Popular Science Monthly/Volume 84/May 1914/The Measurement of Environic Factors and their Biologic Effects
|THE MEASUREMENT OF ENVIRONIC FACTORS AND THEIR BIOLOGIC EFFECTS|
DIRECTOR, DEPARTMENT OF BOTANICAL RESEARCH, CARNEGIE INSTITUTION OF WASHINGTON
THE simpler forms of plants in earlier geologic times lived in swamps and along seashores and under the equable conditions furnished did not attain anything beyond a primitive and elementary development. The chief bar to escape from the restricted moist habitats consisted in the fact that the life cycle of the plant included alternating generations in one of which, the sexual generation or gametophyte, reproduction was possible only in the presence of water. Finally, however, the spore which gave rise to this sexual generation began to germinate in place on the other generation and the resulting gametophyte was produced, and remained enclosed in the tissues of the sporophyte as it is among the seed-plants of the present day.
The domination of the sporophyte in this manner vastly increased the possibilities of evolutionary development, and when this plastic self-contained type of plant began to move out over the broad spaces of the world, all the ranges of temperature afforded by the earth's surface, as well as of moisture, illumination, concentration of the solutions in the soils, alkalinity, etc., were encountered to which to-day the manifold types of plants stand in a delicate adjustment.Water was the chief determining factor when the vegetal organism was in a separated-generation stage, and it continued to be the most potent agency in evolution and differentiation as the new combined individual moved away from the swamps and shores to the occupation of the drier slopes of valleys and mountains and finally into the most arid of deserts.
The relation of plants, and of all organisms to water, is, therefore, a fundamental one, and it is to the determination of these and other important environic relations that a large share of the attention of the department of botanical research is directed. If this conception has been properly formulated you will be prepared to receive without surprise the statement that the Desert Laboratory as the principal instrument of research of this department was not established primarily for the purpose of making studies upon desert vegetation as such.
There is no adequate foundation for a science devoted to the organisms which live in arid regions. There is no more a desert botanical science than there is a mountain astronomy. The physicist seeks and selects a place for the operation of his instruments in which observations and experiments may be carried on to the best advantage. The biologist takes one of his laboratories to the seashore and another to the desert, because here in these places organisms carry on the various processes in tissues of diverse structure and at different rates, and extended facilities for experimentation and widened angles of observation are made possible.
If to these statements as to the purpose and general scope of our researches, a few words be added as to the view-point taken as to the constitution of living matter itself, we may then profitably proceed to a discussion of the main thesis of the present paper.
Protoplasm is characterized by the fact that it includes an enormous number of compounds of carbon, oxygen and hydrogen, which sustain comparatively simple (chemical) structural relations to each other and most of which are highly unstable. These two features make possible variety and complexity in the mechanical structure and composition of the tissues of plants and animals, give opportunity for the occurrence of a multiplicity of chemical transformations in metabolism, and render all the functions of the organisms highly modifiable.
Any sense of daze we may experience from a contemplation of the number of things, or combinations in protoplasm, however, is not a logical excuse for going into the haze of vitalistic notions upon which much of the pedagogical practise and speculative writing in biological science of the present time is based. Ten, or ten million, the components of protoplasm act in accordance with a few fundamental physico-chemical laws. Complexity of composition yields in importance to the types of energy transformations displayed, and to the external expression of what are known as the biological activities as they may be modified by the environment.The plant may be profitably visualized as an upright cylinder of watery gelatine surrounded by a semi-permeable tubular casing. The lower extremity of this cylinder is ramified into roots which are in intimate contact with moist soil-particles, so that the water in the body of
the plant is practically continuous with that in the substratum. The absorption of water by the plant, or rather the movement of water from the soil into the plant is influenced by the agencies which affect diffusion, osmosis and adsorption anywhere. In other words, the properties of protoplasm, as a mixture of colloids, cause water to move from the substratum into its body. The further movements of water, such as the ascent of sap, may be taken to result similarly from physical conditions. The uppermost part of the gelatinous mass of the plant ramifies as does the lower, but in this case into flattened organs or leaves. Here, as well as everywhere, on the external surfaces of the plant evaporation of water takes place in a manner modified by the specialized character and structure of the surface as well as by the relative humidity, temperature and movements of the air. This transpiration, or loss of water from the exposed surfaces, is a process of such importance that it is impossible for the plant to maintain growth to any extent, or carry out normal development without it. Consequently it has come in for a great deal of attention during the last century. Much of the work has been of a
Fig. 3. Sectional Diagram Showing the Essentials of the Spherical Porous Cup Atmometer with Non-rain-Absorbing Device, as frequently arranged. A rubber stopper in the bottom bears besides the supply tube to the first mercury valve, a larger tube for filling. The latter has a mask 10 serve as zero point and is covered by a loose-fitting cap. Suction through the open tube at extreme left removes the air from the system and fills the whole with water. Mercury in the valves is shown as if evaporation were in progress; when rain occurs the column rises in the right-hand valve and falls in the other.
