THE MIGRATION CIRCLE

226. Purpose.The migration circle is designed to record the invasion of species, since it operates outward from an individual or a group of plants as a center. As migration takes place to a certain degree in all directions, a circle is better adapted to the purpose than the quadrat. From the very nature of invasion, migration circles should always he permanent in order that the yearly advance may be accurately noted. Circles of this character are important aids in the study of any vegetation, except, perhaps, one that has practically become stabilized. Their great value, however, is found in succession, where it is necessary to trace the movement of new individuals away from the original invaders as centers of colonization.

227. Location and method.The size of the migration circle is largely controlled by the density of the vegetation, and in some degree by the height of the species also, since this determines the trajectory of the disseminule. In close formations, a circle of 1–, rarely of 5–meter radius can best be used, but in the more open initial stages of succession a radius of 5, 10, or, in exceptional cases such as open woodland, even 25 meters, affords the best results. The location should always be made with a plant or group of plants of the species to be studied as a center. This migration circle differs from the quadrat in that it is used to show the movement of one, rarely two or three species, and not the position of all the plants within it. The center is permanently fixed by driving a labeled stake with the number of the circle and the data. Two tapes the length of the radius are used for recording. These are provided with the usual eyelets, 5 decimeters apart, and are fastened on a peg in the top of the central stake so that they move readily. At the outer ends they are staked 5 decimeters apart by a tape of this length when the radius is 1 meter, and 1 meter when the radius is 5 or 10 meters. The record forms must be especially prepared on blank sheets about 9 inches square. The scale is 10:1 for circles of 1 meter, and 100:1 for those of 5– and 10–meter radius. In the former, concentric circles are drawn about the center at intervals of 5 decimeters, and radii are drawn to the circumference at the same interval. In the larger circles, the intervals are 1 meter. Each segment of the circle is read by means of the two tapes, and the position indicated with reference to the concentric lines and radii. When but one species is read, a tiny circle is used to denote the position of each plant. If more than one is used, the symbols are those already indicated for the quadrat. One tape is left in place and the other with the segment tape is shifted to a new position, and the resulting segment is read as before. The exact position of the base radius is fixed by a label stake,in order that the segments of successive years may exactly correspond. The record sheet is labeled, dated, and filed. By folding at one edge, it may be filed in the regular field book.

228. The denuded circleis established in the same way as a permanent one. The original position of the individuals of the species under consideration may be recorded or not, depending upon the use to be made of the results. The safest plan is first to read the circle in the usual way, and then to denude it. The latter should be done in such a way as to remove all the disseminules from the surface in so far as possible. It is essential also that this be done before the seeds are mature and begin to be scattered. The central plant or cluster is of course not removed. In special cases, all the plants of the species are allowed to remain to serve as centers of colonization. The successive yearly readings of the denuded area are made exactly as for a permanent circle. Permanent and denuded circles, like quadrats, should always be established near each other so that they permit of ready comparison under similar conditions.

229. Photographsof migration circles furnish the most detail when the camera is placed just behind the central group in such a way as to show its relation to the other individuals or clusters of the circle. In the denuded circle, or when the plants stand out conspicuously from the bulk of the vegetation, it is not necessary to use guidons, but in other cases the latter greatly increase the value of the picture. Factor readings are less important for migration circles than for quadrats and transects. The factors of principal importance are those that deal with migration and ecesis, i. e., wind, water-content, and soil temperatures. The former may be determined for both circles in common, but the conditions that affect ecesis must be observed separately for each.

230. Value of cartographic methods.Chart, map, and photograph are records indispensable to the systematic study of vegetation. They serve not merely to preserve the facts ascertained, and to permit their ready comparison, but they also put a premium upon accurate methods, and consequently bring to light many points otherwise overlooked. For ecology, they have the value which drawings possess in taxonomy, in that they make clear at a glance what pages of description fail to indicate. They are the fundamental material of comparative phytogeography, and in all careful vegetational study their use is no longer optional but obligatory.Hence it is obvious that cartographic methods should be clear and simple, and that they should be uniform, so that charts and maps of widely separated formations may be directly compared without difficulty. It is not to be expected that uniform methods will come into general use immediately, but a proper appreciation of the obligation that rests upon every ecologist to make his results both easily comprehensible and usable will serve to produce this very necessary result. In the treatment that follows, as elsewhere, no attempt is made to describe the general cartographic methods used by other ecologists, notably Flahault. The methods employed by the author form a complete system, which has proved valuable, and for various reasons it alone is discussed here.

231. Standard scale.The question of the scale to which charts and maps are to be made is of primary importance. The general principle is that the ratio between area and drawing should be as small as possible. Moreover, charts and maps of the same character should always be drawn to the same scale, unless a good reason to the contrary exists. The ideal scale is 1:1, which is manifestly an impossibility. This is approached most nearly in the quadrat chart where the scale is 10:1. Charts of definite areas are made on a scale as large as possible, while maps of formations, regions, etc., are necessarily drawn upon a very small scale. General maps designed to show the distribution of species and formations, or the vegetation of continents, are usually not drawn with reference to a scale at all. While it is manifestly impossible to use the same scale for charts and maps, it is feasible and desirable that they be constructed upon scales readily convertible into each other. This is most satisfactorily accomplished by means of the decimal system, and the various type scales are 10:1, 100:1, 1000:1, etc. The first two or three scales are used for charts of quadrats, transects, and circles; the remaining ones are employed in making maps of large areas. No attempt has been made to draw an absolute line between charts and maps, but an endeavor is made to restrict the term chart to the record of the number and position of plants, while maps deal with the arrangement and location of formational areas. It is hardly necessary to point out the reasons why all charts and maps should be based upon the decimal system of scales. Experience will furnish the very best of arguments.

232. Color scheme.The first requisite for the graphic representation of formations, regions, etc., is that each class of formations be invariably indicated by the same color. It is also necessary that the colors and shades be easily distinguishable, and it is at least desirable that they be referred to the different classes in some consistent sequence. Uniformity in all thesepoints is greatly to be desired at the hands of all ecologists. Here, as in the case of the standard scale, uniformity will be found the more desirable the more impossible it is made by ignoring it. In the use of color to represent regions and provinces, on maps too small to indicate formations, the color of each division is represented by the color of its dominant formation; thus the prairie province is colored ochroleucus on account of the color used to represent prairie formations, the boreal-subalpine zone atrovirens on account of the typical coniferous forests, etc. No endeavor has been made to take account of the various types of formations, e. g., the different coniferous forests, as this is a problem to be worked out for more local maps in various shades of dark green, etc. The following color scheme which has been based upon the points made above is proposed as a satisfactory solution of the problem. The color standard used is that of Saccardo’s Chromotaxia.

