Popular Science Monthly/Volume 81/July 1912/Some Features of the Root-Systems of the Desert Plants
| SOME FEATURES OF THE ROOT-SYSTEMS OF THE DESERT PLANTS |
By Dr. W. A. CANNON
DESERT LABORATORY
THE roots of the desert plants are of interest, in part because of their relation to the physiological activities of the shoot, and in part because of their own physiology. There is a close relation between the character of the roots of the desert plants and the distribution of the plants, and probably with many other activities of the plants, as, for example, the formation of the leaves, of the flowers and the taking on of new growth. What the precise relation may be between the root systems and the adaptation of desert plants to desert surroundings is not known, nor, for that matter, the relation of the roots of the plants of the more humid regions to their distribution, or to their origin. Also the special relation of the roots of plants to the substratum has not been extensively investigated, as, for example, the character of root development as related to the precise per cent, of water content, or to the temperature. The lack of quantitative experimental studies on roots in soil is to be attributed in large part to the difficulty in studying the soils. If certain activities of the roots, or the significance of root character to many features of the plants' activities, are to be understood, it will be necessary to do quantitative experimental work on plants growing in the soil, and not, as heretofore extensively done, growing under highly artificial conditions.
It is popularly supposed that the roots of the desert plants are very long—that is, that they penetrate the ground to great depths, and from this that the length of a root-system is in some way a measure of the aridity of a locality. It is difficult to say how this idea arose, which really is without adequate foundation, because a relatively small amount of work has been done on the roots of the plants either of the humid regions or of the deserts, in the field. It is probable, however, that the few excavations that have been made have been carried on in those places where it chanced that the roots penetrated to great depth. But it is in exactly these places where the most favorable moisture conditions of the given locality are to be found, namely, where the soil is deep, giving an opportunity for the penetration of water to a great depth, as in the bottoms, or along the banks of stream ways—arroyas in our southwest, ouedes in southern Algeria, or weds in the eastern Sahara. From this it is to be seen that the localities referred to, instead of being typically intense deserts, are, on the other hand, the most favorable situations as regards moisture.
It is difficult, at present, to state under what conditions the roots of the desert plants are formed, owing mainly to the lack of experimental evidence. But by a system of reasoning backwards from the mature root we can possibly picture to ourselves something of these conditions. In the first place, if we examine the root-systems of desert plants, in the field, during the season of drought, we shall find it very difficult, if not impossible, to find any portions which show vegetative activity, although it may be possible at the same time to demonstrate a certain, even if low, rate of transpiration. On the other hand, if the root-systems of the desert plants are examined during the rainy periods, there will be no difficulty whatever in finding fresh growth, new rootlets of whatever kind. But that this is not the whole story is evidenced by the fact that in winter many of the plants native to the southwest do not form new roots, or, at least, I have not been aide to find new roots. In spite of this fact, such plants as the flat opuntias do, in winter, absorb water and very promptly after rains. This is shown by the thickening of the fleshy and flat stems. It is therefore probable that a certain amount of heat as well as of moisture is required to bring about the formation of fresh roots. In addition to these two factors, there is probably another one, namely, aeration of the soil. Whether this is mainly concerned with the formation of the roots or of the position occupied by the roots in the soil is not known. It seems highly probable in certain cases, particularly in fleshy plants like the cacti and some liliaceous forms, that the amount of air in the soil must be of importance in determining the position occupied by the roots. So far as observation goes, the roots formed may be classed in at least two categories: (1) They constitute the extension of the roots previously formed and (2) they may appear on much older roots, but are of limited growth. It is supposed that in the main the greatest amount of water taken into the plant comes through the roots of the first kind, so that the place of water absorption as the roots grow, ever becomes farther from the stem, and the problem of water transportation is ever an increasingly difficult one. This last one is probably to be considered a very important matter on the desert where the evaporation rate is often very high, caused by the low relative humidity, by high temperature and by air currents. It is conceivable that, given favorable conditions, a large proportion, possibly all, of the roots of this character might remain alive, but, as a matter of fact, in desert plants, as before noted, it is difficult during the dry seasons to find any living roots of this class. As one result of this we find that the extension of the root-systems as a whole, away from the central plant axis, goes on relatively slowly, and probably most of the rootlets formed in any season perish before the close of the succeeding dry season.