As was fully expected, the exact calibrations have yielded data upon which new conceptions have been erected and new generalizations formulated. Among these may be mentioned that of "incipient desiccation." When the water-loss from an evaporimeter and from a plant is followed throughout the course of a June day at the Desert Laboratory, it is found under certain conditions that the midday maximum of temperature is accompanied by a maximum loss of water from the instrument. When this was compared with the loss from a plant it was seen that in certain cases the increased loss of water from the latter toward the middle of the day was checked.
All other means of interpretation of this lessened water-loss, including a consideration of the partial closure of the stomata, as determined by Lloyd, failing to explain the fluctuation in the middle of the day, recourse was had to determining the amount of water actually present in the leaves at such times. This revealed a deficiency. Briefly put, water was being lost from the membranes of the plants faster than it was being supplied to them with the result that vaporization slackened. This condition was designated as incipient desiccation and, as it is not accompanied by any external indications, its discovery was taken to be of great importance, both scientifically and economically, since the efficiency of the leaf as a food-forming organ decreases notably as the incipient drying stage is reached and long before externally visible, wilting or flagging is shown. The skilful agriculturist will, therefore, irrigate his crops not when they wilt but when the proportion of water in the leaves falls below a certain point.
Still another feature of relative transpiration and incipient drying remained to be detected and measured. Evaporation, of course, tends to render heat latent and hence keep down the temperature of leaves. Variations in transpiration should therefore be accompanied by characteristic temperatures. A calorimeter for the requisite measurements was designed by Mrs. E. B. Shreve. Leaves were put into a chamber filled with turpentine which penetrated the tissues quickly and realized the temperatures at once. The temperatures were found to meet expectancies, even in the stages of incipient desiccation, where the lessened water-loss was accompanied by the development of a degree of heat which might affect the efficiency of the leaf in food-formation.
Returning to the figure of the plant as a cylindrical mass of colloids, it is to be said that the water which enters the plant at its roots does not move as in a tube directly to the upper end where it is transpired. The cylinder may in effect be enlarged or variously developed to such shape that a surplus of water accumulates and if the supply be cut off from below the amount on hand may be such that the plant lives for a season, a decade or even longer upon the water on hand. I have carried out a series of measurements upon this phase of the water relations of plants during the last six years and find that many plants of arid regions in South America, North America and Africa show such accumulation of water. The sap of such plants under normal conditions shows about the concentration which gives an osmotic pressure of three to twelve atmospheres. When the supply is cut off the loss of water continues with the result that the concentration increases four or five times. The desiccation of these plants, however, is not simply that of drying out. The rate of loss decreases much more rapidly than would be justifiable on the facts of amount of water present," and one is led to infer that the plant again to be thought of as a cylinder of jelly undergoes changes of its colloids which tend to prevent transpiration. Whether such changes are reversible as in incipient desiccation or not is a matter yet to be determined. Without going into detail at all it may be said that the continued depletion of the store of water of a succulent is responsible for many important features in the life-cycle of the plant, in growth and reproduction and in survival (see Fig. 4).
The consideration of the facts brought to light in a study of the balances or accumulations of water in plants formed a basis for an analysis of the conditions of parasitism in the higher forms. This is primarily a water-relation. When one plant as, for example, the mistletoe, is parasitic on another, such conditions must be present as to cause water to flow from the host to the tissues of the parasite carrying substances in solution. The experimentation in this subject consisted, first of all, in forming artificial parasitical relations between plants in order to obtain
Fig. 4. Graphs Showing the Water Relations of a Large Cylindrical Cactus when Separated from a Source of Supply for Extended Periods. The lower line shows the weight decreasing from 38 kg. to 28.8 kg. in five years. The upper line shows the rate of daily loss which fell from 10.6 g. to 3.5 g. during this time. The rate of loss is not directly proportional to the succulence or amount of water present.
a number of couples which might be joined as host and dependent, and also equally important to find others which might not enter into this relation.