233. Formation and vegetation mapsare detailed maps of a single formation or a series of them, showing the formational limits, and when the scale is not too small, the ecotones of zones and consocies. In the caseswhere the topography is level, as sometimes happens in mapping single formations, the chain and pedometer must be used to ascertain the size of the different areas. Indeed in all mapping of vegetation, the methods of surveying are directly applicable. Over large areas, however, it is not necessary that limits be drawn with mathematical accuracy, and for the purposes of the ecologist, the plane table and camera are satisfactory substitutes for the surveyor’s transit, at least in the present aspect of the subject. When the formation or group of formations is commanded by an elevation of some height, the latter is used as a base. A plane table is established upon it and the topographical and vegetational features are recorded in the usual way. This map is usually supplemented by a series of views from the same base. Indeed it has come to be recognized that a complete series of photographs of this kind give a more valuable record than the plane table, and that the construction of an accurate map from them is an easy matter. Since the camera saves much time and energy also, it is used almost exclusively to furnish the data for map making. In hilly, and especially in mountainous regions, the photographic method is indispensable. Its application is extremely simple. A central hill or mountain is selected, and from it a series of views is taken so that the edge of one exactly meets the edge of the other. This is an extremely important matter, and demands much nicety of judgment. The camera is kept in the same spot, and after each exposure it is turned as the operator looks through it until a landmark at one edge just passes from view at the other. As soon as the new position is determined, the tripod screw is turned to hold the box firmly in position. In case of a slight jar, the exact position should again be obtained. If the series is accurately made, the resulting prints will give a complete panoramic view of the region, without overlap or omission. For this purpose, a 6½ × 8½ camera is desirable, since the topographic and vegetational features are larger and stand out more distinctly. A large camera requires fewer changes of position, and hence saves time and reduces the chance of error. A 4 × 5 camera serves the purpose sufficiently well, though it requires a little more care in operation on account of the greater number of exposures necessary. This may be avoided in some degree by the use of a wide-angle lens if the depth of the area is not too great. Whatever camera may be used, a telephoto lens is a very desirable adjunct, since it enables one to choose between three different sizes of the view without changing the position of the camera. To avoid possible confusion, the exposures are always made from right to left, and the plates are used in the numerical order of their holders. For the same reason the landmarks are described and numbered in their proper order. The prints obtained are mounted on a card in sequence. The view map may be preserved in this form, or it may be reduced orenlarged by making a copy to the size desired. Outline maps of topography may be traced from the resulting negative, and the formations filled in by means of the proper colors. The most satisfactory method, however, is to have the original views or the copy printed “light” and to color the formations just as they appear there, with all the wealth of topographic and vegetational detail. If a detailed topographic map alone is desired, this is traced directly from the large copy.

234. Continental maps.A method of determining the general outlines of regions, provinces, and vegetational zones as a preliminary to their detailed study has been used successfully for several years.[22]This is based upon provincial and continental maps on which are traced the geographical areas of the species of genera typical of the various formations. Detail topographic maps of the prairie province and the North American continent have been used for this purpose. A number of the facies of extensive and representative formations of the different portions of the continent are selected and grouped according to genera. One map is devoted to each genus, unless the number of species is large. In this case a number of maps are used, since the limits are apt to become confused. The range of each species is determined from all the reliable sources, and a corresponding line is drawn upon the map to delimit its geographical area. The limits of the area of each species are drawn in a different color, and the name of the species printed in the same color in the legend. Although this work has as yet been done only for the trees of North America, and for the grasses and principal species of the prairie province, it promises to constitute a final method for the limitation of vegetational divisions. It is clear that if the original data concerning ranges are accurate, the increasing study of formations will do little more than rectify the detailed course of the limiting line, since in most cases facies and formations coincide in distribution. The limiting line or ecotone of a zone or province is a composite obtained from the limits of certain representative facies and principal species, and checked by the limits of species typical of the contiguous vegetations. Thus, the boreal-subalpine zone is clearly outlined by combining the limits ofPopulus tremuloides,Larix americana,Pinus banksiana,Abies balsamea,Picea mariana,Picea canadensis, andBetula papyracea, and checking the results by the areal limits of the hardwoods and grasses to the southward.

PHOTOGRAPHY

235.The camera is an indispensable instrument for the ecologist. Although it has too often been employed to give an air of thoroughness to work of no ecological value, it is as important for recording the structure of vegetation as the automatic instrument is for the study of the habitat. No ecologist is equipped for systematic field investigation until he is provided with a good camera and has become skilful in its use. For this reason, it is felt that a few hints concerning photographic methods and their application in ecology may not be out of place. No written advice can take the place of experience, but certain elementary suggestions and cautions will greatly shorten the apprenticeship of one who does not have the good fortune to be taught by a professional photographer. To the student of ecology, the camera is not a toy. It must be understood and operated with as much thoroughness as any other instrument, and when this is done, the results will be equally certain and desirable.

Fig. 56. 4 × 5 long focus “Korona” camera (series V).

Fig. 57. 5 × 7 long focus “Premo” camera.

236. The camera and its accessories.Although two cameras are desirable whenever it is possible to obtain them, a single one will meet all the requirements of field work. This should be 4 × 5 inches in size, since it is much more convenient and will do all the work that a larger camera can. In the comparatively few cases in which larger views are needed, the 4 × 5 negatives can be readily enlarged. The smaller instrument is less expensive in operation because of the cheapness of the plates, and it gives a negative of the proper size for lantern slides and for reproduction. A 6½ × 8½ camera is valuable in special cases, such as making a series of photographs for maps. In the writer’s own experience, the 6½ × 8½ camera, although used exclusively at first, has been almost completely supplanted by the 4 × 5. The best field camera is of the folding type with a good stout box. It must be what is known technically as a long focus instrument, which enables small objects to be taken natural size and permits the use of a telephoto lens. It should be provided with a swing and also a reversible back by which the position of the plates can be changed instantly. The lens must be of the telephoto pattern, which makes it possible to use the front or back lens either alone or in combination. The chief advantage of this is that the image, when distant, may be made of three different sizes without changing the position of the camera. Generally speaking, the high-priced rapid lenses are the best, since it is exceptional to get the desired length of exposure in vegetation, on account of the ease with which the plants move in the wind. Before buying such a lens it is desirable to test its rapidity and depth of focus, since it is not necessarily better than some of the lenses furnished with good cameras. The lens should be provided with an iris diaphragm capable of being stopped down to 128 or 256. The shutters furnished with the ordinary lenses are satisfactory, since “snap-shots,” i. e., instantaneous exposures, are practically never possible for plants. The automatic shutter of the “Premo” camera is an especially convenient form. All shutters should be carefully tested before using to determine the exact time value of the exposures indicated. It is not uncommon for the exposure at 1 second, or at other points, to have a value quite different from the one indicated. When this is the case, it is evident that it can not be known too soon. The camera should have at least a half-dozen double plate-holders. These are numbered consecutively so that the figure uppermost when the holder is in the camera will indicate the number of the plate exposed. A carrying case is desirable on a long trip when all the plate-holders must be taken, but ordinarily it is a disadvantage, since the camera box will carry two or three holders. The camera cloth should be as small and light as possible, and at the same time opaque. The most satisfactory one for thefield is the rubber cloth. The tripod should be a happy combination of lightness and stability, a condition more nearly reached by the aluminum tripod than by any other. It should have not less than three joints in order to facilitate the use of the long focus upon objects near the ground.

