Chapter IV: The Plant Formation (7)

=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 of _Pseudotsuga mucronata_, another of _Picea engelmannii_, and a third of _Picea 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 that _Peltigera_ and _Cladonia_, which grow on the forest floor, and _Evernia_, _Ramalina_, and _Physcia_, which are found on the trees, are merely constituent species of the forest formation. The same is true of _Cladonia_, _Urceolaria_, and _Parmelia_, 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 of _Parmelia_, _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.

=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.

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 different senses, it is proposed to term this primary division of the formation, a _consociation_, or better, a _consocies_. 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.

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 term _society_ is 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, and are 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 a _community_. As they are used here, _family_ and _community_ become 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.

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 definite phenomena 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 _-etum_ is 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_, as _Asterile_, _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_, as _Opulasteranum_, _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 _-etum_ in 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:

_Picea-Pseudotsuga-hylium_ formation (_-ium_) _Paronychia-Silene-chalicium_
_Picetum_ consocies (_-etum_) _Paronychietum_
_Opulaster-Ribesanum_ layer (_-anum_)
_Opulasterile_ society (_-ile_) _Androsacile_
_Thalictrare_ community (_-are_) _Festucare_
_Pirolon_ family (_-on_) _Arenarion_

=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, it is 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.

CLASSIFICATION AND RELATIONSHIP

=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.

=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.

I. Hydrophytia: water plant formations

1. ocean: oceanium: oceanad,[44] oceanophilous, etc.

2. sea: thalassium

surface of the sea: pelagium

deep sea: pontium

3. lake: limnium, limnad

4. pond, pool, tiphium, tiphad

5. stagnant water: stasium: stasad

6. salt marsh: limnodium, limnodad

7. fresh marsh: helium

8. wet meadow: telmatium

9. river: potamium

10. creek: rhoium

11. brook: namatium

12. torrent: rhyacium

13. spring: crenium

14. warm spring: thermium

15. ditch: taphrium

16. sewer: laurium

17. swamp forest: helohylium

18. swamp open woodland: helodium

19. meadow thicket: helodrium

20. bank: ochthium

rock bank: petrochthium

sand bank: ammochthium

mud bank: pelochthium

21. rocky seashore: actium

22. sandy seashore: agium

23. sandbar: cheradium

24. tank: phretium

II. Mesophytia: middle plant formations

_a._ Sciophytia: shade plant formations

26. forest: hylium

27. grove: alsium

28. orchard: dendrium

29. canyon: ancium

30. open woodland: orgadium

31. thicket: lochmium

_b._ Heliophytia: sun plant formations

32. meadow: poium

33. pasture: nomium

34. culture land: agrium

35. waste place: chledium

III. Xerophytia: dry plant formations

36. desert: eremium

37. sand-hills, sandy plain: amathium

38. prairie, plains: psilium

39. dry, open woodland: hylodium

40. dry thicket: driodium

41. dry forest: xerohylium

42. gravel slide: chalicium

43. sandbar: syrtidium

44. sand draw: enaulium

45. blowout: anemium

46. strand: psamathium

47. dune: thinium

48. badlands: tirium

49. hill, ridge: lophium

50. cliff: cremnium

51. rock field: phellium

52. boulder field: petrodium

53. rock, stone: petrium

54. humus marsh: oxodium

55. alkali area: drimium

56. heath, dry meadow: xeropoium

57. moor: sterrhium

58. alpine meadow: coryphium

59. polar barrens: crymium

60. snow: chionium

61. wastes: chersium

Particular formations are indicated by means of floristic distinctions. Thus, _Populus-hylium_ is the aspen forest as distinguished from the _Picea-Pseudotsuga-hylium_, or the balsam-spruce forest; and the _Bulbilis-psilium_, or buffalo-grass prairie, from the _Bouteloua-Andropogon-psilium_, or grama-bluestem prairie. Similarly, the aspen formation of the Old World and of the New may be distinguished as _Populus-tremula-hylium_ and _Populus-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 the habitat 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 term _mictium_ (μικτόν, mixture) is here proposed for the designation of all mixed formations, whether they arise from succession or from juxtaposition. Thus, the _Mentzelia-Elymus-mictium_ is the transition between the _Mentzelia-Pseudocymopterus-chalicium_ and the _Elymus-Muhlenbergia-chalicium_. Similarly, the _Populus-Picea-mictium_ and the _Pinus-Pseudotsuga-mictium_ are transition stages in the development of the _Picea-hylium_. On the other hand, the _Andropogon-Bulbilis-mictium_ is 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.

