THE FORMATION IN DETAIL

Fig. 74. Numerical alternation ofPinusandPseudotsugaupon east and west slopes.

All species that alternate show a variation in abundance from one area to another. Frequently, the difference is slight, and may be ignored, except in determining abundance. Very often, however, the variation is so great that a facies may be reduced, numerically, to the rank of a principal species, or one of the latter to a secondary species. This phenomenon is distinguished asnumericalalternation. It arises from the fact that the similar areas are sufficiently different to affect the abundance, without producing complete suppression. It is probable that this result is due almost entirely to competition.Astragalus crassicarpusgrows on all the slopes of the prairie formation, but on some it has the abundance of a facies, while on others it is representedby a few scattered individuals. This difference is much more striking in separate examples of the same formation, particularly when a normal facies is reduced to the numerical value of a secondary species. This is a matter of great importance in the study of formations, for it has doubtless often resulted in mistaking a consocies for a formation.

Alternation furnishes the logical basis for what may be called comparative phytogeography. The latter is of much broader scope than the old subject of geographical distribution, for it treats not only of the distribution of formations and associations as well as of species, but it also seeks to explain this by means of principles drawn from the relation between habitat and vegetation. When the latter come to be fully based upon physical factor investigations, and upon the effects of migration and competition as shown in alternation, the comparative study of formations will represent the highest type of phytogeographical activity.

346. The rank of the formation.There have been as many different opinions in regard to the application of the term formation as there are concerning the group which is to be called a species. In taxonomy, however, the concept of the species is purely arbitrary, and agreement can not be hoped for. In vegetation, on the contrary, the connection between formation and habitat is so close that any application of the term to a division greater or smaller than the habitat is both illogical and unfortunate. As effect and cause, it is inevitable that the unit of the vegetative covering, the formation, should correspond to the unit of the earth’s surface, the habitat. This places the formation upon a basis which can be accurately determined. It is imperative, however, to have a clear understanding of what constitutes the difference between habitats. A society is in entire correspondence with the physical factors of its area, and the same is true of the vegetation of a province. Nevertheless, many societies usually occur in a single habitat, and a province contains many habitats. The final test of a habitat is an efficient difference in one or more of the direct factors, water-content, humidity, and light, by virtue of which the plant covering differs in structure and in species from the areas contiguous to it. A balsam-spruce forest shows within itself certain differences of physical factors and of structure. The water-content will range from 20–25 per cent, and the light from .02–.003. One portion may consist chiefly ofPseudotsuga mucronata, another ofPicea engelmannii, and a third ofPicea parryana, or these species may be intermingled. If, however, this forest is compared with the gravel slide, which touches it on one side, and the meadow thicket, which meets it on another,the physical factors and the species both demonstrate that it is the forest, and not its parts, which corresponds to a distinct physical entity, the habitat. This test of a formation is superfluous in a great many cases, where the physiognomy of the contiguous areas is conclusive evidence of their difference. It is evident also that remote regions which are floristically distinct, such as the prairies and the steppes, may possess areas physically almost identical and yet be covered by different formations. This point is further discussed under classification.

The existing confusion in the matter of formations is due to two causes. The first arises from the fact that much ecological work has been hasty. Little or no attention has been given to development, and in consequence rudimentary and transitory stages of succession have often been described as formations. Mixed areas in particular have caused trouble. In the second place, there has been a marked tendency to minimize the need of thoroughness and training by calling every slightly different area a formation. A failure to recognize the primary value of alternation has also contributed materially to this. Alternating facies, and principal species, when separated from each other, have often been mistaken for formations. This is a danger that must be fully appreciated and guarded against. In practically all regions, the same formation is represented by numerous scattered areas, all showing greater or less differences arising from alternation. This is especially true of thickly populated regions where virgin areas are rare. The fact that twenty-five miles intervene to-day between two small stretches of primitive prairie is permitted to unduly emphasize their differences. It requires the study of a number of such examples to counteract this tendency, and to cause one to see clearly that they must have been at one time merely so many bits of the prairie formation.