Fig. 58. 5 × 7 “Korona” Royal camera.

237. Choice of a camera.There is not a great deal of choice between the moderate-priced cameras of the various makers. A field camera is restricted to certain special uses, and hence is more serviceable when attachments useful only in portraiture or instantaneous work are absent. Even the ray filter, which has some value in the indoor photography of flowers, is useless in the field on account of the long exposure required. From considerable experience, “Premo” and “Korona” cameras have been found to be very satisfactory instruments. Doubtless the same statement would be found true of all the standard makes, but they have not been used by the writer. “Premo” cameras are made by the Rochester Optical Co., Rochester, N. Y., and “Korona” cameras by the Gundlach-Manhattan Optical Co., Rochester, N. Y. When two or more cameras are used, the best results can be obtained if they are of the same make, since the details of operation are then the same. The reduced liability of making a blunder is often offset by the fact that a different pattern will permit of a wider range of use. Any standard brand of plates will produce good negatives when skilfully used; at least, this has been proved in the case of the Cramer, Hammer, Seed, and Stanley brands. Every professional photographer has his favorite brand of plate, but the ecologist will do well to give the various kinds a thorough trial, and then to invariably use the one which gives him the best results. Thus, while it seems to be less popular with the profession than the others mentioned, the writer has obtained at least as satisfactory resultswith the Stanley plate as with the others, and consequently now uses it exclusively, since it is cheaper. The one important point is to make a final choice only after personal experience, and then to always use plates of the same brand, and preferably of the same rate of speed.

238. The use of the camera.To the ecologist, objects to be photographed fall into two categories, viz., those that move, and those that do not move. For practical purposes, areas sufficiently distant to render the movement imperceptible belong to the latter, as well as those, such as rock lichens, many fungi, etc., which can not be stirred by ordinary winds. The treatment accorded the two is essentially different. A fundamental rule of ecological photography is that detail must receive the first emphasis. The ecological view should be a picture as well as a map, however, but when one must be sacrificed, artistic effect must yield to clearness, and accuracy, i. e., technically speaking, contrast must give way to detail. Leaving apart the necessity of securing a sharp focus, which holds for all work, detail or definition depends directly upon the aperture of the diaphragm. Detail is increased by decreasing the size of the aperture. This in turn increases the length of time necessary for a proper exposure, and consequently the danger that the plant will be moved in the midst of the exposure. When the movement is negligible, the invariable rule should be to reduce the aperture to its smallest size, and to expose for a corresponding time. In all cases where the plants are close enough to show even a slight blurring on account of the action of the wind, the time of exposure must be reduced, in the hope that a short period of quiet will suffice for it. This reduction in time must be compensated by increasing the aperture of the diaphragm, and hence the amount of light which strikes the plate. The proper balance between the two is a matter of considerable nicety. It depends much upon the vagaries of the wind, and can readily be determined only after considerable experience. Although regions naturally differ somewhat in the nature of their winds, much experience in prairie and mountain regions warrants the primary rule that views of vegetation and plants subject to movement are not to be attempted on windy or cloudy days when it can possibly be avoided. Even on reconnaissance, a poor picture is no better than none at all, while in resident work a time will come sooner or later which will permit the making of a view satisfactory in all respects. There may be occasional instances when one is rewarded for keeping the camera trained on a particular spot for hours, and for wasting several plates in the hope that still moments will prove to be of the requisite duration. As a regular procedure, however, this has nothing to commend it.

Various methods have been tried to reduce or eliminate the trouble caused by the wind. Canvas screens have been used for this purpose with somebenefit. When the picture is worth the trouble, a tent may be erected to afford a very efficient protection. This is too prodigal of time and energy, however, to be practicable under the usual conditions. Flashlight exposures on still nights are sometimes feasible, but the disadvantages connected with them are too great to bring them into general use. The best procedure is to bide one’s time, and to take quadrats, transects, and other detail areas, as well as many plant groups, at a time that promises to be most favorable. Single plants can often be moved in the field so that they are protected from the wind, or so that they are more strongly lighted. Slender, or feathery plants are usually very difficult to handle out of doors. The best plan is to photograph them in a room that is well and evenly lighted, or, best of all, in a stable, roomy tent.

239. The sequence of details.No photographer ever escapes blunders entirely. At the outset of his work, the ecologist must fully realize this, and accordingly plan a method of operating the camera which will reduce the chance of mistake to a minimum. The usual blunders which every one makes sooner or later, such as making two exposures on one plate, drawing the slide before closing the shutter, allowing the light to strike the plate through the slit in the holder, etc., can be all but absolutely avoided by a fixed order of doing things. This order will naturally not be the same for different persons; it is necessary merely that each have his own invariable sequence. The following one will serve as an illustration. As a preliminary, the plate-holders are filled, after having been carefully dusted, and the slides are uniformly replaced with the black edge inward. It is a wise precaution to again see that all the slides are in this position before leaving the dark room. This will ensure that a black edge outward always means that the plate has been exposed. The tripod is first set up and placed in what seems about the proper position. The camera is next attached to it, and the front and back opened. The bellows is pulled out, a short distance for views, and a longer one for detail pictures, and fastened. It is necessary to move the diaphragm index to the largest aperture and to open the shutter at “time.” The next steps are to orient the view or object, and to bring it into sharp focus upon the ground glass. The first is accomplished by moving the entire instrument, changing the position of the tripod legs, swinging the camera upon the tripod, or by raising or lowering the lens front. It is often desirable also to change the position of the object on the plate by use of the reversible back. In views with much distance, the foreground is brought into sharp focus. In close views, especially of quadrats, the swing is used to increase the distance for the foreground, and the focus is made upon the center. After focusing, the shutter is closed, the indicator set at the timedesired, and the diaphragm “stopped down” as far as possible. Plate-holder 1 is slipped into place, care being taken not to move the camera by a sudden jar. The camera cloth is dropped above the holder and allowed to hang down over the slide end. The slide is drawn and put on top of the instrument, the black edge always up. The exposure is made and the slide replacedwith the black edge outward. This point should receive the most critical attention, as a blunder here will often cause the loss of two negatives. The plate-holder is returned to the receptacle, or merely placed in the back of the camera, which is then closed. The number of the plate, the name of the view or object, the condition of the light, the length of exposure, and the aperture of the diaphragm, as well as the date, are recorded in a notebook for this purpose. The shutter is then opened at “time,” the diaphragm thrown wide open, and the front of the camera closed. When distances are short, the camera is often carried upon the tripod. As a rule, however, it is usually removed, and the tripod folded. In making subsequent pictures, the plates should always be used in their numerical order.

