CHAPTER IX
REGULATING THE TRANSPIRATION

WATER that has entered the soil may be lost in three ways. First, it may escape by downward seepage, whereby it passes beyond the reach of plant roots and often reaches the standing water. In dry-farm districts such loss is a rare occurrence, for the natural precipitation is not sufficiently large to connect with the country drainage, and it may, therefore, be eliminated from consideration. Second, soil-water may be lost by direct evaporation from the surface soil. The conditions prevailing in arid districts favor strongly this manner of loss of soil-moisture. It has been shown, however, in the preceding chapter that the farmer, by proper and persistent cultivation of the topsoil, has it in his power to reduce this loss enough to be almost negligible in the farmer's consideration. Third, soil-water may be lost by evaporation from the plants themselves. While it is not generally understood, this source of loss is, in districts where dry-farming is properly carried on, very much larger than that resulting either from seepage or from direct evaporation. While plants are growing, evaporation from plants, ordinarily called transpiration, continues. Experiments performed in various arid districts have shown that one and a half to three times more water evaporates from the plant than directly from well-tilled soil. To the present very little has been learned concerning the most effective methods of checking or controlling this continual loss of water. Transpiration, or the evaporation of water from the plants themselves and the means of controlling this loss, are subjects of the deepest importance to the dry-farmer.


Absorption

To understand the methods for reducing transpiration, as proposed in this chapter, it is necessary to review briefly the manner in which plants take water from the soil. The roots are the organs of water absorption. Practically no water is taken into the plants by the stems or leaves, even under conditions of heavy rainfall. Such small quantities as may enter the plant through the stems and leaves are of very little value in furthering the life and growth of the plant. The roots alone are of real consequence in water absorption. All parts of the roots do not possess equal power of taking up soil-water. In the process of water absorption the younger roots are most active and effective. Even of the young roots, however, only certain parts are actively engaged in water absorption. At the very tips of the young growing roots are numerous fine hairs. These root-hairs, which cluster about the growing point of the young roots, are the organs of the plant that absorb soil-water. They are of value only for limited periods of time, for as they grow older, they lose their power of water absorption. In fact, they are active only when they are in actual process of growth. It follows, therefore, that water absorption occurs near the tips of the growing roots, and whenever a plant ceases to grow the water absorption ceases also. The root-hairs are filled with a dilute solution of various substances, as yet poorly understood, which plays an important tent part in the ab sorption of water and plant-food from the soil.

Owing to their minuteness, the root-hairs are in most cases immersed in the water film that surrounds the soil particles, and the soil-water is taken directly into the roots from the soil-water film by the process known as osmosis. The explanation of this inward movement is complicated and need not be discussed here. It is sufficient to say that the concentration or strength of the solution within the root-hair is of different degree from the soil-water solution. The water tends, therefore, to move from the soil into the root, in order to make the solutions inside and outside of the root of the same concentration. If it should ever occur that the soil-water and the water within the root-hair became the same concentration, that is to say, contained the same substances in the same proportional amounts, there would be no further inward movement of water. Moreover, if it should happen that the soil-water is stronger than the water within the root-hair, the water would tend to pass from the plant into the soil. This is the condition that prevails in many alkali lands of the West, and is the cause of the death of plants growing on such lands.

It is clear that under these circumstances not only water enters the root-hairs, but many of the substances found in solution in the soil-water enter the plant also. Among these are the mineral substances which are indispensable for the proper life and growth of plants. These plant nutrients are so indispensable that if any one of them is absent, it is absolutely impossible for the plant to continue its life functions. The indispensable plant-foods gathered from the soil by the root-hairs, in addition to water, are: potassium, calcium, magnesium, iron, nitrogen, and phosphorus,--all in their proper combinations. How the plant uses these substances is yet poorly understood, but we are fairly certain that each one has some particular function in the life of the plant. For instance, nitrogen and phosphorus are probably necessary in the formation of the protein or the flesh-forming portions of the plant, while potash is especially valuable in the formation of starch.