The succulents with their accumulations of water offered suitable material and, using these as artificial hosts a number of species were caused to live on them parasitically for months or even years. Having this material, analyses of the sap or watery solutions of the two plants led, as might be expected, to the result that one plant which would live parasitically on another must have a more highly concentrated sap. Not all plants with a high concentration of sap may become parasitic on all those of low concentration, however, for other reasons, some of them seasonal, morphological, etc. (see Fig. 5).
Fig. 5. A Joint of Opuntia Blakeana Rooted Parasitically in a Cavity in the Body of Carnegiea gigantea, under which condition it has existed for three years
The difficulties in dealing with the mechanical features presented by the soil are such that it has not yet been possible to construct an instrument which would give data analogous to absorption by roots as does the evaporimeter for transpiration by leaves. Developments in this matter are to be hoped for, however. Meanwhile the studies of Dr. Cannon on root-systems and the distribution of water in the soil have yielded some generalizations of no little value in the consideration of the aspects of the vegetation of a region. Among these it is to be mentioned that the treelessness of the immense stretches of western prairie and probably of steppes everywhere is a matter dependent upon the distance below the surface at which the so-called "ground water" or "water table" lies. Trees and forests may be established in such regions when the supply of moisture in the upper layers of the soil are increased by irrigation, conservation of rainfall or whatever artificial means may be employed.
Living matter is a thermal engine in which the energy of various substances is released very slowly by oxidation processes. It is also self organizing and substances of various kinds entering into its solutions may be reduced and their components rearranged in the form of characteristic constituents and products and in turn become fuel for the engine. Many of these reducing processes are carried on in the presence of light in the plant. Potassium and calcium nitrate, for example, yield nitrites and later ammonia in such a process, and acetic, glycollic, propionic, malic, tartaric and citric acid are hroken up, yielding formaldehyde, carbon dioxide and other substances.
The greatest addition to the potential energy of the plant, however, is that in which carbon dioxide from the air enters leaves and in the ensuing process carbohydrates result and oxygen escapes. This photosynthesis is perhaps the most fundamental of all processes in the world of living things, since it is with this action initially that the construction of nearly all organic material is begun. Various theories have been proposed to account for the procedure from the entrance of carbon dioxide into the plant to the formation of sugars, but none of these will stand the test of our critical experiments. Their inadequacy may be ascribed to the fact that the function of the light in the matter is not yet clear. At present we may only say that upon the entrance of carbon dioxide into a leaf it probably unites with potassium to form the bicarbonate, in which salt it is more easily broken up than as if it remained a free acid. Here are then bicarbonate and water in the presence of chlorophyll, the green coloring substance of the plant. The spectrograph of this substance reveals the fact that rays of certain wave-lengths are absorbed by it. In other words, these rays impinging on the chlorophyll change the nature of its electronic movement and cause some disintegration of its structure. The disturbance, whatever it may be, is communicated to the bicarbonates, and to the water, which are reduced, the free oxygen escaping and some simple carbonhydrate resulting. So far we may proceed in complete harmony with known facts. Between this and the appearance of hexoses in the leaf is a wide gap. Once bridged and the full effect of light in the entire course of photosynthesis made out, it may be possible to simulate a process which now takes place only in living tissue, and make available to the race a source of energy all but limitless in its potentialities.