Puzzling cases of mixture resulting from position occur toward the limits of facies which occupy extensive areas. _Bouteloua oligostachya_, and _Andropogon scoparius_ extend 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 a single 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, _Bouteloua_ and _Andropogon_ are 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.

EXPERIMENTAL VEGETATION

=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 vegetation is designated the _method of natural habitats_. In contrast with this is the _method 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 the _method of control habitats_, as carried on in the plant house.

METHOD OF NATURAL HABITATS

=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.

METHOD OF ARTIFICIAL HABITATS

=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. Modifications of 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

=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. _Simple_ cultures 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. _Mixed_ cultures 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 some _heterochronous_ cultures, in which seeds are also sown after the plants have appeared. Mixed cultures are distinguished as _layered_ cultures, when the 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 of _ecad_ cultures, and _factor_ cultures. 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 which _Cuscuta_ or parasitic fungi are used to place certain species at a disadvantage. _Permanent_ cultures 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.

=362. Details of culture methods.= All competition cultures have been made 1 meter square. In other words, they are quadrats, and they are treated exactly as denuded quadrats in the field with respect to factor readings, charts, and photographs. In the writer’s studies, germination tests were made of a large number of species, and those selected which showed a high per cent of germinability. Since this was the first experimental study of competition, this test was deemed necessary, but it is quite evident that no such selection is made in nature. Consequently, when the seeds used are known to be fresh, a germination test is usually superfluous. Considerable care was taken also to select species known to be vigorous growers, with the result that practically all the species used for experiment were ruderal or subruderal. The species employed, and the kinds of cultures in which they were grouped were as follows:

1. _Simple culture of Helianthus annuus._ The culture plot was divided
into four equal parts; 12 seeds were planted in one, 25 in another, 50
in the third, and 100 in the fourth.

2. _Mixed culture of Helianthus annuus, Panicum virgatum, and Elymus
canadensis._ Twenty-five seeds each of _Helianthus_ and _Panicum_ were
planted alternately at equal distances in one-half of the plot, while
the other half was planted similarly with _Helianthus_ and _Elymus_.

3. _Mixed culture of Solidago rigida and Onagra biennis._ Over
one-half of the plot were scattered 50 seeds of _Solidago_ and 100 of
_Onagra_; over the other, 100 and 200 seeds respectively.

4. _Layered culture of Laciniaria punctata, Bidens frondosa, Salvia
pitcheri, Cassia chamaecrista and Kuhnia glutinosa._ Fifty seeds of
each species were scattered more or less uniformly over the entire
plot.

5. _Layered culture of Silphium laciniatum, Datura stramonium and
Lactuca ludoviciana._ Fifty seeds of _Datura_ and _Lactuca_, and 25 of
_Silphium_ were sown uniformly in one-half of the plot. In the other
half, 25 holes were made at equal intervals, and one seed of each of
the three planted in each hole.

6. _Ecad culture of Oenothera rhombipetala (xerophytic), Verbascum
thapsus (mesophytic), and Penthorum sedoides (hydrophytic)._ One
hundred seeds of _Oenothera_ and 200 each of _Verbascum_ and
_Penthorum_ were scattered over the plot.

7. _Heterochronous culture of Helianthus annuus and Datura
stramonium._ One hundred seeds of Helianthus were scattered over one
half, and the same number of Datura seeds over the other half of the
plot. In both, also, 50 seeds were sown in one 4–inch circle, and 25
seeds in a second circle at some distance. A month later, 100 seeds of
_Helianthus_ were sown in the _Datura_ plot, and _vice versa_.

8. _Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra,
Datura, Penthorum, Solidago and Verbascum._ The plot was divided into
9 squares and in each were sown 50 seeds of one of these plants.

9. _Community culture._ The sowing was made exactly as for the family
culture, except that 20 seeds of each plant were used. In the middle
of each square, 5 seeds of a different species were planted. For the
_Helianthus_, _Kuhnia_, and _Panicum_ groups, _Onagra_ was used; for
_Bidens_, _Onagra_, and _Datura_, _Helianthus_ was used, and for
_Penthorum_, _Solidago_, and _Verbascum_, _Panicum_.