There is a constant movement of the indispensable plant nutrients after they have entered the root-hairs, through the stems and into the leaves. This constant movement of the plant-foods depends upon the fact that the plant consumes in its growth considerable quantities of these substances, and as the plant juices are diminished in their content of particular plant-foods, more enters from the soil solution. The necessary plant-foods do not alone enter the plant but whatever may be in solution in the soil-water enters the plant in variable quantities. Nevertheless, since the plant uses only a few definite substances and leaves the unnecessary ones in solution, there is soon a cessation of the inward movement of the unimportant constituents of the soil solution. This process is often spoken of as selective absorption; that is, the plant, because of its vital activity, appears to have the power of selecting from the soil certain substances and rejecting others.


Movement of water through plant

The soil-water, holding in solution a great variety of plant nutrients, passes from the root-hairs into the adjoining cells and gradually moves from cell to cell throughout the whole plant. In many plants this stream of water does not simply pass from cell to cell, but moves through tubes that apparently have been formed for the specific purpose of aiding the movement of water through the plant. The rapidity of this current is often considerable. Ordinarily, it varies from one foot to six feet per hour, though observations are on record showing that the movement often reaches the rate of eighteen feet per hour. It is evident, then, that in an actively growing plant it does not take long for the water which is in the soil to find its way to the uppermost parts of the plant.


The work of leaves

Whether water passes upward from cell to cell or through especially provided tubes, it reaches at last the leaves, where evaporation takes place. It is necessary to consider in greater detail what takes place in leaves in order that we may more clearly understand the loss due to transpiration. One half or more of every plant is made up of the element carbon. The remainder of the plant consists of the mineral substances taken from the soil (not more than two to 10 per cent of the dry plant) and water which has been combined with the carbon and these mineral substances to form the characteristic products of plant life. The carbon which forms over half of the plant substance is gathered from the air by the leaves and it is evident that the leaves are very active agents of plant growth. The atmosphere consists chiefly of the gases oxygen and nitrogen in the proportion of one to four, but associated with them are small quantities of various other substances. Chief among the secondary constituents of the atmosphere is the gas carbon dioxid, which is formed when carbon burns, that is, when carbon unites with the oxygen of the air. Whenever coal or wood or any carbonaceous substance burns, carbon dioxid is formed. Leaves have the power of absorbing the gas carbon dioxid from the air and separating the carbon from the oxygen. The oxygen is returned to the atmosphere while the carbon is retained to be used as the fundamental substance in the construction by the plant of oils, fats, starches, sugars, protein, and all the other products of plant growth.

This important process known as carbon assimilation is made possible by the aid of countless small openings which exist chicfly on the surfaces of leaves and known as "stomata." The stomata are delicately balanced valves, exceedingly sensitive to external influences. They are more numerous on the lower side than on the upper side of plants. In fact, there is often five times more on the under side than on the upper side of a leaf. It has been estimated that 150,000 stomata or more are often found per square inch on the under side of the leaves of ordinary cultivated plants. The stomata or breathing-pores are so constructed that they may open and close very readily. In wilted leaves they are practically closed; often they also close immediately after a rain; but in strong sunlight they are usually wide open. It is through the stomata that the gases of the air enter the plant through which the discarded oxygen returns to the atmosphere.

It is also through the stomata that the water which is drawn from the soil by the roots through the stems is evaporated into the air. There is some evaporation of water from the stems and branches of plants, but it is seldom more than a thirtieth or a fortieth of the total transpiration. The evaporation of water from the leaves through the breathing-pores is the so-called transpiration, which is the greatest cause of the loss of soil-water under dry-farm conditions. It is to the prevention of this transpiration that much investigation must be given by future students of dry-farming.


Transpiration

As water evaporates through the breathing-pores from the leaves it necessarily follows that a demand is made upon the lower portions of the plant for more water. The effect of the loss of water is felt throughout the whole plant and is, undoubtedly, one of the chief causes of the absorption of water from the soil. As evaporation is diminished the amount of water that enters the plants is also diminished. Yet transpiration appears to be a process wholly necessary for plant life. The question is, simply, to what extent it may be diminished without injuring plant growth. Many students believe that the carbon assimilation of the plant, which is fundamentally important in plant growth, cannot be continued unless there is a steady stream of water passing through the plant and then evaporating from the leaves.