The formulation of plans for this work has necessitated a large number of measurements of intensity and of the reducing effect of light under various conditions and in various places in which experimentation might be carried on, by Dr. Spoehr. Among the noteworthy results of such calibration is the demonstration that the blue-violet rays, direct and skylight, is greater at the Desert Laboratory on a shoulder of the Tucson mountains (2,700 feet) sheltered from the prevailing winds and resultant dust, than on the summit of the Santa Catalina mountains (9,250 feet) in which the skylight is less, but the direct light passes through a layer of air a mile less in thickness than that which reaches the laboratory. Watery vapor and dust particles may account for some of the absorption of light on the summit of the forested mountain slopes. Our concern with light as a factor of environment however by no means ends with the part it plays in the reduction and combination processes. Its ionization effects are discernible in respiration in all of its separate stages. None of the measurements of the action of this important environic component have proven more interesting than those which have been carried out by Professor H. M. Richards and Dr. Spoehr upon the reduction of the acids in plants. Although formed and present in minute quantities in all plants, yet they accumulate and are present in such quantities in the succulent cacti that facile conditions for experimentations are found. The accumulation goes on during darkness so that at daybreak these plants may contain as much as ten times as much acid as at sunset, the diminution during the day being due to the action of light, the disintegration of the acid resulting in formaldehyde and carbon dioxide. Now growth has long been held to be directly retarded by light, it being supposed that the blue-violet rays especially exerted a fixing or destructive action on living matter which prevented growth. That such action did not take place was established by my own experiments on etiolation previous to 1903. The fact remains, however, that growth-expansions and elongations generally go on more rapidly at night than in daytime, and in the determination of the daily fluctuation of acidity we believe to have hit upon the cause of the difference in the rate of growth by day and by night.
Growth is correlated with hydratation, or increase of the water absorbing capacity and consequent swelling of living matter and cell walls in which osmotic pressure must also play a part. Acids may cause such swelling and increase, and this effect would accumulate with the increasing acidity through the night until daybreak, when light begins to break up the acids, and growth-extension would slacken. Light does, therefore, in finality, retard growth not by its action on the components of living matter as formerly supposed, but by breaking up the compounds which increase the water-absorbing power of protoplasm. The controlling environmental features in the growth and development of vegetation are water-absorption orand temperature.
Some isolated processes of plants, the course of which runs for a short time, such as the action of enzymes upon the starch, which may be accumulated in a tuber or a seed, the germination of seeds or the development of buds, which depends directly upon the hydrolysis of such food material, are found to conform fairly well with van't Hoff's rule by which the rate of activity is about doubled for every rise of 18° F. above the minimum temperature at which it begins. If the entire development of the plant could be interpreted in the same manner the task of estimating the effect of the temperature factor in environment would be a simple one. This is far from the case, however, as any change in temperature may disturb chemical equilibrium in a dozen ways.
The director began a study of this subject in 1900, and first formulated a method by which the total heat-exposure of a locality in which a plant was growing was calculated in hour-degrees, simply as the product of the number of hours the plant stood above the temperature at which growth began and of the averaged intensities during this period. The method was obviously empirical, as it assumed that the rate of growth was the same at all temperatures above its zero point, which might be freezing or above it.
Fig. 6. Graph Showing Rate of Growth of Seedlings of Wheat at Temperatures Between 40° and 108° F., plotted from data given in text-books of plant physiology.
Next, Professor B. E. Livingston, using the exponential law of chemical velocity in the interpretation of temperature effects, found that survival and distribution of some types of vegetation were explainable upon the temperature integrations arrived at in this manner. This method, however, still depends upon averages or summations of temperature and does not evaluate the higher temperatures correctly as the plant grows fifteen to twenty times as rapidly at its optimum temperature as it does within ten or fifteen degrees of its zero or minimum. The nature of the experiments upon environic effects, for which some method of temperature evaluation was necessary, demanded greater exactness, and it was finally decided that the actual activity of some plant should be used as a standard of measurement, as the effect of temperature upon growth is one in which chemical equilibrium is disturbed in a score of ways and is therefore not expressible by any single or simple formula. This will be obvious upon the inspection of the graph which shows the relation of temperature to the growth of the hypocotyle of wheat plants determined by measuring the rate at constant temperatures for 48 hours (Fig. 6).
From this it may be seen that growth of the stems takes place at a rate of about 4 to 6 mm. in 48 hours at temperatures between 40° and 65° F. Above this the rate rises precipitously until the temperature reaches 80° F., and if it becomes warmer than this a drop ensues during the next few degrees of rise, then the increase is resumed and carried until a temperature of 86.5° F. is reached. Any further rise in temperature definitely checks growth, which ceases entirely at temperatures of 108° F.