In this connection, the lichen and moss groups which are found on rocks constitute an interesting problem. It is clear thatPeltigeraandCladonia, which grow on the forest floor, andEvernia,Ramalina, andPhyscia, which are found on the trees, are merely constituent species of the forest formation. The same is true ofCladonia,Urceolaria, andParmelia, which are found among the sedges and grasses of alpine meadows. The physical conditions are essentially those of the formation, and the lichens themselves are more or less peculiar to it. This is particularly true of the forest, in which the two strata, bark and moist shaded soil, are present because of the trees. In the case of granitic rocks, the circumstances are very different. The species of lichens found on the rocks are not peculiar to the formation, but they also occur elsewhere. In the forest,Parmelia,Placodium,Physcia,Rinodina,Urceolaria,Lecanora,Lecidea, etc., occur on the rocks. In the alpine meadows, the rock groups are composed ofParmelia,Gyrophora,Cetraria,Acarospora,Lecanora,Lecidea,Buellia, etc. The stratum itself is physically very different and constitutes a distinct habitat. These groups are really small formations, which are quite distinct from the surrounding forest or meadow. This is proven conclusively in many places in the mountains where areas of the characteristic lichen formations of cliffs are carried by the fall of rock fragments into forest and meadow, where they persist without modification. This also shows clearly that the groups on scattered rocks in the same area are to be regarded as examples of the same cliff formation, except where the differences are evidently to be ascribed to development and not to alternation. Where these rock formations can not be traced to cliffs or magmata with certainty, they must be considered as antedating the vegetation in which they occur. Often, indeed, especially in igneous areas, they are relicts of the initial stage of a primary succession. Finally, they prove their independence of the forest or meadow formation by initiating a distinct succession within these. Crustaceous groups or formations yield to foliose ones, and these in turn give way to formations of mosses, particularly in the forest where the effect of the diffuse light is felt. From the above, the following rule of formational limitation is obtained: any area, which shows an essential difference in physical character, composition, or development from the surrounding formation is a distinct formation.

Fig. 75. Relict lichen formation in a spruce forest, invaded by rock mosses.

347. The parts of a formation.All the parts which make up the structure of a formation are directly referable to zonation and alternation, alone or together, or to the interaction of the two. The principles which underlie this have already been discussed under the phenomena concerned. It is necessary to point out further that the structure may be produced in several ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as primary and alternation as secondary, (4) by primary alternation and secondary zonation, (5) by the interaction of the two, as in layered formations. Though all these methods occur, the first two are relatively rare, and the resulting structure comparatively imperfect. The typical structure of formations can best be made clear by the consideration of a prairie which belongs to the fourth group, and a forest which represents the last.

Fig. 76. Early (prior) aspect of the alpine meadow formation (Carex-Campanula-coryphium), characterized byRydbergia grandiflora.

The major divisions of prairie and forest formations are regularly due to alternation. There is an inherent tendency to the segregation of facies, arising out of physical or historical reasons, or from a combination of both. Not all formations show this, but it is characteristic of the great majority of them. The primary areas which thus arise have been called associations: they are naturally subordinate to the formation. To avoid the confusion which inevitably results from using the word association in two differentsenses, it is proposed to term this primary division of the formation, aconsociation, or better, aconsocies. This term is applied only to an area characterized by a facies, or less frequently, by two or more facies uniformly commingled. The consocies of grassland are determined by grasses, those of forests by trees, etc. From the different position of the facies in these two types of vegetation such areas are readily seen at all times in the forest, but they are often concealed in grassland by the tall-growing principal species of the various aspects. When definite consocies are present, they are often found to mingle where they touch, producing miniature transition areas, and, very rarely, they sometimes leave gaps in which no facies appears.

Fig. 77. Late (serotinal) aspect of the alpine meadow, characterized byCampanula petiolata,Rydbergiain fruit.