As regards the second class of roots referred to, there is quite a different story to tell. These roots are apparently quite as deciduous as the leaves of many plants. It should be noted, however, that nothing has been done in an experimental way to test the longevity of these roots, and it is reasonable to suppose that under some favoring conditions they might endure, possibly becoming converted into large laterals, even if under conditions, which are the usual ones, their life is limited.
It may be well to describe the roots referred to. If we examine a root of such a desert shrub as Franseria, we shall find, along such of the roots as extend in a more or less horizontal direction from the stem of the plant, groups of filamentous rootlets. These occur at about 1 cm. intervals, in varying numbers usually about one half dozen together. They are from two to four cm. long and probably not more than one half millimeter in thickness. The rootlets appear promptly with the coming of the summer rains, and they cease their activity when the soil attains to an unbearably dry condition, as perhaps in adobe soil, 10 per cent, moisture, more or less.
The deciduous rootlets greatly increase the absorption surface without, at the same time, necessitating invasion of new root areas, or of causing a long transfer of water from the place of absorption to the stem. So far as is known, the deciduous rootlets are formed only when there is an abundance of water, and when the temperature is high. These rootlets have been seen on most of the desert shrubs, on all in the vicinity of the Desert Laboratory, and have been observed on a few of the shrubs in southern Algeria. Whether a similar kind of rootlets occurs on perennials in the more humid regions is not known to me.
The deciduous rootlets are thus of great importance to such desert plants as bear them. They appear adventitiously always, and apparently in the same place on the root year after year. In certain species it has been observed that the adventitious roots are formed precociously, but in other forms this is not the case. And again, where such rootlets are not to be found, it appears that they can not be induced.
The extension of the roots of the desert shrubs is various, perhaps in no case exceeding three or four meters. The position in the ground is also not uniform. In most instances the position occupied by the roots is characteristic for the species, but it is likely that the extension of the root-systems varies mainly with the age of the individual.
There are three main types of root-systems to be found in the shrubs of the desert plants of the southwest. (1) Root-systems which extend horizontally from the main plant axis and lie, for their whole course, near the surface of the ground. (2) Root-systems which are characterized by a strongly developed tap root going directly down to a depth determined in part by the character of the soil, in part by the penetration of the rains and in part by the character of the root itself. (3) And roots that not only reach widely, but also penetrate fairly deeply.
The superficial root-system (Type 1) is characteristic of many plants, particularly of the cacti. In some instances all of the roots except the anchoring roots, which, however, may not penetrate more than 50 cm., may not be more deeply placed than from 2 cm. to 5 cm. so that with a cane one can easily remove the root and then with little exertion can strip it from the soil to the base of the stem. Perhaps the root-system of Opuntia arbuscula (?) is the most superficial of any thus far described. In this species the ideal superficial root-system just alluded to finds complete expression. But the giant cactus 
Giant cactus (Carnegiea gigantea), A, Vertical Extension of Giant Cactus. growing in association with a creosote bush (Covillea tridentata), B. The anchoring roots of the cactus and the superficially placed absorbing roots are shown in position. It will be seen that the roots of the creosote bush, which are of the generalized type, occupy a lower position in the soil than those of the cactus. Between the surface of the soil and the dotted horizontal line is the adobe soil, here about 30 cm. in thickness. Below the dotted line is the hardpan, caliche, which is impervious to water and is not penetrated by the roots. also, although it is now reckoned among the trees, has a rootsystem which is essentially superficial. The accompanying figure gives a good idea of the position occupied in the ground by the root-system of a small giant cactus. The plant referred to was 1.2 meters high. The supporting system consisted of a stout root crown from which proceeded a few relatively slender branches, and the main absorption system consisted of long, slender branches and superficial roots which extended as far as three meters from the base of the plant. It may be said, in passing, however, that as the giant cactus becomes large, the anchoring system, sufficient in its younger stages, is no longer strong enough, and the bases of the superficial laterals increase greatly in thickness and form props by which the upright position of the cactus is maintained.