At the time the cultures were started, check plants were sown in pots. The most vigorous seedlings were transplanted singly to large pots, and grown under conditions of water, light, and soil as similar as possible to those of the competition plots. Photographs of check plants and plots were made at the proper intervals, and the plots were charted in quadrats to show the course of competition. The factors which control competition were sought in a critical study of water-content and light values, which is still in process. This work has gone far enough to indicate the correctness of the view[45] that competition is purely physical in character. It has, moreover, been demonstrated that “room” in competition is merely a loose expression for the relation between the number of individuals in a given space, and the amount of water, light, and temperature available in the same space.

GLOSSARY

NOTE: Last terms frequent in compounds are found in their proper place alphabetically. The accent is indicated only in those words accented on the penult; all others are accented on the antepenult, or recessively.

=abundance=, the total number of individuals in an area.

=acospore= (ἀκή, point), a plant with awned disseminules.

=acrophyti´um= (ἄκρον, peak), an alpine plant formation.

=acti´um= (ἀκτή, rocky coast), a rocky seashore formation; =actad=,
plant of a rocky seashore.

=-ad= (-αδης, patronymic suffix), suffix for denoting an ecad.

=adaptable=, able to originate ecads; =adaptation=, the structural
response to stimuli.

=adjustment=, the functional response to stimuli.

=adventicious= (_adventicius_, foreign), invading from distant
formations.

=adventive= (_adventivus_, accidental), established temporarily.

=aggregation=, the coming together of plants into groups.

=agi´um= (ἀγή, beach), a beach formation; =agad=, a beach plant.

=agri´um= (ἀγρός, field), a culture formation; =agrad=, a cultivated
plant.

=aiphyti´um= (ἀεί, permanent), an ultimate formation.

=alsi´um= (ἄλσος, grove), a grove formation; =alsad=, a grove plant.

=alternation=, the heterogeneous arrangement of plant groups and
formations universally present in vegetation.

=amathi´um= (ἄμαθος, sand of the plain), a sand-hill or sandplain
formation; =amathad=, a sand-hill plant.

=ammochthi´um= (ἄμμος, sand, ὄχθη, bank), a sand bank formation;
=ammochthad=, a sand bank plant.

=anci´um= (ἄγκος, mountain glen), a canyon formation; =ancad=, a
canyon plant.

=anemi´um= (ἄνεμος, wind), a blowout formation; =anemad=, a blowout
plant; =anemochore=, a plant distributed by wind.

=-anum= (locative suffix), a suffix denoting a layer.

=apostrophe= (ἀπό, away from, στροφή, a turning), the arrangement of
the row of chloroplasts parallel to the rays of light.

=apparent noon=, the time when the sun crosses the meridian, i. e.,
sun noon as distinguished from noon, standard time.

=-ard= (ἄρδον, water of the land), combining term for water-content;
=ardium=, a succession due to irrigation.

=ardesiacus=, slate colored.

=-are= (locative suffix), suffix denoting a community.

=aspect= (_aspectus_, appearance), the seasonal impress of a
formation, e.g., the spring aspect.

=association=, the arrangement of individuals in vegetation.

=atmometer= (ἀτμός, vapor), an instrument for measuring evaporation.

=atropurpureus=, dark purple.

=atrovirens=, dark green.

=autochore= (αὐτός, self), motile plants, or those with motile spores;
=autochthonous= (χθών, ground), native.

=avellaneus=, drab.

=barrier=, a physical or biological obstacle to migration or ecesis.

=bathyphyti´um= (βαθύς, low), a lowland plant formation.

=blastochore= (βλάστη, growth), a plant distributed by offshoots.

=-bole= (βολή, a throw), combining term for propulsion; =bolochore=, a
plant distributed by propulsion.

=broti´um= (βροτός, mortal), a succession caused by man; =brotochore=,
a plant distributed by man.

=caeruleus=, pale blue.

=caesius=, eye-blue.

=camni´um= (κάμνω, cultivate), a succession due to cultivation.

=carphospore= (κάρφος, scale), a plant with disseminules possessing a
scaly or chaffy pappus.

=carpostrote= (καρπός, fruit), a plant migrating by means of fruits.

=centrospore= (κέντρον, spur), a plant with spiny disseminules.

=chalici´um= (χάλιξ, gravel), a gravel slide formation; =chalicad=, a
gravel slide plant.

