Popular Science Monthly/Volume 79/December 1911/The Water Relations of Desert Plants
|THE WATER RELATIONS OF DESERT PLANTS|
By Dr. D. T. MacDOUGAL
DESERT BOTANICAL LABORATORY, TUCSON, ARIZ.
WHILE working as a student in the laboratory of Professor 0. P. Jenkins at DePauw University twenty-three years ago, a typewritten schedule of experiments in plant physiology by Professor J. C. Arthur was placed in my hands as a guide in some practical work that was to extend throughout the collegiate year. The program in question probably constituted the first attempt of its kind in an American school, and its series of demonstrations may be taken to represent with fair accuracy the concepts and assumptions which might be safely presented to a student at that time.
Sachs and his students had made contributions of immense importance in growth, organogeny, irritability and tropisms in general, but the first serious efforts at analysis of the physical phenomena underlying the action of organisms may be assigned to Pfeffer and DeVries. Pfeffer established the principles of osmosis by a study of the behavior of crystalloidal substances toward membranes, the results of which were published in 1877, and in the same year DeVries brought out his contributions on turgidity. To the latter we owe the first systematic analysis of turgor, and of the mechanism by which the rigidity and firmness of soft-bodied organisms are maintained and by which movements are executed. The plasmolytic method for the detection of the differential action of substances and membranes, and the establishment of the principle of isotonic coefficients were also the work of DeVries. Both of these authors were intent on finding the solution of problems in plant physiology, in which they were notably successful, but their results form the basis of the dissociation theory of Arrhenius, and theory of pressure in solutions of van't Hoff, which together may be regarded as the basis of modern physics and chemistry.
It seems highly characteristic of research in plant physiology that devotion to many of its problems may lead the student far afield from botany, or the stricter domain of biology. The worker in this subject frequently finds it necessary to build cantilever bridges across chasms which yawn in front of him to find that the farther ends of his spans comes down to the solid ground of chemistry, physics, climatology or geology. At present, however, he has come upon rifts which he can not cross without aid from the farther side.
The conclusions of DeVries and Pfeffer, impressive as to their inclusiveness and with some of their applications ranging far ahead of the science of the time-yielded methods of practical calibration of a large number of biological processes and set physiology in the way of becoming an exact science. The water-relations of the organism have always stood out as a subject of great importance, and as the main aspects are presented with less complication in plants, where the essential features are not complicated by a circulatory system, it has naturally followed that the principal contributions have been made by workers who attacked the problems involved from a botanical point of view.
Osmotic action, being earliest and best known, has had thrown upon it the entire burden of the explanation of the water-relations, and all of the mechanical action of the organism which might in any manner be attributed to pressures originating by the action of electrolytes. One contemplates departures from it, as set out in text-books, with regret; but some very substantial modifications of our conceptions with regard to these matters are long overdue.
The simpler phenomena of swelling and of changes of form due to the imbibition action of wood, starch and other material in a colloidal condition found place even in my preliminary directions for work: it was well recognized, however, that secretion, excretion and the accumulation of water anywhere in an organism were not fully comprehensible on the theory of osmotic action, and I can still recall that while trying out the simple tests in plant physiology which had been outlined for me, and which were calculated to give an encouraging sense of sufficiency to the student, the professor of biology was leading us into a consideration of the action of the epithelial cells and of other tissues which presented many features not explainable by osmosis. However much this inadequacy may have impressed my teacher, candor compels me to say that it did not bear too poignantly upon me. and I was willing to leave these as well as many other troublesome things to such all-embracing causes as "special physiological action" or any other convenient bogie, as being entirely too mysterious for a beginner.