The seasonal changes of a formation, which are called aspects, are indicated by changes in composition or structure, which ordinarily correspond to the three seasons, spring, summer, and autumn. The latter affect the facies relatively little, especially those of woody vegetation, but they influence the principal species profoundly, causing a grouping typical of each aspect. For these areas controlled by principal species, but changing from aspect to aspect, the termsocietyis proposed. They are prominent features of the majority of herbaceous formations, where they are often more striking than the facies. In forests, they occur in the shrubby and herbaceous layers, andare consequently much less conspicuous than the facies. A close inspection of the societies formed by principal species shows that they are far from uniform. Since they usually fail to exhibit distinct parts, it becomes necessary to approach the question of their structure from a new standpoint. Such is afforded by aggregation, which yields the simplest group in vegetation, i. e., that of parent and offspring. This is so exactly a family in the ordinary sense that there seems to be ample warrant for violating a canon of terminology by using the word for this group, in spite of its very different application in taxonomy. It has already been shown that aggregation further produces a grouping of families, which may properly be called acommunity. As they are used here,familyandcommunitybecome equally applicable to the association of plants, animals, or man. Both families and communities occur regularly in each society of the formation, and they represent its two structures. In some cases, all the families are grouped in communities, two or more of which then form the society. Very frequently, however, families occur singly, without reference to a community, and the two then constitute independent parts of the same area. This is typically the case wherever gregarious species are present, since these are merely family groups produced by aggregation.

Fig. 78.Calthetum(Caltha leptosepala), a consocies of the alpine bog formation.

Fig. 79.Iridile(Iris missouriensis), a society of the aspen formation.

Objection may be made that this analysis of formational structure has been carried too far, and that some of the structures recognized are mere interpretations, and not actual facts. Such a criticism will not come from one who has got beyond the superficial study of formations, for he will at once recognize that certain probable features of structure have not been considered. On the other hand, the ecologist or the botanist who has not made a careful investigation from the standpoints of development and structure will naturally refrain from expressing an opinion, until he has obtained an acquaintance at first hand with the facts. Over-refinement is the usual penalty of intensive work. The unbiased investigator, however, will not be misled by the suddenness with which new concepts appear. It seems plausible that the structure of a formation, if not as definite, is at least nearly as complex as that of an individual plant. Few botanists will insist that the refinement of tissues and tissue systems has been carried further than the differentiation of the plant warrants. Yet, if these had been defined within a period of a few years rather than slowly recognized during more than a century, they would have been called seriously in question. As a matter of fact, the consocies, under the term association, and the society, under various names, have been recognized by ecologists for several years. They are definitephenomena of alternation which can be found anywhere. The family and the community, though the latter is less distinct in outline, are equally valid structures, the proof of which anyone can obtain by thorough methods of study.

348. Nomenclature of the divisions.The suffix-etumis used to designate a consocies of a formation, e. g.,Picetum,Caricetum, etc. When two or more species characterize the area, the most important, or more rarely, the two are used. The termination used to designate a society is-ile, asAsterile,Sedile,Rosile. The suffix which denotes the community is-are, and for the family, it is-on, viz.,Giliare,Bromare,Bidenton,Helianthon, etc. Layers are indicated by the affix-anum, asOpulasteranum,Verbesina-Rudbeckianum, etc. It is evident that these suffixes, like the terms to which they refer, must be used always for the proper divisions if they are to have any value at all. There has been a marked tendency, for example, to use-etumin connection with the names of groups of very different rank. It is hardly necessary to point out that such a practice does not promote clearness. The following tabular statement will illustrate the application of both terms and suffixes:

349. The investigation of a particular formation.A comprehensive and thorough study of a formation should be based upon as many examples of it as are accessible. The example which is at once the most typical and the most accessible is made the base area. This plan saves time and energy, reduces the number of instruments that are absolutely necessary, and establishes a common basis for comparison. The inquiry should be made along four lines, all fundamental to a proper knowledge of the formation. These lines are: (1) the determination of the factors of the habitat, (2) a quadrat and a transect study of the structure of the formation, (3) a similar investigation of development, (4) a floristic study of the contiguous formation, with special reference to migration. The sequence indicated has proven to be the most satisfactory, and is to be regarded as all but absolutely essential. Naturally, this applies only to the order in which the various lines are to be taken up, as they are carried on together when the work is fully under way. Since instrument and quadrat methods have already been given in detail, itis unnecessary that they be repeated. Similarly, the questions which pertain to structure and development and to the surrounding vegetation are considered in detail in the pages which precede.

