Distribution of the Digested Food.—After being changed to the soluble form, this material is ready to be used in growth, either in the leaf, in the stem, or in the roots. With other more complex products it is then distributed throughout all the growing parts of the plant; and when passing down to the root, it seems to pass more readily through the inner bark, in plants which have a definite bark. This gradual downward diffusion through the inner bark of materials suitable for growth is the process referred to when the “descent of sap” is mentioned. Starch and other products are often stored in one growing season to be used in the next season. If a tree is constricted or strangled by a wire around its trunk (Fig. [118]), the digested food cannot readily pass down and it is stored above the girdle, causing an enlargement.
Assimilation.—The food from the air and that from the soil unite in the living tissues. The “sap” that passes upwards from the roots in the growing season is made up largely of the soil water and the salts which have been absorbed in the diluted solutions (p. [67]). This upward-moving water is conducted largely through certain tubular canals of the young wood. These cells are never continuous tubes from root to leaf; but the water passes readily from one cell or canal to another in its upward course.
The upward-moving water gradually passes to the growing parts, and everywhere in the living tissues, it is, of course, in the most intimate contact with the soluble carbohydrates and products of photosynthesis. In the building up or reconstructive and other processes it is therefore available. We may properly conceive of certain of the simpler organic molecules as passing through a series of changes, gradually increasing in complexity. There will be formed substances containing nitrogen in addition to carbon, hydrogen, and oxygen. Others will contain also sulphur and phosphorus, and the various processes may be thought of as culminating in protoplasm. Protoplasm is the living matter in plants. It is in the cells, and is usually semi-fluid. Starch is not living matter. The complex process of building up the protoplasm is called assimilation.
Respiration.—Plants need oxygen for respiration, as animals do. We have seen that plants need the carbon dioxide of the air. To most plants the nitrogen of the air is inert, and serves only to dilute the other elements; but the oxygen is necessary for all life. We know that all animals need this oxygen in order to breathe or respire. In fact, they have become accustomed to it in just the proportions found in the air; and this is now best for them. When animals breathe the air once, they make it foul, because they use some of the oxygen and give off carbon dioxide. Likewise, all living parts of the plant must have a constant supply of oxygen. Roots also need it, for they respire. Air goes in and out of the soil by diffusion, and as the soil is heated and cooled, causing the air to expand and contract.
The oxygen passes into the air-spaces and is absorbed by the moist cell membranes. In the living cells it makes possible the formation of simpler compounds by which energy is released. This energy enables the plant to work and grow, and the final products of this action are carbon dioxide and water. As a result of the use of this oxygen by night and by day, plants give off carbon dioxide. Plants respire; but since they are stationary, and more or less inactive, they do not need so much oxygen as animals do, and they do not give off so much carbon dioxide. A few plants in a sleeping room need not disturb one more than a family of mice. It should be noted, however, that germinating seeds respire vigorously, hence they consume much oxygen; and opening buds and flowers are likewise active.
Transpiration.—Much more water is absorbed by the roots than is used in growth, and this surplus water passes from the leaves into the atmosphere by an evaporation process known as transpiration. Transpiration takes place more abundantly from the under surfaces of leaves, and through the pores or stomates. A sunflower plant of the height of a man, during an active period of growth, gives off a quart of water per day. A large oak tree may transpire 150 gallons per day during the summer. For every ounce of dry matter produced, it is estimated that 15 to 25 pounds of water usually passes through the plant.
When the roots fail to supply to the plant sufficient water to equalize that transpired by the leaves, the plant wilts. Transpiration from the leaves and delicate shoots is increased by all the conditions which increase evaporation, such as higher temperature, dry air, or wind. The stomata open and close, tending to regulate transpiration as the varying conditions of the atmosphere affect the moisture content of the plant. However, in periods of drought or of very hot weather, and especially during a hot wind, the closing of these stomates cannot sufficiently prevent evaporation. The roots may be very active and yet fail to absorb sufficient moisture to equalize that given off by the leaves. The plant shows the effect (how?). On a hot dry day, note how the leaves of corn “roll” towards afternoon. Note how fresh and vigorous the same leaves appear early the following morning. Any injury to the roots, such as a bruise, or exposure to heat, drought, or cold may cause the plant to wilt.
Water is forced up by root pressure or sap pressure. (Exercise 99.) Some of the dew on the grass in the morning may be the water forced up by the roots; some of it is the condensed vapour of the air.
The wilting of a plant is due to the loss of water from the cells. The cell walls are soft, and collapse. A toy balloon will not stand alone until it is inflated with air or liquid. In the woody parts of the plant the cell walls may be stiff enough to support themselves, even though the cell is empty. Measure the contraction due to wilting and drying by tracing a fresh leaf on page of notebook, and then tracing the same leaf after it has been dried between papers. The softer the leaf, the greater will be the contraction.
Storage.—We have said that starch may be stored in twigs to be used the following year. The very early flowers on fruit trees, especially those that come before the leaves, and those that come from bulbs, as crocuses and tulips, are supported by the starch or other food that was organized the year before. Some plants have very special storage reservoirs, as the potato, in this case being a thickened stem although growing underground. (Why a thickened stem? p. [84].) It is well to make the starch test on winter twigs and on all kinds of thickened parts, as tubers and bulbs.