Osmosis has indeed brought us far, and the briefest review will demonstrate the tremendous strides that have been made by its application. Our conceptions of turgidity and of processes which depend directly upon cell-pressures are so well-established as to be subject to but slight possible modification. It is not so, however, with many other phases of the physiology of the cell. The greater mass of an organism is colloidal, complex as to constitution, diverse as to reaction to acids, alkalies and electrolytes in general, and lastly having highly specific inter-actions among its constituents. It is bodies or masses of this kind that are to be dealt with when considering the action and morphology of the chromosome, chlorophyll bodies and cell-organs in general, as well as the nature and action of the membranes of the living organism The opinion is hazarded that further advances in cell-mechanics will await some more definite physical knowledge of the colloidal bodies whose evolutions and involutions are the center of the morphological interest in cytology. A systematization of the water-relations of these bodies, and of the changed qualities resulting from contact and action of other cell-constituents is demanded: determination of chemical structure is of ultimate importance, but not so immediately necessary to the physiologist, who would now welcome a return from the chemist and physicist of the service rendered them earlier by botanists.
The water-relations, now as earlier, hold the center of the stage in physiology, especially in plants. In a final analysis it might be truly said that it is to the immanence of this subject that the establishment of the Desert Laboratory is due. It may be profitable to discuss some of the problems which present themselves to those of us whose activities center at that institution, and to take a glance at the living material which has developed under water-conditions quite unlike those of this and other regions with a moist climate. I am confident that I speak with the concurrence of my colleagues when I say that whatever results of importance we may have accomplished must be attributed largely to the living plants available for our work and the environmental conditions which furnish a background for our experimentation.
If organic response to environic factors is to be taken as a potent means to evolution some striking features might be expected in the southwestern deserts; and when one looks up and down the slopes of Tumamoc hill, or across the washes to the bajadas of the Tucson mountains, types of vegetation not seen in regions with more moisture are seen everywhere. Furthermore, it needs only a brief acquaintance with the desert to know that the animals which find food and shelter in this vegetation show structures and habits equally pronounced.
Two general types of plants may be seen away from the streamways: one comprises species of annuals and perennials with retarded stems, branches reduced to spines, small, narrow, hardened and waterproofed leaves, which send their roots to only a moderate depth in the soil occupying a kettle-shaped mass, being of the generalized type of Cannon. It may be explained at this point that the moisture of desert soils available to plants is in the more superficial layers which are wetted by the rains. The spinose plants now under discussion contain a very small proportion of water: their bodies are hard, with a minimum of development of cortex or pith, and they hold only a small amount of sap in the protoplasts or suspension colloids of the cells. This juice, however, is characterized by the fact that it generally contains a very large proportion of salts or compounds which exert osmotic pressure. The state of the cells may be determined by the use of plasmolytic methods in which the strength of a solution, such as cane sugar or potassium nitrate, which will balance the solution in the cell is measured, or by extracting a certain measured amount of living material which has been crushed with distilled water and after the freezing point of this extract has been found the original pressure may be calculated. The simpler process of squeezing out sap and testing its freezing point can not be used in a large number of instances since the highest pressures that can be applied fail to bring out the scanty sap from some species. The use of such methods at the Desert Laboratory demonstrates that the leaves of the creosote bush (Covillea, or Larrea) (Fig. 1) may have
Fig. 1. Covillea (Larrea), the Creosote Bush, the most widely distributed Shrub with Restricted Surfaces in American Deserts. The leaves show osmotic pressures equivalent to over 75 atmospheres.
osmotic pressures of 75 atmospheres, the upper parts of the stems 35 to 60 atmospheres, and the basal portions of the stems 35 to 50 atmospheres. Fitting, by the use of plasmolytic methods on plants in the Algerian deserts at Biskra, found pressures in leaves of plants of this type of over a hundred atmospheres. These pressures would support a column of water 250 to 300 feet high.
It is notable that plants of this type are constantly in absorbent contact with the soil, and apparently continue to derive some water from it even in the driest times, as evinced by the fact that they wilt quickly when taken up. Such forms are very difficult to transplant. A misapprehension as to the influence of concentration of sap upon transpiration has long been current, and I must plead guilty to some participation in statements tending to perpetuate the mistake. It may be easily found, however, that even the maximum pressures noted above would not retard transpiration as much as ten per cent, from that which would take place with a sap of distilled water. One of my reviewers has recently made a variation of this mistake in suggesting that acidity would have a retarding effect on water loss. No foundation exists for such a supposition.