There are several plants which illustrate the pronounced forms of the tap root, among which, in southwestern Algeria, may be cited the Tamarix, and certain other small shrubs, and in our own southwest such a form as palo christi, or Christ's thorn. Zizyphus also, which occurs both in southern Algeria and in the southwestern part of the United States, has a pronounced tap root. I will refer especially to the rootsystem of palo christi (Koerberlinia spinosa). The Koerberlinia spinosa is a close-growing, spinous shrub without leaves at any stage, which grows in the bottoms, or along the sides of the bottoms, in the vicinity of Tucson. The young plants have roots which strike directly downward, giving off almost no laterals within one meter of the surface of the ground. The depth to which the tap root attains has not been determined. As the plant becomes older a sucker is sent out close to the surface, from which there springs up a daughter plant. Adventitious roots occur along the course of this sucker, particularly where the daughter shoot arises. Occasionally the connection between the daughter shoot and the mother plant is not destroyed, and the adventitious roots in that case are not very numerous nor very long. Sometimes, however, the connection between the plant and offspring is broken and the adventitious roots, or one of the adventitious roots, strike straight down and behave precisely as the main root of the parent plant. That is, in this case, as in the Zizyphus and Tamarix, the root-system is an obligate deeply penetrating one, for which reason the species is confined to such localities as provide sufficient depth of earth.
The third type of root-system, which may be called a generalized type, is such as is possessed by most of the plants growing in the vicinity of the Desert Laboratory, and in fact by most of the desert plants. Perhaps it would be clearer to state this in another way, namely, that the plants which cover the greatest area in the arid region are such as have the generalized type of root-system. It will only be necessary to refer to the root-system of the creosote bush of the southwest for an example of this type. The roots of the creosote bush extend outward from the main stem for a distance of about three meters, less in small plants, and reach downward, either directly or at an angle, to a depth which is usually determined by the character of the soil. On the mesa, where the soil is usually less than one half meter in depth, the roots of the creosote bush do not exceed that depth, but in the beds of the washes, or rather on the flood-plains of the washes, where the soil is deeper, they have been known to attain a depth of over two meters. From this it is seen that the generalized type of root-system is more flexible than either of the other two types given, and it follows, other conditions being equal, that species with the generalized type of roots may also have a wider local distribution.
It is interesting to note that the most arid portions of an arid country are the areas which are above the flood places of the washes. In the southwest these are usually the mesas. In southern Algeria, for instance, these excessively dry areas are the regs, or the hamadas. It is to lower-lying areas, washes and the flood-plains of the washes that drainage from the higher ground flows, and also where particles of soil from the higher ground are deposited through water or through wind action. And the result is that the low-lying areas have deeper soils and more water than the upland.
We therefore conclude from what has just been said that the most arid portions of such deserts as those in the southwest of the United States are on the higher lands, and the less arid portions in the lower lands—the flood-plains or the washes—and that it is only in the less arid areas that plants with pronounced tap roots occur.
It should be definitely pointed out that the foregoing classification of roots is applicable only to such deserts as that of the Tucson region, where a portion of the flora consists of plants with a water balance. In the more arid regions, such, for example, as southern Algeria, fleshy plants are almost entirely absent, and root-systems characteristic of such plants are consequently not to be found. We therefore have in the most intensely arid desert plants with two general types of root-systems only, namely, the generalized type and that form which has a well-developed tap root. In southern Algeria, for example, species of the genus Haloxylon have a modified generalized type of root-system, and this species occupies the plains—the reg or hamada—where the soil is least abundant and hence where the water relations are least favorable. In the hollows of the plains where soil has accumulated to some extent, and along the washes or oueds, we find plants with the main root especially well developed. In fact, it is only where the soil is actually or relatively deep that such forms as Tamarix, Zizyphus or other relatively large forms all having long deep roots, are to be found. From the character of the roots of plants from the plains of southern Algeria, as well as the roots of plants from southwestern United States it is to be seen, therefore, that if any type of root is entitled to be called the xerophytic type, it is the generalized form, and not the deeply penetrating tap-root form which is thus seen to be the peculiarity of plants which grow where conditions are relatively favorable.
Turning now to consider briefly the environment of the roots of desert plants, we should note, in the first place, that the root environment of these plants is not at all well understood. This, of course, comes partly from the fact, as before pointed out, that the soil is difficult to study. However, certain features of the soil, such as the water content, the temperature, and certain other features, which are best known, can be treated briefly.
As a general thing the rains of the desert do not penetrate the soil to any considerable depth. In the Tucson region, where the rainfall does not exceed 30 cm., the penetration of the ground is usually not over 50 cm., although this varies with the variation in the character of the soil. The water table usually lies so deep that the water is not available to the plants. On the mesa, in the vicinity of Tucson, for example, the water table is frequently 25 meters, or more, deep, but on the flood-plain of the Santa Cruz River, it varies from 3 to 10 meters. Under earlier conditions, which need not be described in this place, it is probable that the water table on the flood-plain of the Santa Cruz River, nearer the surface than at present, was tapped by the roots of the larger plants, for instance, the mesquite, growing there. The depth to water in other desert regions, as for instance, southern Algeria, is very variable, but usually great. For example, at the daya of Tilrempt, on the northern edge of the Sahara, the water lies between 50 and 90 meters deep, while in the vicinity of Ouargla it is frequently no deeper than 1.5 to 2 meters. In the latter case, however, the water is highly charged with salts.