This plant was fixed upon because it is widely grown from subtropical to subarctic localities, reliable measurements have been made of its rate, and it may accompany nearly all of the experimental cultures made in our researches. It is proposed, therefore, to integrate the temperature factor in climate in terms of growth of wheat. Any other suitable species might be used as well. The scheme in brief consists in fixing upon an averaged rate of growth between 40° and 65° F. and then for the five-degree intervals up to the optimum and upper limit. The sheet in which the thermograph has made its tracing of the course of the temperature is now ruled into figures bounded by a
Fig. 7. Thermograph Chart Ruled for Integration in Terms of Rate of Growth. The areas of the separate figures are to be determined by a planimeter and multiplied by the factor expressing the rate of growth prevalent during the period covered by the figures. Record from Coastal Laboratory, June 16-23, 1913.
base line twenty-five degrees below the upper limit of the temperature to be estimated, by arcs of the curves marking the hours, and by the crooked line traced by the pen as a result of changes in temperature, as shown on the accompanying sheet (Fig. 7).
The areas of such figures for temperatures is multiplied by the factor 4.5 for temperatures between 40° and 65° F., and by 20 for temperatures between 65° and 70° F., and by 45 for temperatures
Fig. 8. Growth-values During the First Six Months of Two Years at the Coastal Laboratory, Carmel, Calif. The solid line shows temperature effects in 1912, the broken line during 1913.
between 70° and 75° F., by 70 for temperatures between 75° and 80° F., by 78 for temperatures between 80° and 86° F., the optimum, etc. Time does not suffice to mention the necessary corrections, or the studies being made for the refinement of the method, but attention may be called to the estimation of the temperature factor by this method at our two main experimental localities during the first four and six months of two years at the Desert Laboratory, and the Coastal Laboratory (Fig. 8). The facts displayed in the accompanying figure go far to explain the divergent action of species under observation in the two places. Studies are now in progress for the redetermination of the factors expressing the rate of growth under the ordinary swing of daily temperatures and for an exacter application of the results.
There is much reason to believe that in the integration of the temperatures and moisture relations by the methods outlined we will be able to identify the causation of the remarkable evolutionary departures exhibited by the beetles in Professor Tower's experiments, and steps are now being taken for the necessary analytical tests for the standardization of temperature effects in terms of protoplasmic activity.
Another phase of temperature effects—concerning energy release in protoplasm—has been studied by Professor Ellsworth Huntington, whose analysis of the records of piece-workers in factories established the fact that the least amount of work is accomplished with open air temperatures below the freezing point and in the neighborhood of zero Fahrenheit. The amount increases slowly, however, up to 28° F., then rapidly to 38°, slowly to 48° and more slowly to the optimum of 58° F., above which the rate and amount declines as the weather becomes warmer. It may seem a far cry from the growth of a wheat plant in California to the muscular action of a factory operative in New England, but both are directly dependent upon the fundamental characters of living matter, especially in its relation to temperature.
The distribution and grouping of organisms on the world's surface is conditioned by the agencies which participate in moving them from place to place and by the presence of conditions suitable for their survival and existence. If all species inhabited every place suitable for them, geography, so far as vegetation is concerned, would be a subject about which many closed chapters might be written. They do not, however, as they have not been carried to all the places in which they might survive, and secondly, the conditions comprising the environic complex are slowly but surely changing, reversibly or irreversibly, practically everywhere on all land surfaces. Under such conditions the dynamics of plant geography assumes an importance not yet realized.
So far we have discussed the results of analyses of our plantations and experimental settings. The geographer, however, needs to have defined for him the principles governing the variations in the various environmental components and of course temperature is an agency which has been drawn upon to account for some of the major features of distribution in geologic as well as present time. Methods and practises that have become conventionalized estimate temperatures by altitude and latitude, the actual data obtained by instrumentation being compiled as mean temperatures, and the averages of maxima and minima. I need but to refer to the measurements of temperature in terms of growth discussed previously to illustrate the inadecpiacy of these data for agricultural operations.