Fig. 80.Eritrichiare(Eritrichium aretioides), a community of the alpine meadow formation.

350. Bases.Formations may be grouped with reference to habitat or kind, development or position. Classification upon the basis of habitat places together formations which are similar in physiognomy and structure. Developmental classification is based upon the fact that the stages of a particular succession are organically connected or related, though they are normally different in both physiognomy and structure. Grouping with respect to position is made solely upon occurrence in the same division of vegetation. The formations thus brought together usually possess neither similarity of kind or structure, nor do they have any necessary developmental connection. Habitat and developmental classification are of fundamental value; regional arrangement is more superficial in character. All serve, however, to emphasize different relations, and, while the developmental system expresses the most, they should all be used to exhibit the vegetation of a region, province, or zone.

Fig. 81.Pachylophon(Pachylophus caespitosus), a family of the gravel slide formation.

351. Habitat classification.In arranging formations with reference to habitats, the direct factors, water and light, can alone be used to advantage. Such a system is fundamental, because it is founded upon similarity of habitat and of structure. Proposed groupings based upon nutrition-content, or upon the division of factors into climatic and edaphic, have elsewhere[42]been shown to be altogether of secondary importance, if not actually erroneous. The basis of the habitat grouping is water-content, which is supplemented by light whenever the factor is decisive. The primary divisions thus obtained are water, forest, grassland, and desert, which are characterized respectively by associations of hydrophytes, mesophytes, hylophytes, poophytes, and xerophytes respectively. Within these, formations are arranged according to the type of habitat, i. e., pond, meadow, forest, dune, etc. These divisions comprise all formations which belong to the type by virtue of their physiognomy and structure. Such formations differ from each other very considerably or completely in the matter of floristic, i. e., component species, but they still belong to the same type. A dune formation in the interior and one on the coast may not have a single species in common, and yet they are essentially alike in habitat, development, and structure.

352. Nomenclature.The names of formations are taken from the habitats which they occupy. Each formation should have a vernacular and a scientific name. The latter is especially important since it ensures brevity and uniformity, and obviates the obscurity and confusion that arise from vernacular terms in many tongues. Scientific names have been made uniformly from Greek words of proper meaning by the addition of the suffix-ium(εῖον), which denotes place.[43]The following list gives the English and the scientific name of the various habitats, and their corresponding formations, and indicates the primary divisions into which these fall.

Particular formations are indicated by means of floristic distinctions. Thus,Populus-hyliumis the aspen forest as distinguished from thePicea-Pseudotsuga-hylium, or the balsam-spruce forest; and theBulbilis-psilium, or buffalo-grass prairie, from theBouteloua-Andropogon-psilium, or grama-bluestem prairie. Similarly, the aspen formation of the Old World and of the New may be distinguished asPopulus-tremula-hyliumandPopulus-tremuloides-hylium, respectively. In all formational names, the facies alone should be used. Frequently, a single facies will suffice for clearness. As a rule, however, the two most important facies should be employed; in rare cases only is it necessary to use the names of three. When it is desirable to refer to two or more examples of the same formation, a geographical term is added, e. g., (1)Populus-hylium(Crystal Park), (2)Populus-hylium(Cabin Canyon).

353. Developmental classification.This is based upon succession as the record of development. Upon the basis of development, all the formations which belong to the same succession are classed together. They are arranged within each group in the sequence found in the particular succession. From its nature, developmental classification is of primary importance in exhibiting the history of vegetational changes. It has less value than thehabitat system for summarizing the essential structure of a vegetation, inasmuch as it places the emphasis upon historical rather than structural features. It is evident that both deal with the same formations, and that the difference is merely one of viewpoint. The habitat classification is simpler in that it considers only those formations actually on the ground, while development has regularly to take into account stages which have disappeared. The groups of the developmental system, and the arrangement of formations within them have already been indicated under the nomenclature of succession (sections 326 and 327).