240. The time of exposureis obviously the most critical task in the manipulation of a camera. The time necessary for a proper exposure varies with the season, the hour, the condition of the sky, the light intensity of the formation, the color and size of the area to be photographed, and, finally, of course, with the aperture of the diaphragm. Fortunately for the ecologist, the variation in light intensity during the season, and even during the greater part of the day, is not great, and can ordinarily be ignored. The beginner will make the most progress by determining the exposure demanded by his instrument for taking a general view in full sunlight and with the smallest stop of the diaphragm. In standard cameras with lenses of ordinary rapidity, this is usually about one second. This will serve as a basis from which all other exposures may be reckoned until one has worked through a wide range of conditions and can recall just what time each view requires. On completely cloudy days the time required is five to ten times that necessary on a clear day; filmy clouds and haze necessitate an exposure of two or three seconds. The more open forest formations demand an exposure of about five to ten seconds on a sunny day, while the deeper ones require two or three times as long. A close view requires more time than a distant one, since the light-reflecting surface is much smaller. Quadrats require two or three seconds, and individual groups frequently take a longer time. The color of the vegetation plays an important part also: a dark green spruce forest requires twice as long an exposure as the aspen forest, and a grassland quadrat takes more time than one located in a gravel slide. In this connection, it is hardly necessary to point out that the lighted side ofobjects should always be taken, never the shaded one. The exposures indicated above are based upon the smallest stop. The reasons for using this whenever possible have already been given. When a larger stop is necessary, the exposure is decreased to correspond; for example, a quadrat that takes three to four seconds at 256 can be taken at 64 in one second. As a rule, the sun should not be in front of the camera, but, when necessary, views can be made in this position if the sun is prevented from shining directly into the lens.

241. Developingis as important as exposing. Indeed, it may well be considered more important, since a properly exposed plate may be spoiled in developing, while an under-exposure or over-exposure may be saved. Owing to the ease with which plants move in the wind, the ecologist is obliged to reconcile himself to many under-exposures, which can be converted into good negatives only by skilful developing. Every base station should have a good dark room, equipped with running water when possible, a good ruby lantern, and the proper trays and chemicals. Prepared developing solutions are alluring because of their convenience, but after an extended trial of several kinds, the writer has reached the conviction that pyrogallic acid, or “pyro,” is by far the most satisfactory in working with vegetation. Of almost innumerable formulae, the following gives excellent satisfaction and is convenient to use.

For developing, equal parts of I and II are mixed, and a few drops of a 10 per cent solution of potassium bromide added, unless there is reason to suspect that the plate has been seriously underexposed. The fixing bath is a concentrated solution of sodium hyposulphite, “hypo,” to which a few drops of acetic acid are added. It should be replaced every week or two, depending upon how much it is used. A tray of water is kept at hand for bringing out the detail in underexposed negatives, and a second tray is used for washing. The “pyro” and the bromide solution should always be within reach, the former for accelerating, and the latter for retarding the development of unsatisfactory plates.

The image will begin to show on a properly exposed plate within one to three minutes after it has been put in the developer. If the image appears almost instantly, and then recedes quickly, the plate is badly overexposed,and should be thrown away. In case it “comes up” less quickly, indicating that it is not greatly overexposed, it can be saved by the addition of more bromide. When the image does not show till the end of five to ten minutes, the plate has been underexposed. It is then necessary to add more “pyro,” taking care not to pour it on the plate, and, after the image appears with its striking contrast, to leave the plate in water until as much detail as possible is brought out in the shadows. In the case of a normal exposure, when greater detail is desired, the negative is left for some time in water, and when contrast is sought more “pyro” is used. Negatives with unusual detail lack “snap”; they are “flat,” and fail to make artistic pictures. Contrast, on the other hand, often obscures detail, and the best results can only be obtained by a happy combination of the two. The most important maxim in developing is that the process shall be continued until the image has become indistinct. The universal tendency of the beginner is to remove the negative the moment the outlines grow dimmer, and the result is a thin, lifeless negative. It is almost impossible to develop too far, if the image is not allowed to disappear. Negatives of this sort are “thick,” and though they print more slowly, produce brilliant pictures. A large quantity of the developing solution is used with single plates in small trays, and is allowed to act without rocking the tray. Much time is saved, however, by developing several plates together, and to avoid using a large quantity of the solution, the tray is gently rocked from time to time. This movement is particularly necessary at the beginning, in order that the plates may be covered evenly, and at once. Fifty cubic centimeters of the solution will develop three or four 6½ × 8½ plates, and twice as many 4 × 5’s. After the developer has once been used, it is kept for several days to restrain overexposed plates. As soon as the plate is developed, it is rinsed in water, and placed in the fixing fluid, until the white opaqueness is entirely removed. The “hypo” is then washed out by immersing the negatives for one to two hours in running water. If the latter can not be secured, the water in which they are placed should be changed frequently. The negatives are then air-dried within doors, in a place free from dust. Finally, they are filed away in negative envelopes, each bearing the name and number of the negative, and preferably also, the time and other exposure data.

242. Finishing.On account of the time demanded by other field tasks, it has not been found desirable to make and finish prints in the field. This, with the making of lantern slides, enlargements, etc., may well be turned over to a professional photographer. It is the custom to make a proof of each negative to meet the casual needs that arise in the field. For thispurpose, solio “seconds” are used, since they are both cheap and satisfactory. When an urgent demand for a finished print does arise, it is met by using “velox” paper, which can be exposed in the dark room, and then developed and fixed exactly like a plate. Two standard papers for views are “solio” and “platina.” The former gives brown tones, and is used for contrast and brilliancy, hence it is especially good for printing from negatives that have too much detail and too little contrast. “Platina,” on the contrary, yields soft gray tones, and softens contrasts.