The obvious and popularly accepted assumption that low valleys are warm and that ridges are cold wind-swept habitats, arising from the conception of surface temperatures as in the main a function of altitude has been followed much too far, and the geographer who bases his generalizations on assumptions of this kind will be due to encounter some extremely disturbing anomalies, some of which have come in for examination and measurement at the Desert Laboratory by Dr. Shreve. Forested slopes and bare rocky surfaces do not lose heat at the same rate at night or warm up at the same rate in the daytime. The air cooled on the bare slopes flows down the declivity, collecting in the valleys beneath as would water. Consequently the main building of the Desert Laboratory, 400 feet above the station at the base of Tumamoc hill, is ordinarily 15° to 20° F. warmer than the plantation below at night. This exercises marked influence on the organisms inhabiting these places. In the Santa Catalina mountains in which much of our experimentation is done, this inversion of temperature and collection of cold air operates to give valleys climates equivalent in temperature to the great ridges half a vertical mile higher, in illustration of which it is cited that the cacti, characteristic of warm places, spread highest on the crests of the mountain ridges. The divergence of the temperatures from the normal rate to be expected from increase of temperature may be also illustrated by the following facts:
During the low extremes which characterized the climate of the southwest during the first month of the present year the minima were as follows:
|Desert Laboratory (2,700 feet)||17||°||F.||....|
|Breeding plantation (2,300 feet)||1||°||F.||18||°||F.|
|Xero-montane plantation (6,000 feet)||-6||°||F.||7||.6°||F.|
|Rim of Bear Canon (7,000 feet)||.5°||F.||3||°||F.|
|Montane plantation (8,000 feet)||-2||°||F.||±||.5°||F.|
If now these places are plotted on a scale in which the vertical element was magnified, but the fall in temperature was computed on the basis of one degree for about every three hundred feet in elevation, it may be seen that the 2,300 feet locality diverges 17° F. from the expectancy by reason of cold air drainage, the 6,000 feet locality is 13.6° F. colder, the 7,000 feet location 2.5° F. colder and the 8,000 feet location 2.5° F. colder. The difference between the mountain top at 8,000 feet and the Desert Laboratory as correspondent to within the limit of possible error, but between these two places ranges the vegetation from the subtropics to the pines under conditions of temperature largely influenced by the relief and orography. The facts which have been brought to light in this single thermometric traverse of a valley and up a mountain slope shows the need of extended surveys for the purpose of evaluating the temperature factor as an agency affecting the distribution of organisms, and when our generalizations can be broadly based and rationally formulated we may also be in a position to furnish the paleontologist with criteria for the better interpretation of the occurrence of the plants and animals found in ancient deposits.
The number of things to be considered in the study of the comparative effects of the different climatic complexes represented at our various plantations make necessary long extended observations coupled with analytical tests before we may hope to reach any conclusions that may be concisely expressed or widely applied.
The results obtained by the studies which have just been brought to a stage of completion of the Salton Sea furnish us with some very suggestive facts as to the influence of the substratum upon organisms. The waters of this sea when it stood at maximum level in 1907 contained about one fourth of one per cent, of dissolved material, principally salts of sodium, potassium and magnesium. The recession of the lake by evaporation left bare a new strip of strand each year, which was saturated with a solution differing from that of the preceding year by about one fifth of one per cent, in concentration with some changes in relative value of the various substances, especially the calcium and magnesium. Accompanying such conditions, an Aster, a Prosopis, a Scirpus and an Atriplex have shown variations not hitherto seen in these species. Especial interest attaches to these occurrences from the fact that there are seven species endemic below the level which the ancient body of water, Blake Sea, must have reached three or four centuries ago. In other words, these seven species are found nowhere but on the beaches of the lake and there is a strong presumption that this restricted occurrence is due to their origination in the place and that they have not spread beyond the ancient sea-bed.
The facts brought out in the foregoing discussion are presented not so much to denote actual progress in our researches as to illustrate the character of some of the problems under consideration. The inquiry as to the integrity of purpose and validity of results of such work is a question which may rightly be directed toward every project which absorbs funds and consumes the time and energy of the investigator and the worker. Something of the wider purposes and fundamental character of the problems attacked are suggested by the results and plans which have been discussed. In addition it seems necessary to say to those who mistakenly attribute a directly economic purpose to the Desert Laboratory that none of its facilities are devoted to agricultural experimentation. This is a function especially pertaining to the government, and so far as our own is concerned a function that is most efficiently carried out. It is clear, however, that the data being accumulated at the Desert Laboratory may in time constitute an important contribution to the physical and biological principles to be considered in the occupation and utilization of arid regions. When it is taken into account that the world-wide progress of civilization with its attendant extended occupancy of the surface of the earth has brought the race to a point where it must consider seriously methods for the more intensive use of the areas already occupied, and also bring into usefulness the arid areas which comprise one fifth of the total land area, the importance of this possibility may be realized.
- Formal abstract of lecture given before the trustees of the Carnegie Institution of Washington, in connection with the Annual Meeting, December 1, 1913.