354. Regional classification.The grouping of formations with respect to the divisions of vegetations is chiefly of geographical value. It indicates a certain general relationship, but its principal use is to summarize the structure of the vegetative covering of a region. The arrangement of formations in the various divisions is made with reference to the outline of North American vegetation (section 341). This is naturally based upon the identity of altitude and latitude zones. In the study of mountain countries, it is often desirable to group formations with reference to altitude alone. In this case, the grouping is based upon the following divisions: (1)bathyphytia, lowland plant formations; (2)mesiophytia, midland formations; (3)pediophytia, upland formations; (4)pagophytia, foot-hill formations; (5)orophytia, subalpine formations; (6)acrophytia, alpine formations; (7)chionophytia, niveal formations.

355. Mixed formations.These are mixtures of two, rarely more, adjacent formations, or of two consecutive stages of the same succession. Mixed formations are really transitions in space or in time between two distinct formations. Theoretically, they are to be referred to one or the other, according to the preponderance of species. Actually, however, they often persist in an intermediate condition for many years, and it becomes necessary to devote considerable attention to them. In some cases, there is good reason to think that the species of two contiguous formations have become permanently associated, and thus constitute a new formation. This is often apparently true in succession, when the change from one stage to the next requires a long term of years, but it is really true only of the very rare cases in which a succession becomes stabilized in a transition stage. When the mixture is due to development, the formations concerned are often quite dissimilar, e. g., grassland and thicket, thicket and forest. If it is the result of position, the formations are usually similar, i. e., both are grassland, thicket, or forest, since the plants of the lower level are regularly assimilated or destroyed, when invasion occurs at two levels. The termmictium(μικτόν,mixture) is here proposed for the designation of all mixed formations, whether they arise from succession or from juxtaposition. Thus, theMentzelia-Elymus-mictiumis the transition between theMentzelia-Pseudocymopterus-chaliciumand theElymus-Muhlenbergia-chalicium. Similarly, thePopulus-Picea-mictiumand thePinus-Pseudotsuga-mictiumare transition stages in the development of thePicea-hylium. On the other hand, theAndropogon-Bulbilis-mictiumis a mixture produced by the mingling of two contiguous prairie formations. In the future development of this subject, it will probably become desirable to name mixed formations on the basis of origin, but at present this is unnecessary. Both in classification and in description they should be considered between the formations which give rise to them, and this will at once indicate their origin.

Fig. 82. A mixed formation of aspens and spruces (Populus-Picea-mictium), preceding the final spruce forest of a burn succession.

Puzzling cases of mixture resulting from position occur toward the limits of facies which occupy extensive areas.Bouteloua oligostachya, andAndropogon scopariusextend from the prairies through the sand-hills and plains, and into the foot-hills of the Rocky mountains. Their abundance at once raises a question as to the validity of the prairie, sand-hill, plain, and foot-hill formations. If these two grasses were controlling, and equally characteristic throughout, then the entire stretch would have to be regarded as asingle formation. Since they are often absent, or mixed with other facies of greater importance, they can not be considered the sole tests of the formation. This view is reinforced by the fact that prairie, sand-hill, plains, and foot-hill all have their characteristic principal and secondary species, in addition to facies that are more or less typical. In certain formations, doubtless,BoutelouaandAndropogonare relicts, in others invaders, while in the formations actually constituted by them they are dominant. The final solution of such problems is quite impossible, however, until the comparative study of large areas can be based upon the accurate detailed investigation of the component formations.

356. Scope and methods.The experimental study of the formation as a complex organism rests upon methods essentially similar to those discussed under experimental evolution. The scope of the two fields is practically the same, moreover, in that both deal with the experimental development of an organism and the structures that result. The actual problems are naturally very different, since the formation is a complex of individual plants, but the fundamental basis of habitat, function, and structure is common to both. However, the functions now to be considered are aggregation, invasion, competition, etc., and the structures, zones, consocies, societies, communities, and families. The latter may properly be regarded as adaptations called forth by the adjustment, i. e., aggregation, migration, ecesis, etc., of the formation to the physical factors of the habitat. As consequences of measured factors, formational adjustment and adaptation must themselves be carefully measured and recorded. For these purposes, the methods of quadrat and transect, of chart, photograph, and formation herbarium are used. Invaluable as they are for any scientific inquiry into vegetation, such methods form the very foundation of experimental study in which accuracy is the first desideratum.