243. Concept and purpose.A formation herbarium is a collection of exsiccati, in which the species are arranged with respect to their position in the formation, instead of being grouped in genera and families. Its primary purpose is to furnish a record of the constitution and the structure of a formation or a series of formations. At the same time, it affords the basal material for developing the subject of comparative phytogeography. It is impossible for one ecologist to visit many remote regions, to say nothing of spending a period sufficient for obtaining even a fair knowledge of the vegetation. He can at the best acquire an acquaintance with but few regions at first hand. In consequence, a method that brings a vegetation to him, with its structure carefully wrought out by years of study, is of the highest value. Time, as well as distance, sets a narrow limit to the number of formations which one man can investigate critically in a lifetime. It is no longer possible for a botanist to explore vast regions, and to bring back results which have anything more than a very general value. This fact, far from restricting the comparative study of vegetation, will serve to make it more accurate and systematic. The exact results of numerous resident investigators, expressed in formation herbaria, with the proper series of quadrat maps and photographs, will be worked over by men who are themselves specially acquainted with a particular vegetation. Comparisons will be founded upon a definite basis, and the relationship of various vegetations can then be expressed in precise rather than general terms. It is hardly too sweeping to assert that accurate work in the field of comparative phytogeography can be done only in this fashion. The value of formation herbaria in class work is evident. On account of the limitations of time and distance, classes can touch but few formations, and these at every time except the growing period. For these reasons, an accurate and complete formational record that can be consulted or studied at any time is almost indispensable to class study in the development and structure of formations.

244. Details of collecting.Formational collections, unlike the ordinary sets of exsiccati, can not be made upon the first visit to a region, or by a single journey through it. The determination of formation limits, and of developmental stages, of aspects, layers, abundance, etc., must necessarily precede, a work which alone takes several years. Moreover, collecting itself requires more than one year in a region containing numerous formations. This is exemplified by theHerbaria Formationum Coloradensium.[23]The preliminary study for this was made from 1896–1899, the collecting was done chiefly in 1900 and 1901, while additional numbers were added in 1902–3. For the purposes of the formation herbarium, specimens should be collected and pressed in such fashion as to show all the ecological features possible. Plants must be collected both in flower and in fruit, with the underground parts as perfect as may be. Seedlings and rosettes should be included whenever present. In pressing, one or two leaves should be arranged with the lower side uppermost to admit of the ready comparison of both surfaces. Opened flowers are valuable for flower biology, while seeds and fruits are desirable for showing migration contrivances. The ferns, mosses, and lichens of the formation should be fully represented, together with the more important fungi and algae. The number of photographs taken for each herbarium should be limited only by considerations of time and expense. The ideal series consists of a general view of each formation, showing its physiographic setting, nearer views of each of its aspects, detail views of its consocies, societies, and layers, and flower portraits of all the constituent species. Such a series can only be obtained by residence through a long term of years, and in most cases general and aspect views, with portraits of the facies and a few of the striking principal species, must suffice. Quadrat and transect charts, together with formational maps, are extremely desirable, and, indeed, all but indispensable.

245. Arrangement.The arrangement of species within each formation herbarium is based upon the structure of the vegetation. The primary groupings are made with reference to time of appearance and abundance; when definite zones, associations, or layers are present, they must likewise be taken into account. In the Colorado collection, the first division is into three aspects based upon the period of flowering (aspectus vernalis,aestivalis,autumnalis). Within each aspect, the species are arranged with respect to abundance in the groups, facies, principal species, and secondary species. Each group is placed in an ordinary manila cover, which bears a printed label indicating the aspect and the group. The species labels give,in addition to the name, date, and place of collection, the phyad or vegetation form, the geographical area, the rank of the species, the aspect, and the formation. To these may well be added data concerning migration contrivances, seed production, pollination, period of flowering, etc. The photographs are mounted on the usual herbarium sheets, and placed in the proper order in the various groups, and a similar disposition is made of quadrat and transect charts, and such physical factor summaries as seem desirable.

246. Succession herbaria.The arrangement of formation herbaria may follow the classification of formations with respect to character, region, or development. The first is the most convenient for purposes of instruction, and has distinct advantages in permitting a close comparison of the vegetation of different habitats. The second basis, which is the one used in theHerbaria Formationum Coloradensium, is peculiarly adapted to mountain vegetation in which the zones are usually very distinct. The arrangement of herbaria in a developmental series, however, is the most logical and the most illuminating, since the structure of the ultimate formations is not only made plain, but the stages in their development are also laid bare. Such succession herbaria are the natural outgrowth of formational ones. Indeed, the latter should be made merely the starting point for these in all regions where the causes which bring about successions are active. Where weathering is still an important factor, as in mountains, the initial and intermediate formations which lead to the final grassland or forest are often in evidence. After a formation herbarium of each stage has been made in the way indicated, a succession herbarium is obtained merely by arranging the various herbaria in the sequence of the developmental stages. Thus, in the Colorado collection, the subalpine formations are arranged according to altitude in the following series: (1) the pine formation, (2) the gravel slide formation, (3) the half gravel slide formation, (4) the aspen formation, (5) the balsam-spruce formation, (6) the spruce-pine formation, (7) the meadow thicket formation, (8) the brook bank formation. Of these, five belong to the same succession, and it is possible to indicate the development of the spruce-pine forest by arranging these five formations in their proper order in a succession herbarium, as follows: (1) the gravel slide formation, (2) the half gravel slide formation, (3) the pine formation, (4) the balsam-spruce formation, (5) the spruce-pine formation.

Development and Structure

247. Vegetation an organism.The plant formation is an organic unit. It exhibits activities or changes which result in development, structure, and reproduction. These changes are progressive, or periodic, and, in some degree, rhythmic, and there can be no objection to regarding them as functions of vegetation. According to this point of view, the formation is a complex organism, which possesses functions and structure, and passes through a cycle of development similar to that of the plant. This concept may seem strange at first, owing to the fact that the common understanding of function and structure is based upon the individual plant alone. Since the formation, like the plant, is subject to changes caused by the habitat, and since these changes are recorded in its structure, it is evident that the terms, function and structure, are as applicable to the one as to the other. It is merely necessary to bear in mind that the functions of plants and of formations are absolutely different activities, which have no more in common than do the two structures, leaf and zone.

