[Illustration: Experiment to show that oxygen is given off by green plants in the sunlight.]
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.
[Illustration: Section of a twig of box elder three years old, showing three annual growth rings. The radiating lines (_m_) which cross the wood (_w_) represent the pith rays, the principal ones extending from the pith in the center to the cortex or bark. (From Coulter's _Plant Relations_.)]
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.
[Illustration: Experiment to show that the skin of the potato (a stem) retards evaporation.]
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.
[Illustration: Experiment to show that food material passes down in the inner bark.]
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.
[Illustration: Experiment to show osmosis of sugar (right hand tube) and non-osmosis of starch (left hand tube).]
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.
In a similar manner, protein seems to be changed and transferred to various parts of the plant. Some forms of protein substance are _soluble_ and others _insoluble_ in water. White of egg, for example, is slightly soluble, but can be rendered insoluble by heating it so that it coagulates.
Insoluble proteins are digested within the plant; how and where is but slightly understood. In a plant, soluble proteins pass down the sieve tubes in the bast and then may be stored in the bast or medullary rays of the wood in an insoluble form, or they may pass into the fruit or seeds of a plant, and be stored there.
[Illustration: Diagram to show the areas in a plant through which the raw food materials pass up the stem and food materials pass down.]
What forces Water up the Stem.--We have seen that the process of osmosis is responsible for taking in soil water, and that the enormous absorbing surface exposed by the root hairs makes possible the absorption of a large amount of water. Frequently this is more than the weight of the plant in every twenty-four hours.
Experiments have been made which show that at certain times in the year this water is in some way forced up the tiny tubes of the stem. During the spring season, in young and rapidly growing trees, water has been proved to rise to a height of nearly ninety feet. The force that causes this rise of water in stems is known as _root pressure_.
The greatest factor, however, is transpiration of water from leaves. This evaporation of water in the form of vapor seems to result in a kind of suction on the column of water in the stem. In the fall, after the leaves have gone, much less water is taken in by roots, showing that an intimate relation exists between the leaves and the root.
Summary of the Functions of Green Plants.--The processes which we have just described (with the exception of food making) are those which occur in the lives of any plant or animal. All plants and animals breathe, they oxidize their foods to release energy, carbon dioxide being given off as the result of the union of the carbon in the foods with the oxygen of the air. Both plants and animals digest their food; plants may do this in the cells of the root, stem, and leaf. Digestion must always occur so that food can be moved in a soluble condition from cell to cell in the plant's body.
[Illustration: Leaf of sundew closing over a captured insect.]