The length of time that the water in available amounts remains in the soils following storms is a variable one. In the vicinity of Tucson the soils of the river flood-plain, and of Tumamoc Hill, remain moist for a period exceeding six weeks, but the deeper levels are moist for a somewhat longer period. It has been stated that in the vicinity of Tucson, at a depth of about 20 cm., the soil is sufficiently moist to be of benefit to plants throughout the year. However, it should be said that if the activities of the perennials, or of the annuals, can be taken as indicators, the period of maximum activity, which should indicate the optimum water content of the soil, is not of long duration, perhaps not exceeding six weeks, which would include the rainy season.
A relatively small amount of work has been done on the temperature of desert soils. For a period of about five years there has been kept at the Desert Laboratory a continuous temperature record at two depths—15 cm. and 30 cm. But only a relatively few observations have been made at a depth of 2.5 cm. As a general result of the soil temperature studies it can be said that at the depth of 15 cm. the greatest diurnal range, which usually occurs in March and July, is 12° F. The extreme yearly range at this depth is 73° F. In January the temperature begins to rise, and rises gradually until the last of March, when the rate becomes accelerated, so that by the last of spring the soil approaches the temperature characteristic of summer. The highest temperatures occur in July just before the midsummer rains. When the rains come the temperature falls 5° or 10°. The minimum for the year is reached in December.
Soil temperatures at the 30 cm. level are very different from those just given above. In the first place, the daily range in temperature is usually not over 2°, and the maximum not above 4°. The minimum temperature at a depth of 30 cm. occurs in March. In the first part of April the soil begins to get warm and the temperature arises until the rains of midsummer. The fall in temperature of the soil occurs during seven months of the year and the rise in temperature of the soil at this depth occurs during five months of the year.
Unfortunately the temperatures for 2.5 cm. depth have not been taken throughout the year, but are available for spring months only. So far, however, it would seem that the variations in the temperature of the soil at that depth are considerable. For example, one day in the spring the variation at the depth of 30 cm. was 3° F. At the depth of 15 cm. it was 11° F., while at the depth of 2.5 cm. it was 40° F. The greatest difference in maximum temperature at any moment was on April 15, when there was a difference of 23.5° F. between the upper two levels.
From what has been said regarding the soil temperatures it will appear at once that at any moment during the daytime the roots of the desert plants are subject to a very large temperature stress. Those roots which penetrate most deeply, where probably moisture is the greatest, are in the coldest soil, while such roots as lie near the surface of the soil, where the moisture conditions are least favorable, are in the warmest soil. We therefore have the interesting paradox that roots placed where there is the most water are not so advantageously placed, physiologically speaking, as those roots where there is least water, for the reason that low temperature retards absorption. This is probably of considerable importance to perennials whose root-systems live throughout the year, but its exact effect has not been studied.
While speaking of the temperature of the soil, it may be interesting to glance briefly at the effect on the development of the root-systems of desert annuals which is brought about by a variation in the relation of the temperatures of the soil and of the air. Briefly stated, the case is as follows: In the "Root Habits of Desert Plants"[1] the root-systems of the winter annuals are described as being easily distinguished from the root-systems of the summer annuals, because among other features the former have a more prominently developed tap root, and a poor development of laterals which are generally filamentous, or at least extremely slender. The summer annuals, on the other hand, have root-systems which resemble the generalized type, above described, of certain perennials: that is, the laterals are developed well, they are frequently rather coarse and the main root is often forked, thus the absorbing surface of the summer annuals is apparently greater than those of winter. The apparent reason for this difference is as follows: When the rains of summer come, the air temperatures fall disproportionately to the decrease in temperature of the soil, so that the soils are moist and relatively warm while the air is moist and relatively cool. In winter, on the other hand, the soils are always cooler than the air, which sometimes may be very warm. Under the first conditions the root absorption is favored, but under the latter conditions root absorption is not favored—conditions which lead to a strikingly different development in the two types of plants.