Almost any ordinary branching plant with broad leaves will, if forced to carry out its development under arid conditions, show some of the features of the type of desert plants described, and it is customary to assume that the causal conditions responsible for such forms are the desert factors: that we have here a direct adaptation or environic response which has become heritable in the strictest and fullest sense. This is a matter that deserves the fullest consideration. Meanwhile it will be perfectly safe to assume that such spinose forms represent the simplest or most elementary specializations of desert plants, and species with the most diverse morphological constitution may show alterations of this character. The sclerophylls of the American desert include species of Prosopis, Acacia, Calliandra, Parkinsonia, Cercidium, Olneya of the leguminous plants, Covillea and Zizyphus of the Zygophyllaceæ, Fouqueria, Lycium, Koehberlinia, Condalia, Manzanita, Franseria, Jatropha, Sapindus, Vauquelinia, Quercus, Aster and others.
Southwestern America has been arid for an extremely long period, not uniformly so, however. The researches of Professor Ellsworth Huntington, in which evidence has been obtained from ruins of structures built by man, of geological terraces, lake beds, strands and drainage lines in Central Asia, Palestine, and America, and also by the examination of the structure of the big trees of California, seem to justify the conclusion that variations in climate with regard to temperature and moisture have taken place within the last two thousand years that would be of profound biological importance. It seems fair to assume that similar oscillations, each movement of which might extend over a few hundred years, have taken place previously.
It is under these conditions therefore that we are to think of the evolution of the desert vegetation of the southwest, and present knowledge compels us to believe that much of it originated somewhere within the limits of the region which is arid at the present time. Perhaps the most important constitutents of this indigenous specialized flora are the cacti, which must have originated somewhere in the Mexican highlands in the Tertiary or later. This group is known to contain over a thousand species, and now extends through South America, its distribution offering some most highly localized occurrences of species. So rapid has been its evolution, and so wide the amplitude of its departures from the prototypes, that the relationship of the group is very difficult to determine.
Chief interest in the present connection lies in the fact that in the evolutionary movement the members of the group have undergone all of the specializations of the spinose forms in addition to a number of others of even more sweeping morphological importance. Stems have been reduced and branching restricted: leaves are retained by some; in others, such as the prickly pears, they appear only as rudiments dropping off before maturity, while in others, such as the great melon cacti and the sahuaro, they are not visibly represented at all. So far does the general reduction go in the Echinocacti or melon cacti that the adult plant consists of a short stem, a few inches, or at most less than two yards high, unbranched, and bearing only two types of spines which may be taken to represent the rudiments of atrophied organs, or specialized organs, largely according to the morphological prejudices of the observer. These plants represent the climax of specialization to desert conditions and the end result of the influence of aridity on the development of land vegetation.
In these succulents which constitute the highest group of desert plants, the cortex and medulla of the stems are exaggerated to an enormous extent and the greater bulk of the plant consists of a parenchymatous tissue with mucilaginous cell-contents, which gives to the
Fig. 2. A Group of Echinocacti and Ibervillea, isolated for Determination of the Rate of Water Loss. Succulents of this type have osmotic pressures of less than 12 atmospheres and absorb water only from soils containing large proportions of moisture. Some of these individuals have been without an external supply for thirty-eight months.
plant the physical qualities of a huge roll of jelly. The comparison may be made more inclusive, however. As the spinose plants have a sap high in electrolytes, mineral salts, or of substances showing osmotic activity, so these plants are rich in suspension colloids, and simulate a mass of gelatine capable of taking in and holding great quantities of water. The most sketchy knowledge of the colloids prepares one to learn that the sap of these plants shows a very low osmotic pressure under ordinary conditions of growth. The melon cacti of Arizona have a drinkable sap which shows but 3 to 5 atmospheres of pressure, the
great tree cactus with its mucilaginous juice varies from 7 to 10 atmospheres and the opuntias (cylindrical) as high as 10 to 12 atmospheres (Fig. 2.) These values are to be contrasted with those given above for the spinose forms, which are seven to thirty times as great and with such ordinary broad-leaved shrubs as. the lilac, in which pressures from 20 to 30 atmospheres are the rule.