248. Vegetation essentially dynamic.As an organism, the formation is undergoing constant change. Constructive or destructive forces are necessarily at work; the former, as in the plant, predominate until maturity, when the latter prevail. Consequently, it no longer seems fruitful to classify the phenomena of vegetation as dynamic or static. The emphasis which has been placed upon dynamic aspects of vegetation has served a useful purpose by calling attention to the development of the latter. Although it is a quarter of a century since Hult, and more than a half century since Steenstrup, by far the greater number of ecological studies still ignore the problem of development. This condition, however, can be remedied more easily by insisting upon an exact understanding of the nature of the formation than in any other way. It is entirely superfluous to speak of dynamic and static effects in the plant, and the use of these terms with reference to the formation becomes equally unnecessary as soon as the latter is looked upon as an organism. The proper investigation of a formation can no more overlook development than structure, so closely are the two interwoven. Future research must rest squarely upon this fact.

249. Functions and structures.The functions of a formation are association, invasion, and succession: the second may be resolved into migration and ecesis, and the third, perhaps, into reaction and competition. Formational structures comprise zones, layers, consocies, societies, etc., all of which may be referred to zonation, or to alternation. The term associationhas been used in both an active and a passive sense. In the former, it applies to the inevitable grouping together of plants, by means of reproduction and immobility. Passively, it refers to the actual groupings which result in this way, and in this sense it is practically synonymous with vegetation. Invasion is the function of movement, and of occupying or taking possession; with association, it constitutes the two fundamental activities of vegetation. It is the essential part of succession, but the latter is so distinctive, because of the intimate relation of competition and reaction, that clearness is gained by treating it as a separate function which is especially concerned with development. Association, zonation, and alternation are structural phenomena, which are in large part the immediate product of habitat and function, and in a considerable degree, also, the result of ancestral or historical facts. It is a difficult matter to determine in what measure the last factor enters, but it is one that must always be taken into account, particularly when the physical factors of the habitat are inadequate to explain the structures observed. Structurally, association regularly includes both zonation and alternation. As there are certain typical instances in which it exhibits neither, the treatment will be clearer if each is considered separately.

250. Concept.The principle of association is the fundamental law of vegetation. Indeed, association is vegetation, for the individual passes into vegetation, strictly speaking, at the moment when other individuals of the same kind or of different kinds become grouped with it. It is then (and the same statement necessarily holds for vegetation) the coming together and the staying together of individuals and, ultimately, of species. A concrete instance will illustrate this fact. In the development of the blowout formation of the Nebraska sand-hills (Redfieldia-Muhlenbergia-anemium), association begins only when the first plant ofRedfieldia flexuosais joined by other plants that have sprung from it, or have wandered in over the margin of the blowout. Henceforth, whatever changes the blowout formation may undergo, association is a settled characteristic of it until some new and overwhelming physical catastrophe shall destroy the associated individuals. It will readily be seen that association does not depend upon particular individuals, for these pass and others take their place, but that it does depend essentially upon number of individuals.

Association involves the idea of the relation of plants to the soil, as well as that of plants to each other. It is synonymous with vegetation only when the two relations are represented, since there may be association suchas that of a parasite with its host, which does not constitute vegetation. But it will be seen that the relation of the parasite to the host is practically identical with the relation of the plant to the soil or stratum, and the two concepts mentioned above become merged in such a case. From this it follows that association results in vegetation only when the two ideas are distinct. The concept of association contains a fact that is everywhere significant of vegetation, namely, the likeness or unlikeness of the individuals which are associated. In the case of parasite and host, this unlikeness is marked; in vegetation, all degrees of similarity obtain. As will be evident when the causes which lead to association are considered, alternate similarity and dissimilarity of the constituent individuals or species is subordinate as a feature of vegetation only to the primary fact of association.

Since association contains two distinct, though related, ideas, it is of necessity ambiguous. It is very desirable that this be avoided, in order that each concept may be clearly delimited. For this reason, the act or process of grouping individuals is termedaggregation, while the word association is restricted to the condition or state of being grouped together. In a word, aggregation is functional, association is structural; the one is the result of the other. This distinction makes clear the difference between association in the active and passive sense, and falls in with the need of keeping function and structure in the foreground.

251. Causes.In considering the causes which produce association, it is necessary to call in evidence the primary facts of the process in concrete examples of this principle. These facts are so bound up in the nature of vegetal organisms that they are the veriest axioms. Reproduction gives rise immediately to potential, and ultimately, in the great majority of cases, to actual association. The degree and permanence of the association are then determined by the immobility of the individuals as expressed in terms of attachment to each other or to the stratum, such as sheath, thallus, haustoria, holdfasts, rhizoids, roots, etc. The range of immobility is very great. In terrestrial plants, mobility is confined almost entirely to the period when the individual lies dormant in the seed, spore, or propagative part, which is alone mobile. In aquatic spermatophytes, the same is true of all attached forms, while free floating plants such asLemnaare mobile in a high degree, especially during the vegetative period. Among the algae and hydrophilous fungi, attached forms are mobile only in the spore or propagative condition, while the motile forms of the plancton typify the extreme development of mobility. The immediate result of reproduction in an immobile species is to produce association of like individuals, while in the case of a mobile species reproduction may or may not lead immediatelyto association. We may lay down the general principle that immobility tends to maintain the association of the individuals of the same generation, i. e., the association of like forms, while mobility tends to separate the similar individuals of one generation and to bring unlike forms together. With the mobile algae, separation of the members of each generation is the rule, unless the individuals come to be associated in a thallus, or are grouped in contact with the substratum. Flowering plants that are relatively immobile, especially in the seed state, drop their seeds beneath and about the parent plants, and in consequence dense association of the new plants is the rule. In very many cases, however, this primitive tendency is largely or completely negatived by the presence of special dissemination contrivances, which are nearly, if not quite, as effective for many terrestrial plants as the free floating habit is for algae. From this point, the whole question of mobility belongs to migration, just as the adjustment between the parent plants and their offspring, or between plants established and the mobile plants to be established, belongs to competition.

If association were determined by reproduction and immobility alone, it would exhibit areas dissimilar in the mass of individuals, as well as areas dissimilar in the kinds of individuals. Some areas would be occupied by plants of a single species, others by plants of several or many species. This tendency of association to show differences is, however, greatly emphasized by the fact that vegetation is fundamentally attached to and dependent upon a surface that exhibits the most extreme physical differences. For this reason, new differences in association appear, due not only to the morphological differentiation of vegetation forms, but also to the changes in the degree and manner of association produced directly by the different habitats. Association might then be defined as a grouping together of plant individuals, of parents and progeny, which is initiated by reproduction and immobility, and determined by environment. It is a resultant of differences and similarities. In consequence, association in its largest expression, vegetation, is essentially heterogeneous, while in those areas which possess physical or biological definiteness, habitats and vegetation centers, it is relatively homogeneous. This fundamental peculiarity has given us the concept of the formation, an area of vegetation, or a particular association, which is homogeneous within itself, and at the same time essentially different from contiguous areas, though falling into a phylogenetic series with some and a biological series with others. From its nature, the plant formation is to be considered the logical unit of vegetation, though it is not, of course, the simplest example of association.