Of one thing we are fairly sure: if the upward stream of water is wholly stopped for even a few hours, the plant is likely to be so severely injured as to be greatly handicapped in its future growth.

Botanical authorities agree that transpiration is of value to plant growth, first, because it helps to distribute the mineral nutrients necessary for plant growth uniformly throughout the plant; secondly, because it permits an active assimilation of the carbon by the leaves; thirdly, because it is not unlikely that the heat required to evaporate water, in large part taken from the plant itself, prevents the plant from being overheated. This last mentioned value of transpiration is especially important in dry-farm districts, where, during the summer, the heat is often intense. Fourthly, transpiration apparently influences plant growth and development in a number of ways not yet clearly understood.


Conditions influencing transpiration

In general, the conditions that determine the evaporation of water from the leaves are the same as those that favor the direct evaporation of water from soils, although there seems to be something in the life process of the plant, a physiological factor, which permits or prevents the ordinary water-dissipating factors from exercising their full powers. That the evaporation of water from the soil or from a free water surface is not the same as that from plant leaves may be shown in a general way from the fact that the amount of water transpired from a given area of leaf surface may be very much larger or very much smaller than that evaporated from an equal surface of free water exposed to the same conditions. It is further shown by the fact that whereas evaporation from a free water surface goes on with little or no interruption throughout the twenty-four hours of the day, transpiration is virtually at a standstill at night even though the conditions for the rapid evaporation from a free water surface are present.

Some of the conditions influencing the transpiration may be enumerated as follows:--

First, transpiration is influenced by the relative humidity. In dry air, under otherwise similar conditions, plants transpire more water than in moist air though it is to be noted that even when the atmosphere is fully saturated, so that no water evaporates from a free water surface, the transpiration of plants still continues in a small degree. This is explained by the observation that since the life process of a plant produces a certain amount of heat, the plant is always warmer than the surrounding air and that transpiration into an atmosphere fully charged with water vapor is consequently made possible. The fact that transpiration is greater under a low relative humidity is of greatest importance to the dry-farmer who has to contend with the dry atmosphere.

Second, transpiration increases with the increase in temperature; that is, under conditions otherwise the same, transpiration is more rapid on a warm day than on a cold one. The temperature increase of itself, however, is not sufficient to cause transpiration.

Third, transpiration increases with the increase of air currents, which is to say, that on a windy day transpiration is much more rapid than on a quiet day.

Fourth, transpiration increases with the increase of direct sunlight. It is an interesting observation that even with the same relative humidity, temperature, and wind, transpiration is reduced to a minimum during the night and increases manyfold during the day when direct sunlight is available. This condition is again to be noted by the dry-farmer, for the dry-farm districts are characterized by an abundance of sunshine.

Fifth, transpiration is decreased by the presence in the soil- water of large quantities of the substances which the plant needs for its food material. This will be discussed more fully in the next section.

Sixth, any mechanical vibration of the plant seems to have some effect upon the transpiration. At times it is increased and at times it is decreased by such mechanical disturbance.

Seventh, transpiration varies also with the age of the plant. In the young plant it is comparatively small. Just before blooming it is very much larger and in time of bloom it is the largest in the history of the plant. As the plant grows older transpiration diminishes, and finally at the ripening stage it almost ceases.

