food material may be passed in a soluble form until it comes to a place where food storage is to take place, then it can be transformed to an insoluble form (starch, for example); later, when needed by the plant in growth, it may again be transformed and sent in a soluble form through the stem to the place where it will be used.