252. Aggregation.As indicated under the causes of association, the process by which groups of individuals are formed depends entirely upon reproduction and migration. In short, aggregation is merely a corollary of movement. The simplest example of this process occurs in forms likeGloeocapsa,Tetraspora, and others, where the plants resulting from fission are held together by means of a sheath. Though called a colony, such a group of individuals is a family in the ordinary sense. Practically the same grouping results in the case of terrestrial plants, especially spermatophytes, when the seeds of a plant mature and fall to the ground about it. The relation in both instances is essentially that of parent and offspring, although the parent soon disappears in the case of annuals, while among the algae its existence is regularly terminated by fission. The size and the density of the family group are determined by the number of seeds produced, and by their mobility. These are further affected by the height and branching of the plant, and by the position of the seeds upon it. The disseminules of immobile species fall directly beneath the parent, and the resulting group is both uniform and definite. A similar arrangement is caused likewise by offshoots. An increase in mobility brings about a decrease of aggregation, since the disseminules are carried away from the parent plant. Perfectly mobile forms rarely produce family groups for this reason. It is evident, however, that mobile perennials sometimes arrange themselves in similar fashion in consequence of propagation by underground parts. Consequently, it is possible to state the law of single aggregation, viz., that immobility promotes the grouping of parent and offspring, and mobility hinders it.

If all species were immobile, the family group would be characteristic of vegetation. Since the great majority are more or less mobile, aggregates of this sort are the exception rather than the rule. Mobility not only decreases the number of offspring in the family group, but it also spreads disseminules broadcast to enter dissimilar groups. It leads directly to mixed aggregation, by which individuals of one or more species invade the family group. Once established, the newcomers tend also to produce simple groups, thus causing an arrangement corresponding essentially to a community. Such collections of family groups are extremely variable in size and definition. This arises in part from the nature of simple aggregation, and in part from the varying mobility of different species. Mobility alone often produces similar communities by bringing together the disseminules of different plants, each of which then becomes the center of a mixed group. In the case of permobile species, several disseminules of each may be brought together. The resulting area, though larger, is practically the same. At present, it is difficult to formulate the law for this method of grouping. It may be stated provisionally as follows: mixed aggregation is the direct result of mobility, and the greater the mobility the more heterogeneous the mixture.

The constitution of all the major areas of a formation is to be explained upon the basis of aggregation by the two methods described. The relative importance of family groups and communities differs for every formation, and the exact procedure in each can be obtained only by the detailed study of quadrats. The problem is further complicated by competition and reaction, particularly in closed vegetation. For this reason, aggregation can be studied most satisfactorily in a new or denuded area, where these processes are not yet in evidence.

253. Categories.In the analysis of association, it must be kept clearly in mind that the concrete examples from which all generalizations must be drawn are often in very different stages of development, and are of correspondingly different ages. For this reason it has seemed best to consider the primary relations of association in general in this place, leaving the treatment of the effects of invasion, succession, alternation, and zonation to be taken up under these topics.

Various categories of association may be distinguished, according to the dominant physical factor concerned or the point of view taken. These will fall into two series, as we consider the relation of plant to plant with reference to some object or characteristic, or the grouping of plants together in response to some dominant factor. In the first series may be placed association with reference to substratum, to the ground (occupation), and to invasion; in the second belong light and water-content association. It should be noted that these are all actual associations in nature, and not concepts such as the vegetation form, within which plants from widely different associations may be classified. Naturally, it does not follow that it is not logical or valuable to group together those plants, such as hydrophytes, sciophytes, hysterophytes, etc., which have a common relation to some factor, but belong to different formations.

254. Stratum association.Plants manifest independent or dependent association with reference to the stratum to which they are attached and from which they derive food or support. Independent association is exhibited by those holophytic species of a formation which are entirely independent of each other with respect to mechanical support or nutrition. It is characteristic of the greater number of the constituent species of formations. Dependent association is manifested in the relation between host and parasite, stratum and epiphyte, support and liane. Warming[24]has distinguished sixkinds of associations: parasitism, helotism, mutualism, epiphytism, lianism, and commensalism. Commensalism corresponds to the primary principle of association which has given rise to vegetation. Homogeneous commensalism is the term applied to social exclusive plants, in which the patch is composed of a single species. Such association is extremely rare in nature, and if the most minute forms be considered, probably never occurs. On the other hand, heterogeneous commensalism, in which individuals of more than one species are present, is everywhere typical of vegetation. Warming regards saprophytism merely as a specialized kind of parasitism, an opinion that may well be defended. Helotism, however, is also a mere modification of parasitism, if it is not indeed parasitism pure and simple. Mutualism is an altogether vague concept, including parasites, epiphytes, and endophytes of doubtful physiological relation. Pound and Clements[25]treated lianes, parasites, and saprophytes as vegetation forms, relating herbaceous creepers and twiners to the lianes, and dividing the fungi and lichens into nine groups. Whatever the value of these divisions may be from the standpoint of vegetation forms, they represent the same relation between plant and nutritive stratum, and with respect to association should be merged in one group. Schimper[26]was the first to perceive the essential similarity of all such groups from the standpoint of association. He terms these plant societies (Genossenschaften), retaining the four groups already established, lianae, epiphyta, saprophyta, and parasiticae. It is evident that dependent association comprises extremely divergent forms, from the slightly clinging herb, such asGalium, to the most intense parasite. The distinction, however, is a clear one, if restricted to that relation between plants in which one acts as a mechanical support or stratum or as a nutritive host for the other.

255. Ground association.The first division of formations into open and closed was made by Engler and Drude.[27]Open formations were defined as those having incomplete stability and heterogeneous composition, while closed formations have a more definite uniform stamp. What is true of formations is equally true of vegetation, so that association may be regarded as open or closed with reference to the density and thoroughness with which the plants occupy the ground. In open association, the ground is slightly or partially occupied, readily permitting the entrance of new plants without the displacement of those already present. Such an arrangement is characteristic of the early stages of a formation, or of a succession of formations. It produces unstable open formations, which arise, usuallyafter denudation, in sand-hills, blowouts, gravel slides, dunes, flood plains, burned areas, etc. In closed association, occupation of the ground is complete, and the invasion of new species can occur only through displacement. Closed association results in stable, closed formations, such as forest, thicket, meadow, and prairie. As open association characterizes the early stages of a succession of formations, so closed association is peculiar to the later or last stages of all such successions. In short, open formations represent certain phases of the development of vegetation, while closed formations correspond to the relatively final structural conditions. It is a fundamental principle of association that every succession from denudation, or from newly formed soils, begins with open formations and ends with a closed formation. The causes leading up to open and closed association are intimately connected with development, and hence are considered under invasion and succession.