Eighth, transpiration varies greatly with the crop. Not all plants take water from the soil at the same rate. Very little is as yet known about the relative water requirements of crops on the basis of transpiration. As an illustration, MacDougall has reported that sagebrush uses about one fourth as much water as a tomato plant. Even greater differences exist between other plants. This is one of the interesting subjects yet to be investigated by those who are engaged in the reclamation of dry-farm districts. Moreover, the same crop grown under different conditions varies in its rate of transpiration. For instance, plants grown for some time under arid conditions greatly modify their rate of transpiration, as shown by Spalding, who reports that a plant reared under humid conditions gave off 3.7 times as much water as the same plant reared under arid conditions. This very interesting observation tends to confirm the view commonly held that plants grown under arid conditions will gradually adapt themselves to the prevailing conditions, and in spite of the greater water dissipating conditions will live with the expenditure of less water than would be the case under humid conditions. Further, Sorauer found, many years ago, that different varieties of the same crop possess very different rates of transpiration. This also is an interesting subject that should be more fully investigated in the future.

Ninth, the vigor of growth of a crop appears to have a strong influence on transpiration. It does not follow, however, that the more vigorously a crop grows, the more rapidly does it transpire water, for it is well known that the most luxuriant plant growth occurs in the tropics, where the transpiration is exceedingly low. It seems to be true that under the same conditions, plants that grow most vigorously tend to use proportionately the smallest amount of water.

Tenth, the root system--its depth and manner of growth--influences the rate of transpiration. The more vigorous and extensive the root system, the more rapidly can water be secured from the soil by the plant.

The conditions above enumerated as influencing transpiration are nearly all of a physical character, and it must not be forgotten that they may all be annulled or changed by a physiological regulation. It must be admitted that the subject of transpiration is yet poorly understood, though it is one of the most important subjects in its applications to plant production in localities where water is scaree. It should also be noted that nearly all of the above conditions influencing transpiration are beyond the control of the farmer. The one that seems most readily controlled in ordinary agricultural practice will be discussed in the following section.


Plant-food and transpiration

It has been observed repeatedly by students of transpiration that the amount of water which actually evaporates from the leaves is varied materially by the substances held in solution by the soil-water. That is, transpiration depends upon the nature and concentration of soil solution. This fact, though not commonly applied even at the present time, has really been known for a very long time. Woodward, in 1699, observed that the amount of water transpired by a plant growing in rain water was 192.3 grams; in spring water, 163.6 grams, and in water from the River Thames, 159.5 grams; that is, the amount of water transpired by the plant in the comparatively pure rain water was nearly 20 per cent higher than that used by the plant growing in the notoriously impure water of the River Thames. Sachs, in 1859, carried on an elaborate series of experiments on transpiration in which he showed that the addition of potassium nitrate, ammonium sulphate or common salt to the solution in which plants grew reduced the transpiration; in fact, the reduction was large, varying from 10 to 75 per cent. This was confirmed by a number of later workers, among them, for instance, Buergerstein, who, in 1875, showed that whenever acids were added to a soil or to water in which plants are growing, the transpiration is increased greatly; but when alkalies of any kind are added, transpiration decreases. This is of special interest in the development of dry-farming, since dry-farm soils, as a rule, contain more substances that may be classed as alkalies than do soils maintained under humid conditions. Sour soils are very characteristic of districts where the rainfall is abundant; the vegetation growing on such soils transpires excessively and the crops are consequently more subject to drouth.

The investigators of almost a generation ago also determined beyond question that whenever a complete nutrient solution is presented to plants, that is, a solution containing all the necessary plant-foods in the proper proportions, the transpiration is reduced immensely. It is not necessary that the plant-foods should be presented in a water solution in order to effect this reduction in transpiration; if they are added to the soil on which plants are growing, the same effect will result. The addition of commercial fertilizers to the soil will therefore diminish transpiration. It was further discovered nearly half a century ago that similar plants growing on different soils evaporate different amounts of water from their leaves; this difference, undoubtedly, is due to the conditions in the fertility of the soils, for the more fertile a soil is, the richer will the soil-water be in the necessary plant-foods. The principle that transpiration or the evaporation of water from the plants depends on the nature and concentration of the soil solution is of far-reaching importance in the development of a rational practice of dry-farming.