It has already been shown that nature’s own experiments in the production of new forms furnish the best material for experimental evolution. This statement is equally true of experimental vegetation. The formation of new habitats by weathering and transport, and the denuding of old ones, yield experimental plots of the greatest value. This is likewise the case in the great majority of formations, where invasion or competition is active. These are the phenomena that must be considered in any careful study of vegetation, but in taking them up from the experimental standpoint, greater attention must be paid to detail, and the changes must be followed closely for a longer time. The method that makes use of existing changes in vegetationis designated themethod of natural habitats. In contrast with this is themethod of artificial habitats, in which the habitat itself is definitely modified, or a group of species actually transferred to a different habitat. Many problems of vegetation can be attacked with greater success under control than in the field. This is particularly true of competition, in which results can be obtained most readily by means of themethod of control habitats, as carried on in the plant house.

357. Natural experiments.Every family as well as every community constitutes an experiment in competition; the same statement necessarily holds for the larger groups, society, consocies, and formation, which are composed of families and communities. The last also make it possible to study competition in two typical instances, viz., in the family, where the individuals are of one kind, and in the community, where they belong to two or more different species. The community, moreover, is a product of invasion, and it furnishes material for the study of this function, as well as for that of aggregation and competition. Practically every formation shows some invasion, but as a rule stable formations contain so few invaders that they are relatively unimportant in this connection. Invasion is most active in transition areas and in mixed formations, whether produced by juxtaposition or by succession, and its study in these places yields by far the largest number of valuable results.

As typical complete invasion, a succession is the best of all natural experiments in aggregation, migration, ecesis, and competition. This is especially true of the initial stages in which changes in the number and position are most readily followed. The methods used in studying successions have been given elsewhere. In addition, it should be pointed out that one of the first tasks in taking up the ecological investigation of a region is to make a careful search for all new and denuded areas, as well as for those in which succession is taking place. The phenomena in these areas can not be explained until the habitats and formations have been worked over critically, but the facts must be collected at the earliest possible moment, since the stages of the succession are constantly changing, while the stable formations are not.

358. Modification of habitat.As the final factors in ecesis and competition, water, light, and temperature control the grouping of plants into vegetation. An efficient change in one of these, or in all of them, brings about a visible adjustment in the structure of the plant group concerned. Modificationsof water-content and light are readily produced in the field by drainage, irrigation, shading, clearing, etc. In fact, all the changes of habitat indicated under experimental evolution serve equally well to initiate experiments in experimental vegetation; indeed, the same experiment covers both fields. It is impracticable, however, to modify the temperature of a habitat without changing its water-content or light, and consequently the influence of temperature can not be determined through experiment by modification. The extent of the area modified should be as large as convenience will permit, in order that the number of individuals may be large enough to indicate clearly the resulting adjustment in position and arrangement. The best results can be obtained where a small separate area of a formation can be modified, e. g., where a small swamp can be drained, or a depression flooded. In the case of light, however, it is usually impossible to clear or to shade a large area, and the study must be restricted to a relatively small group of plants. In regions where lumbering is actively carried on, the consequent clearing initiates invaluable experiments over large areas, and this is likewise true of forest plantations. Modification of a large area has decided advantages in bringing out the changes in the more prominent structural features, but the causes and the details of the adjustment can be worked out much more satisfactorily in a small area.

359. Denuding.The modification of the habitat by denuding is the sole method of initiating succession by experiment. It is consequently of the most fundamental importance in investigating aggregation, ecesis, and competition, as well as the reactions exerted by the invaders of the different stages. The possibilities of denuding an entire habitat or an extensive area are not great, and the investigator must content himself with denuded quadrats, transects, and migration circles, which are small enough to permit a critical study of all the factors in succession. It is of course unnecessary that the denuding be done by the ecologist himself, provided he is able to follow the succession from the very beginning. Accordingly, it becomes possible for him to make the very best use of all those changes wrought by man in which the vegetation is destroyed over considerable areas. These are essentially natural experiments, and at this point the methods of natural and artificial habitats merge.