256. Species guild association.Drude has distinguished a kind of association peculiar to invasion, in which there is a successive or concomitant movement of certain species of a formation into another formation or region, resulting in species guilds (Artengenossenschaften). The association in this case is largely one of community of origin or area, and of concomitant migration. It is especially characteristic of areas adjacent to formational and regional limits. Fundamentally, it is merely the grouping of plants which are invading at the same time, and consequently it differs only in degree from what occurs in every invasion where more than a single individual is concerned. Accordingly, this type of association has little more than historical interest. This must not be construed to mean that it does not occur, but that it differs in no essential from the ordinary grouping of invaders.

257. Light association.The constituent species of formations show two fundamentally different groupings with respect to light. In the one case, the individuals are on the same level, or nearly so, in such a way that each has direct access to sunlight. Such an arrangement is characteristic of most grassland and herbaceous formations. In the case of desert formations, there is often considerable difference in the height of the plants, but the distance between them is so great as to admit of direct illumination of all. This arrangement may be termed coordinate association. In forests, thickets, and many herbaceous wastes, the height and density of certain species enable them to dominate the formation. In a dense forest, the trees receive practically all the light incident upon the formation, and the shrubs, herbs, fungi, and algae of lower habit and inferior position must adapt themselves to the diffuse light which passes through or between the leaves. The sameis equally true of dense thickets and wastes, except that the vertical distance is less, and the diffuseness of the light is correspondingly modified. In these formations, the dominant trees, shrubs, or herbs, the facies, constitute a primary or superior layer. The degree of subordinate association, as a result of which inferior layers will arise, is entirely determined by the density of the facies. In open woodlands, which are really mixed formations of woodland and grassland, the intervals, and usually the spaces beneath the trees also, are covered with poophytes, showing an absence of subordination due to light. This is the prevailing condition in the pine formation (Pinus ponderosa-xerohylium) of the ridges and foot-hills of western Nebraska. When, however, the trees stand sufficiently close that their shadows meet or overlap throughout the day, the increasing diffuseness begins to cause modification and rearrangement of the individuals. By photometric methods, the light in a forest is found to be least diffuse just below the facies, while the diffuseness increases markedly in passing to the ground. The taller, stronger individuals are consequently in a position to assimilate more vigorously, and to become still taller and stronger as a result. Just as these have taken up a position inferior to that of the facies, so the shorter or weaker species must come to occupy a still more subordinate position. This results, not only because the light is primarily weaker nearer the ground, but also because the taller plants interpose as a second screen. The complete working out of this arrangement with reference to light produces typical subordinate association, which finds its characteristic expression in the layering of forests and thickets. Layers tend to appear as soon as open woodland or thicket begins to pass into denser conditions, and up to a certain point, at which they disappear, they become the more numerous and the more marked, the denser the forest.

In the Otowanie woods near Lincoln (Quercus-Hicoria-hylium), layering usually begins at a light value of .1 (1 = normal sunshine in the open). Thornber[28]has found the same value to obtain in the thickets of the Missouri bluffs. In these, again, layers disappear at a value of .005, the extreme diffuseness making assimilation impossible except for occasional mosses and algae. A number of herbaceous plants are present in the spring, but these are all prevernal or vernal bloomers, which are safely past flowering before shade conditions become extreme. In theFraxinus-Catalpa-alsium, all inferior holophytic vegetation disappears between the light value of .004 and that of .003. The spruce-pine formation (Picea-Pinus-hylium) of the Rocky mountains, with a light value of .01, usually contains but a few scattered herbs, mostly evergreen; in some cases there are no subordinate plantsother than mosses and hysterophytes. The lodge-pole pine formation (Pinus murrayana-hylium), with light values often less than .005, is nearly or quite destitute of all but hysterophytic undergrowth. Such extremely dense formations are examples of coordinate association merely, since the formation is reduced to a single superior layer, in which the individuals of the facies bear the same spatial relation to incident light. In layered formations, in addition to the subordinate relation of other species to the facies, there is, of course, a kind of coordinate association manifested in each layer.

258. Water-content association.Schouw[29]was the first to give definite expression to the value of the water-content of the soil for the grouping of plants. He established four groups: (1) water plants, (2) swamp plants, (3) plants of moist meadows, (4) plants of dry soils. The first he termed hydrophytes, introducing the term halophytes to include all saline plants. Thurmann[30]recognized the fundamental influence of water-content upon association, and further perceived that the amount of water present was determined primarily by the physical nature of the soil. He distinguished plants which grow in soils that retain water ashygrophilous, and those found upon soils that lose water readily asxerophilous. Those which seemed to grow indifferently upon either were termedubiquitous. The latter correspond in some measure to mesophytes, but they are really plants possessing a considerable range of adaptability, and do not properly constitute a natural group. Warming[31]proposed the term mesophytes to include all the plants intermediate between hydrophytes and xerophytes. He recognized the paramount value of water-content association as the basis of ecology, and upon this made a logical and systematic treatise out of the scattered results of many workers. Schimper[32]placed the study of vegetation upon a new basis by drawing a distinction between physical and physiological water-content, and by pointing out that the last alone is to be taken into account in the study of plant life, and hence of plant geography. Accepting the easily demonstrable fact that an excess of salts in the soil water, as well as cold, tends greatly to diminish the available water of the soil, i. e., the chresard, it is at once seen why saline and arctic plants are as truly xerophytic as those that grow on rocks or in desert sands. An anomalous case which, however, physical factor records have explained fully, is presented by many plants growing in alpine gravel slides, strands, blowouts, sandbars, etc., in which the water-content is considerable, but the water loss excessive, on accountof extreme heat or reduced air pressure. The effect of these conditions is to produce a plant xerophytic as to its aerial parts, and mesophytic or even hydrophytic as to subterranean parts. Such plants may, from their twofold nature, be termeddissophytes; they are especially characteristic of dysgeogenous soils in alpine regions where transpiration reaches a maximum, but are doubtless to be found in all gravel and sand habitats with high water-content. With these corrections, the concept of water-content association, which owes much to both Warming and Schimper, but is largely to be credited to Thurmann, becomes completely and fundamentally applicable to all vegetation.

Back to Index Next