These purely physical features of the succulents are correlated with habits and modes of activity widely different from those of the spinose forms. The latter penetrate the soil more deeply and are in constant absorbent contact with the soil. The succulents of southwestern deserts, without exception, have a wide-spreading root-system horizontally disposed immediately under the surface of the soil in a layer which is wetted by even a slight precipitation. An increase in moisture is the stimulus which starts the development of myriads of small absorbent rootlets and these have an absorbent capacity which results in the passage of a very large amount of water into the body of the jelly-like plant within a very brief period. (Fig. 3.)
As the rains come to an end the soil moisture soon dries to a limit in which the absorbent elements of the cacti may not act, these die and the plant stands or sits inert, anchored by the heavier roots in a soil with which it bears almost no important physiological relations until the coming of the next rainy season. It was to determine some of the features of the behavior of these plants during periods of extended deprivation of water that my own observations on the water-balance were begun in 1908 and are still being continued.
It is pertinent to say at this point that the halophytes, fleshy plants of seashores and saline areas, are not succulents in the present meaning of the term. These forms contain a large proportion of water, but it is held at high pressures (Cakile as high as 50 atmospheres, according to Lloyd), their transpiration rate corresponds with the proportion of water which they contain and water loss is consequently rapid, and as a further consequence they wilt quickly. An interesting capacity to vary the pressure of the sap in the absorbing organs has been found by English botanists.
The transpiration or loss of water from leaves or green organs of a plant may be roughly compared to the drying out and shrinkage of drops of wet gelatine; but with the modification that comes from the enclosure of the gelatine in small capsules arranged inside a chamber whose bounding walls are fairly water-proof, but which have ventilating openings, hundreds of them to the square millimeter. It would be as if a room were piled full of parchment bags distended with thin mucilage; the walls of the bags would undoubtedly be wet and water vapor would be constantly given off into the air-spaces at a rate very little affected by the composition of the water with which the mucilage is moistened. Furthermore, if the windows were open, the water vapor would be carried out and the total amount remaining lessened constantly.
The accentuated conditions at the Desert Laboratory have been favorable for the observation of a phase of transpiration which has been noted there for the first time. The transpiration of a leaf increases with the rising sun in the morning and the rate is accelerated until sometime in the forenoon, when, with all of the atmospheric factors at an intensity that would facilitate water loss, the rate suddenly drops, with the stomata still open. The theoretical explanation offered for this break by Professor Livingston would assume that the outer walls of the jelly-like cells are coated with a film of water from which evaporation takes place and which is constantly supplied from the cell. When the evaporating power of the air causes a loss in excess of the rate at which the film may be renewed from the cell, the film breaks, and evaporation now may take place from the interstices of the walls only. If the wall of the cell were supposed to be of brick laid in mortar and coated with plaster, the plaster would correspond to the film and the mortar between the bricks to the water from which evaporation could take place after this "incipient drying," as it has been termed, has taken place.
Excessive water loss may proceed with or without the breaking of the film to a point where the turgidity or pressure of the cells is lessened, with the result that the leaf wilts. The wilting point is not a constant, but is mainly the product of the retentivity of the soil and of the evaporating power of the air, both of which may vary widely. The evaporating action of the air may be calibrated exactly at any time, and it is proposed by Professor Livingston that the standard of wilting point for a test species might be one of the most valuable expressions of the agricultural value of a soil.
