A Civic Biology, Presented in Problems

by George W. Hunter

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.

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.

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.

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.

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.

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.

Leaf of sundew closing over a captured insect.

The Venus fly trap, showing open and closed leaves.

Plants with Special Digestive Organs.—Some plants have special organs of digestion. One of these, the sundew, has leaves which are covered on one side with tiny glandular hairs. These attract insects and later serve to catch and digest the nitrogenous matter of these insects by means of enzymes poured out by the same hairs. Another plant, the Venus fly trap, catches insects in a sensitive leaf which folds up and holds the insect fast until enzymes poured out by the leaf slowly digest it. Still others, called pitcher plants, use as food the decayed bodies of insects which fall into their cuplike leaves and die there. In this respect plants are like those animals which have certain organs in the body set apart for the digestion of food.

Assimilation.—The assimilation of foods, or making of foods into living matter, is a process we know very little about. We know it takes place in the living cells of plants and animals. But how foods are changed into living matter is one of the mysteries of life which we have not yet solved.

Excretion.—The waste and repair of living matter seems to take place in both plants and animals. When living plants breathe, they give off carbon dioxide. In the process of starch-making, oxygen might be considered the waste product. Water is evaporated from leaves and stems. The leaves fall and carry away waste mineral substances which they contain.

The embryos of (a) the morning glory, (b) the barberry, (c) the potato, (d) the four o'clock, showing the position of their food supply. (After Gray.)

Reproduction.—Finally, both plants and animals have organs of reproduction. We have seen that the flower gives rise, after pollination, to a fruit which holds the seeds. These seeds hold the embryo. Thus the young plant is doubly protected for a time and is finally thrown off in the seed with enough food to give it a start in life. In much the same way we will find that animals reproduce, either by laying eggs which contain an embryo and food to start it in life or, as in the higher animals, by holding and protecting the embryo within the body of the mother until it is born, a helpless little creature, to be tenderly nourished by the mother until able to care for itself.

The Life Cycle.—Ultimately both plants and animals grow old and die. Some plants, for example the pea or bean, live but a season; others, such as the big trees of California, live for hundreds of years. Some insects exist as adults but a day, while the elephant is said to live almost two hundred years. The span of life from the time the plant or animal begins to grow until it dies is known as the life cycle.

Reference Books

elementary

Hunter, Laboratory Problems in Civic Biology. American Book Company.

Andrews, A Practical Course in Botany, pages 112-127. American Book Company.

Atkinson, First Studies of Plant Life, Chaps. IV, V, VI, VIII, XXI. Ginn.

Coulter, Plant Life and Plant Uses, Chap. V. American Book Company.

Dana, Plants and their Children, pages 99-129. American Book Company.

Mayne and Hatch, High School Agriculture. American Book Company.

Hodge, Nature Study and Life, Chaps. IX, X, XI. Ginn and Company.

MacDougal, The Nature and Work of Plants. The Macmillan Company.

advanced

Apgar, Trees of the United States, Chaps. II, V, VI. American Book Company.

Coulter, Barnes, and Cowles, A Textbook of Botany, Vol. I. American Book Company.

Duggar, Plant Physiology. The Macmillan Company.

Ganong, The Teaching Botanist. The Macmillan Company.

Goebel, Organography of Plants, Part V. Clarendon Press.

Goodale, Physiological Botany. American Book Company.

Gray, Structural Botany, Chap. V. American Book Company.

Kerner-Oliver, Natural History of Plants. Henry Holt and Company.

Strasburger, Noll, Schenck, and Karston, A Textbook of Botany. The Macmillan Company.

Ward, The Oak. D. Appleton and Company.

Yearbook, U. S. Department of Agriculture, 1894, 1895, 1898-1910.