Transpiration for a pound of dry matter

Is plant growth proportional to transpiration? Do plants that evaporate much water grow more rapidly than those that evaporate less? These questions arose very early in the period characterized by an active study of transpiration. If varying the transpiration varies the growth, there would be no special advantage in reducing the transpiration. From an economic point of view the important question is this: Does the plant when its rate of transpiration is reduced still grow with the same vigor? If that be the case, then every effort should be made by the farmer to control and to diminish the rate of transpiration.

One of the very earliest experiments on transpiration, conducted by Woodward in 1699, showed that it required less water to produce a pound of dry matter if the soil solution were of the proper concentration and contained the elements necessary for plant growth. Little more was done to answer the above questions for over one hundred and fifty years. Perhaps the question was not even asked during this period, for scientific agriculture was just coming into being in countries where the rainfall was abundant. However, Tschaplowitz, in 1878, investigated the subject and found that the increase in dry matter is greatest when the transpiration is the smallest. Sorauer, in researches conducted from 1880 to 1882, determined with almost absolute certainty that less water is required to produce a pound of dry matter when the soil is fertilized than when it is not fertilized. Moreover, he observed that the enriching of the soil solution by the addition of artificial fertilizers enabled the plant to produce dry matter with less water. He further found that if a soil is properly tilled so as to set free plant-food and in that way to enrich the soil solution the water-cost of dry plant substance is decreased. Hellriegel, in 1883, confirmed this law and laid down the law that poor plant nutrition increases the water-cost of every pound of dry matter produced. It was about this time that the Rothamsted Experiment Station reported that its experiments had shown that during periods of drouth the well-tilled and well-fertilized fields yielded good crops, while the unfertilized fields yielded poor crops or crop failures--indicating thereby, since rainfall was the critical factor, that the fertility of the soil is important in determining whether or not with a small amount of water a good crop can be produced. Pagnoul, working in 1895 with fescue grass, arrived at the same conclusion. On a poor clay soil it required 1109 pounds of water to produce one pound of dry matter, while on a rich calcareous soil only 574 pounds were required. Gardner of the United States Department of Agriculture, Bureau of Soils, working in 1908, on the manuring of soils, came to the conclusion that the more fertile the soil the less water is required to produce a pound of dry matter. He incidentally called attention to the fact that in countries of limited rainfall this might be a very important principle to apply in crop production. Hopkins in his study of the soils of Illinois has repeatedly observed, in connection with certain soils, that where the land is kept fertile, injury from drouth is not common, implying thereby that fertile soils will produce dry matter at a lower water-cost. The most recent experiments on this subject, conducted by the Utah Station, confirm these conclusions. The experiments, which covered several years, were conducted in pots filled with different soils. On a soil, naturally fertile, 908 pounds of water were transpired for each pound of dry matter (corn) produced; by adding to this soil an ordinary dressing of manure' this was reduced to 613 pounds, and by adding a small amount of sodium nitrate it was reduced to 585 pounds. If so large a reduction could be secured in practice, it would seem to justify the use of commercial fertilizers in years when the dry-farm year opens with little water stored in the soil. Similar results, as will be shown below, were obtained by the use of various cultural methods. It may therefore, be stated as a law, that any cultural treatment which enables the soil-water to acquire larger quantities of plant-food also enables the plant to produce dry matter with the use of a smaller amount of water. In dry-farming, where the limiting factor is water, this principle must he emphasized in every cultural operation.