The action of stomata inevitably comes up in any consideration of transpiration: the beautifully regular structure of these organs, and their delicate action, have led to some extremely fanciful interpretations of their self-regulatory mechanism. Time suffices only to say that the condition of the stomatal openings concerns not only transpiration but also photosynthesis and respiration, and any scheme of automatism for action in response to any one of these processes would at times be highly detrimental to the other functions. Of recent contributions to the physiology of these organs, Lloyd's consideration of the manner in which carbohydrates are drawn into the guard cells and are concerned in the making or loss of turgidity, and also his method of determination of the actual state of the stomata on a leaf at any moment by instantaneous fixation of a strip of detached epidermis must be reckoned to be of great importance. Frances Darwin has recently devised a porometer which measures the rate at which air may be pulled through a leaf from one surface to the other, thus obtaining a basis for the calculation of the average condition of the stomatal openings. Such refinement of methods and perfection of apparatus will permit a much more accurate calibration of leaf action than has been possible hitherto.
The enormous accumulations of water in the bodies of cacti and other succulents raise questions as to the part such liquid may play in the life of the plant and some observations to test the matter were begun in 1908. An afternoon in October, 1909, was spent in felling and cutting up a tree cactus (Carnegiea or Cerus giganteus), near the Desert Laboratory, which consisted of a single cylindrical trunk 18 feet in height. The total weight was nearly a ton, and a section was found to contain over 91 per cent, of water, showing that the entire plant held over seventeen hundred pounds of water, or about five barrels.
It has previously been pointed out that during the dry season these plants sustain only an anchorage relation with the soil, and that absorption ceases wholly. The experiments were therefore planned to detach a number of individuals of the sahuaro (Carnegiea), the melon cactus (Echinocactus), and various opuntias from the soil, place them on suitable supports in the accustomed upright position and thus simply lengthen the dry seasons to which they had been subject. Accidents in nature tear man) individuals loose from the soil and they may not be able to perfect a new root-system for many months, so that the observations closely simulated happenings in the history of the species involved. Some of the test plants were placed in the open air, some in the more equable conditions of a well lighted laboratory room, a few were kept for periods of a few months in constant temperature dark room, and others were exposed to the full blaze of the Arizona sunlight, standing on a base of black volcanic rock, thereby avoiding none of the desiccating effects of the climate.
The formidable armature of the bulky bodies of these plants made their manipulation a matter of some difficulty and discomfort even with the best supports and harness that could be devised. The larger ones were placed on platform scales, where they were allowed to remain undisturbed. The majority, however, were mounted and two or three men were necessary to handle them in the weighings which were made at intervals correlated with the season and the rate of loss.
All individuals showed a high rate of loss when first taken from the soil, the excess being attributed to the evaporation from abraded surfaces of the roots and stems. Next it was found, of course, that the rate of loss was least during the cooler season, at which time an Echinocactus might lose as little as one forty-thousandth of its weight in a day, and on the other hand during many days in the hot dry season the daily loss was one three-hundredth of its total weight. The minimum of the tree cactus was one nine-thousandth of its total and the maximum was about that of the Echinocactus, although not measured under equivalent conditions.
Chief interest in the rate of loss, however, centers about the behavior of these plants from season to season, especially when the amount of water on hand was taken into account. In work of this kind it is found convenient to use a standard of succulency which calculates the number of c.c. of water to 100 sq. cm. of surface. Thus a great melon cactus weighing nearly a hundred pounds had a succulence of 3 on the scale noted, in which condition it transpired water at the rate of 10 g. daily. A year later the succulence had fallen but slightly, being 2.8; the rate of transpiration, however, had decreased to one half, being now but 5 g. daily.