We can only consider briefly the root-air relation, since little work has been done on the soil atmosphere. We therefore do not know the rate of movement of air in the soil, or, for that matter, its composition. It is probable that there is a large per cent, of carbon dioxide where there are a relatively large number of roots of plants, bat as to the diffusion of oxygen from the air into the soil or the diffusion of carbon dioxide from the soil into the air, little appears to be known. Preliminary tests show that there may be more movement of the air in the soil than might at first be supposed, and that varying, even if small, atmospheric pressure may directly affect air movements in the soil. For examples, if a tube 50 cm. long and 2 cm. in diameter be filled with soil composed of sifted sand and adobe—one part of the former to two parts of the latter—it will be found that a water pressure of only 1 cm., or less, will be required to force a continuous stream of air through it. The pressure given is for soil saturated with water. When air-dry, there is almost no resistance. In soils of this composition, therefore, it is probable that ordinary variation in atmospheric pressure is sufficent to induce in it rapid air movements. Preliminary experiments, in which a stream consisting of 20 c.c. of air a minute was passed through the soil where the roots were placed, indicated by the great vigor of the plant, and the relatively extensive root development, that that amount of air was beneficial to development and forwarded growth. Variations in temperature with depth of soil, variations in water content of the soil, are both additional potent factors in modifying the rate of movement of the soil air.
While it is not known in an exact way how the atmosphere of the soil effects the position or certain other features of the root-systems of plants, it seems probable that in certain cases, at least, the effect is pronounced. For example, as has been shown above, the root-systems of the cacti without exception are placed near the surface of the ground. The roots grow in a soil horizon which is not the most moist, but, on the other hand, which although moistened first is also the first to give up its water, and it very likely is the optimum air content of the soil at a critical period which determines the superficial placing of the roots. It is a well-known fact that many bulbous plants require well-drained soil, which is probably only another way of saying that they thrive best in soils having good aeration. Two or three experiments may be cited which may be taken to substantiate the conclusions just stated. For example, there grows in the vicinity of Tucson a cylindro-opuntia (Opuntia arbuscula?) in which the root-system is fleshy, the roots having much the appearance of slender sweet potatoes. It was supposed at first that the fleshy roots of these species was a specific character, which, indeed, may be true, and therefore obligate. Some doubts, however, have been thrown on this conclusion from observations on another species of opimtia (Opuntia vivipera) in which the roots of one and the same individual may be either fibrous or fleshy. Also species resembling Opuntia arbuscula, but possibly another species, which grows in the vicinity of Sacaton, Arizona, appears to have fibrous roots only. It has been found also that the seedlings of many cylindro-opuntias have fleshy roots. This last may be taken to be a temporary or juvenile stage, but probably is not, for reasons which will appear directly. With the above and other observations in mind, specimens of opuntias of several different species have been grown in saturated soils, with the uniform result that the roots formed in the saturated soil were fleshy. This result might be taken to indicate the immediate effect of an abundant water supply, but in the end it may be found that the result, in part at least, may be attributed to the air relation.
There is another relation which has not been referred to and which is of great importance, namely, the osmotic relation. This can be given briefly. A strong impetus to the study of this relation has recently been given by Fitting,[2] who has shown that the shoots of certain desert plants may possess a very dense cell sap, so concentrated in fact that an osmotic pressure as great as 100 atmospheres has been determined, which pressure may even be exceeded. In the cells of the shoots of ordinary mesophytes the usual pressure is said to be from 5 to 11 atmospheres. While it has not been shown that the cell sap of the root hairs of such desert plants as have high osmotic pressures in the cells of the shoots is isosmotic with them, yet it has been assumed that the roots of these plants contain a very dense sap, as is probably the case. There is an apparently direct relation between the dryness of the habitat and the concentration of the plant juices, by reason of which the desert plant can absorb water from an intensely dry soil. As a rule the highest osmotic pressures, therefore, are to be found among perennials living in the driest situations, and during the most arid seasons. From this condition it is of interest to note that it is probably those plants in which the generalized type of root-system is to be found, or a type approaching this, that possess the most highly concentrated cell sap, since it is plants having this form of roots, as was noted above, which occupy the most arid habitats. We may conclude from this additional evidence that, so far as the Tucson desert is concerned, it is not the most deeply penetrating type of roots which are to be considered the desert form par excellence, but, quite the contrary, it is such a root as can both reach out widely and penetrate as deeply as the soil permits find in which there is developed a cell sap of extremely high concentration.