Methods of controlling transpiration

It would appear that at present the only means possessed by the farmer for controlling transpiration and making possible maximum crops with the minimum amount of water in a properly tilled soil is to keep the soil as fertile as is possible. In the light of this principle the practices already recommended for the storing of water and for the prevention of the direct evaporation of water from the soil are again emphasized. Deep and frequent plowing, preferably in the fall so that the weathering of the winter may be felt deeply and strongly, is of first importance in liberating plant-food. Cultivation which has been recommended for the prevention of the direct evaporation of water is of itself an effective factor in setting free plant-food and thus in reducing the amount of water required by plants. The experiments at the Utah Station, already referred to, bring out very strikingly the value of cultivation in reducing the transpiration. For instance, in a series of experiments the following results were obtained. On a sandy loam, not cultivated, 603 pounds of water were transpired to produce one pound of dry matter of corn; on the same soil, cultivated, only 252 pounds were required. On a clay loam, not cultivated, 535 pounds of water were transpired for each pound of dry matter, whereas on the cultivated soil only 428 pounds were necessary. On a clay soil, not cultivated, 753 pounds of water were transpired for each pound of dry matter; on the cultivated soil, only 582 pounds. The farmer who faithfully cultivates the soil throughout the summer and after every rain has therefore the satisfaction of knowing that he is accomplishing two very important things: he is keeping the moisture in the soil, and he is making it possible for good crops to be grown with much less water than would otherwise be required. Even in the case of a peculiar soil on which ordinary cultivation did not reduce the direct evaporation, the effect upon the transpiration was very marked. On the soil which was not cultivated, 451 pounds of water were required to produce one pound of dry matter (corn), while on the cultivated soils, though the direct evaporation was no smaller, the number of pounds of water for each pound of dry substance was as low as 265.

One of the chief values of fallowing lies in the liberation of the plant-food during the fallow year, which reduces the quantity of water required the next year for the full growth of crops. The Utah experiments to which reference has already been made show the effect of the previous soil treatment upon the water requirements of crops. One half of the three types of soil had been cropped for three successive years, while the other half had been left bare. During the fourth year both halves were planted to corn. For the sandy loam it was found that, on the part that had been cropped previously, 659 pounds of water were required for each pound of dry matter produced, while on the part that had been bare only 573 pounds were required. For the clay loam 889 pounds on the cropped part and 550 on the previously bare part were required for each pound of dry matter. For the clay 7466 pounds on the cropped part and 1739 pounds on the previously bare part were required for each pound of dry matter. These results teach clearly and emphatically that the fertile condition of the soil induced by fallowing makes it possible to produce dry matter with a smaller amount of water than can be done on soils that are cropped continuously. The beneficial effects of fallowing are therefore clearly twofold: to store the moisture of two seasons for the use of one crop; and to set free fertility to enable the plant to grow with the least amount of water. It is not yet fully understood what changes occur in fallowing to give the soil the fertility which reduces the water needs of the plant. The researches of Atkinson in Montana, Stewart and Graves in Utah, and Jensen in South Dakota make it seem probable that the formation of nitrates plays an important part in the whole process. If a soil is of such a nature that neither careful, deep plowing at the right time nor constant crust cultivation are sufficient to set free an abundance of plant-food, it may be necessary to apply manures or commercial fertilizers to the soil. While the question of restoring soil fertility has not yet come to be a leading one in dry-farming, yet in view of what has been said in this chapter it is not impossible that the time will come when the farmers must give primary attention to soil fertility in addition to the storing and conservation of soil-moisture. The fertilizing of lands with proper plant-foods, as shown in the last sections, tends to check transpiration and makes possible the production of dry matter at the lowest water-cost.

The recent practice in practically all dry-farm districts, at least in the intermountain and far West, to use the header for harvesting bears directly upon the subject considered in this chapter. The high stubble which remains contains much valuable plant-food, often gathered many feet below the surface by the plant roots. When this stubble is plowed under there is a valuable addition of the plant-food to the upper soil. Further, as the stubble decays, acid substances are produced that act upon the soil grains to set free the plant-food locked up in them. The plowing under of stubble is therefore of great value to the dry-farmer. The plowing under of any other organic substance has the same effect. In both cases fertility is concentrated near the surface, which dissolves in the soil-water and enables the crop to mature with the Ieast quantity of water.

The lesson then to be learned from this chapter is, that it is not aufficient for the dry-farmer to store an abundance of water in the soil and to prevent that water from evaporating directly from the soil; but the soil must be kept in such a state of high fertility that plants are enabled to utilize the stored moisture in the most economical manner. Water storage, the prevention of evaporation, and the maintenance of soil fertility go hand in hand in the development of a successful system of farming without irrigation.