The theoretical explanation of the sudden drop in daily transpiration given above will not suffice for this case which is a comparison of successive seasons. The slowness of the rate of loss would allow ample time for the diffusion from the great water-balance of the plant to take up a deficiency at any given surface. Morphological alterations are not found, and the theoretical explanation that presents itself would be that the colloidal condition of the walls, or inner membranes had been altered. The altered concentration of the cell-sap with its included acids and other substances might well be responsible for a change similar to that which takes place on the surface of a plate of jelly when acted upon by various reagents. A second phase of interest in the acids of the sap was found in their daily variations. Earlier the determinations of the acidity of the sap were made rather at random, with the general result that it was seen to be not affected by progressive desiccation. Within the last few months, however, Professor H. M. Richards has gone into this matter more exactly, with the astonishing discovery that the acidity of these plants is very great in the morning and decreases steadily throughout the day until evening, when it begins to rise and continues to increase until morning. So great is the amplitude of this change that a cactus may contain four times as much acid in the morning as at sunset. It is needless to say that the problem as to the making and fate of this acid is a matter that excites the keenest interest in connection with the respiration and food-construction processes in the plant. At present the change seems to be directly dependent on the course of the temperature. It is to be recalled that the water-holding power of the cell colloids must be notably affected by this variation in the acids of the cell.
The probability of the absorption of water vapor from the air by plants of the desert is one of perennial interest, especially to those who take a sentimental view of desert life. The spines of cacti, especially the large curved and hooked ones of the Echinocacti, will take up water vapor, as has been demonstrated more than once in my work, but the very small amount of moisture thus acquired is not available to the living cells and is quickly lost when the plants are exposed to direct sunlight. The bark of the ocotillo (Fouquiera) will absorb liquid water and yield it to growing tissues, as has been found by Lloyd, the hairs of some south African succulents have been found to absorb moisture, and the fleshy beach plants will absorb either water vapor or liquid water through the leaves, especially when in a desiccated condition. Doubtless the heavy fogs of the California coast are in this way a source of supply for Cakile and other halophytes. In none of these cases, however, is the existence of the plants concerned so dependent upon atmospheric absorption
as has been found by Professor Peirce for the lichens which thrive in the fog channels leading up the valleys and over the passes along the Pacific shore.
Any study of this subject leads in the end to a consideration of the value of the great water-balances described in the life of the plant, and one naturally asks the question as to how long an individual might survive at the expense of its great accumulated supply, and what activity it may carry on while cut off from the customary supply of soil moisture. The replies to these queries vary widely with the species considered. The tree cactus may live a year or as long as two years in the open in Arizona upon its balance. Growth and reproduction are in the main inhibited, however, by any notable depletion. Sometimes, however, the death of the main trunk of a plant leaves a living branch held high in the air, and this may bloom, but this action must be due to the special stimulation of approaching death. (Fig. 4.) The melon cactus may survive one or two seasons in the open, although when given some shade individuals have been seen to live three years, carrying on some apical growth and flower formation with the usual rhythm. The prickly pears survive, grow and carry on reproduction for even longer periods.
So far as physiological usefulness is concerned, stores of water accumulated in tubers, bulbs or thickened underground organs are far more effective than thickened aerial stems or leaves in holding a water-balance available to the plant for extended periods. Ibervillea, the "guarequi" of Sonora, has a thickened stem homologous with the "Big Boot" of California, which is a relative, and it has been cited many times to illustrate observations of an individual which is still alive although detached from a supply since 1902, and has not received any notable addition since 1901. (Fig. 5.) The corms of Brodiæa form new small corms during desiccation, which are plump with the diminished supply on hand, and this process continues until the balance reaches the vanishing point in three or four years. The observations of Professor Campbell show that plants with so little external appearance of water-storage as the liver-worts may hold a balance in mucilage cells, a capacity shared by an extremely large number of species.
A review of the extensive data accumulated establishes the fact that Echinocactus may live for nearly three years at the expense of its water-balance, which may be depleted as much as 50 per cent, before death results. Carnegiea loses nearly 30 per cent, before serious results follow, although the extremely succulent seedlings of this plant may shrink to one third the original weight and still live.
Plants with a large water-balance are especially characteristic of the arid regions of southwestern United States, Mexico, some parts of South America and South Africa. It is notable that the great deserts of central Asia and Asia Minor, as well as the whole of north Africa, have but few native species of this habit. Some succulent Euphorbias are reported from India, but information concerning the occurrence of plants with a large accumulation of water in Australia is very meager. The physical causes which might be operative in inducing this habit in representatives of widely separated families are not known.