The manner of denuding depends in a degree upon the nature of vegetation, but, when time, convenience, and safety are all taken into account, the actual removal of the vegetation as indicated under the denuded quadrat is by far the most satisfactory. Under certain conditions, flooding or burning can be used to advantage, but cases of this kind are infrequent. The purpose of the experiment determines the kind of area to be denuded.Quadrat, transact, and migration circle are equally valuable for ecesis and competition. The quadrat is best adapted to work in a homogeneous area, while the transect is suited to a heterogeneous one characterized by zones, societies, or communities. It is an advantage to replace the denuded transect by a series of denuded quadrats, one for each zone or society, when the transect would be too long for convenience. The denuded migration circle is invaluable for aggregation and ecesis, since it makes possible the study of migration as a distinct function. A series of denuded quadrats, consisting of one or more in the different stages of a succession, furnishes important evidence concerning the development of each stage. By far the best method, however, for making a comparative study of the stages of a succession is the quadrat sequence. A quadrat is denuded each year, thus yielding a complete sequence of miniature stages through the whole course of succession. This method is especially valuable when a succession is represented by a single example, and there is no opportunity of reconstructing it by the comparison of various stages. A quadrat sequence is naturally of the greatest value if begun at the time when the first invaders appear.

360. Modification of the formation by transfer.The study of partial and intermittent invasion into an established vegetation is made through the transfer of a species or group of species by means of seeding or planting. The process differs in no way from that described for experimental evolution, except in so far that an endeavor is made to establish a family or a community, and not merely a few individuals. Transfer makes possible the critical investigation of ecesis under conditions of intense competition, as well as the study of aggregation and the origin of plant groups under these conditions. Perhaps its greatest value is in the experimental study of alternation and zonation, especially the former. It is practically impossible to determine whether alternation, especially when corresponsive, is due to physical or historical causes, i. e., migration and competition, except by means of the reciprocal transfer of the species concerned.

Field cultures for the careful study of ecesis and competition are made by transferring seeds or plants to new or denuded soils. This is practically a combination of the methods of modification and transfer. It has a unique value in making it possible to initiate artificial successions of almost any character that is desired, and to carry them out with the reactions more or less under control. This opens up an extremely important field of experimental inquiry, which promises to put the study of succession upon a much more exact basis. Competition cultures in the field are not essentially different from those under control, and they will be considered under the next method.

METHOD OF CONTROL HABITATS

Fig. 83. Simple culture of floating ecads ofRanunculus sceleratus.

361. Competition cultures.Although it is quite possible to carry on experiments in invasion and succession in the planthouse, the limited space usually available makes this undesirable, except in a few problems where control is necessary. Competition cultures, on the other hand, yield better results in the planthouse than in the field, since the physical factors and the appearance of unwelcome migrants are much more easily controlled. The possibilities of the culture method in the study of competition seem inexhaustible, and the author has found it necessary to confine his own investigations to a few of the fundamental problems. In this work, he has distinguished several kinds of cultures, based chiefly upon the species concerned and the arrangement of the individuals.Simplecultures are those in which a single species is used. The resulting group is a family, and the competition is between like individuals. In such cultures, the problem of the factors in competition is reduced to its simplest terms.Mixedcultures are based upon two or more species, and the problem is correspondingly complicated. As a rule, all the seeds have been sown at the same time in both simple and mixed cultures, but it has been found desirable to make someheterochronouscultures, in which seeds are also sown after the plants have appeared. Mixed cultures are distinguished aslayeredcultures, whenthe species are of very different height. Thus, rosettes have been grown with stemmed plants, tall slender forms with low branching ones, erect plants with twining and climbing plants, etc. Further evidence as to the nature of competition has been sought by means ofecadcultures, andfactorcultures. In the former, plants of different response to water and light are grown together under the same conditions, in order to evaluate the part played by the nature of the plant. In a factor culture, the area is divided into two or more parts which are given different amounts of water or of light, in order to determine the influence of slight variations upon the same competitors. In somewhat similar fashion, an attempt has been made to ascertain the bearing of biotic factors upon competition. Cultures are easily made in whichCuscutaor parasitic fungi are used to place certain species at a disadvantage.Permanentcultures are obtained by allowing the plants to ripen and drop their seeds for several generations, just as in nature. They are indispensable for determining the final outcome of the competition between different species.

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