=cheradi´um= (χέραδος, a sandbar), a wet sandbar formation;
=cheradad=, a wet sandbar plant.

=chersi´um= (χέρσος, dry barren waste), a dry waste formation;
=chersad=, plant of a dry waste.

=chioni´um= (χιών, όνος, snow), a snow formation; =chionad=, a snow
plant; =chionophyti´um=, a niveal plant formation.

=chledi´um= (χλῆδος, rubbish), a ruderal formation; =chledad=, a
ruderal plant.

=chlorenchym= (χλωρός, greenish yellow, ἐνχύμα, infusion), the
chlorophyll tissue of the leaf.

=-chore= (χωρέω, to spread abroad), combining term to denote agent of
migration.

=chresard= (χρῆσις, use), the available water of the soil, the
physiological water-content.

=clitochore= (κλίτος, slope), a plant distributed by gravity.

=clysi´um= (κλύσις, a flooding), a succession in a flooded soil.

=-colus= (κόλος, dwelling in), combining term for habitat forms.

=community=, a mixture of the individuals of two or more species, a
group of families.

=comospore= (κόμη, hair) a plant with hairy or silky disseminules.

=competition=, the relation between plants occupying the same area,
and dependent upon the same supply of physical factors.

=consocies=, that subdivision of a formation controlled by a facies.

=copious=, used of species in which the individuals are arranged
closely but uniformly.

=coryphi´um= (κορυφή, peak), an alpine meadow formation; =coryphad=,
an alpine meadow plant.

=creatospore= (κρέας, ατος, meat), a plant with nut fruits.

=cremni´um= (κρημνός, crag, cliff), a cliff formation; =cremnad=, a
cliff plant.

=creni´um= (κρήνη, spring), a spring formation; =crenad=, a spring
plant.

=crymi´um= (κρυμός, frost), a polar barren formation; =crymad=, a
polar plant; =crymophytic=, pertaining to polar plants.

=crystallochore= (κρύσταλλος, ice), a plant distributed by glaciers.

=cyaneus=, azure.

=cyriodoche= (κύριος, regular), a normal succession.

=dendri´um= (δένδρα, fruit trees), an orchard formation; =dendrad=, an
orchard plant.

=derived=, coming from other formations or regions, not native.

=diphotic= (δι-, two), the two surfaces unequally lighted;
=diphotophyll=, a leaf differentiated into palisade and sponge
tissues owing to unequal illumination.

=diplophyll= (διπλόος, twofold), an isophotic leaf with water-storage
cells in the middle.

=disseminule= (semen, seed), a seed fruit modified for migration.

=dissophyte= (δισσός, double), a plant with xerophytic leaves and
stems, and mesophytic roots.

=-doche= (δοχή, succession), succession.

=drimi´um= (δριμύς, biting, pungent), an alkaline habitat, and the
corresponding formation; =drimad=, a plant of such a formation.

=driodi´um= (δρίος, thicket), a dry thicket formation; =driodad=,
plant of a dry thicket.

=dysgeogenous= (δυς-, bad, γῆ, soil), weathering with difficulty to
form soil.

=ecad= (οἶκος, home), a habitat form due to origin by adaptation;
=ece´sis= (οἰκῆσις, act of coming to be at home), the germination
and establishment of invaders; =ecograph=, an instrument for
measuring a physical factor of a habitat; =ecotone= (τόνος,
tension), the tension line between two zones, formations, consocies,
etc.

=ecballi´um=, (ἐκβάλλω, cut down forests), a succession due to
lumbering.

=echard= (ἔχω, to withhold), the non-available water of the soil.

=edobole= (οἶδος, swelling), a plant whose seeds are scattered by
propulsion through turgescence.

=efficient difference=, the amount of a physical factor necessary to
produce a change in the response.

=enauli´um= (ἔναυλος, hollow channel), a sanddraw formation;
=enaulad=, a sanddraw plant.

=ende´mic= (ἐν, within, δῆμος, district), occurring in a single
formation, or natural region; =ende´mism=, the condition of growing
in but one natural area.

=epistrophe= (ἐπί, towards, στροφή, a turning), the arrangement of the
row of chloroplasts at right angles to the incident light.

=eremi´um= (ἔρημος, desert), a desert formation; =eremad=, a desert
plant.

=estival=, pertaining to summer.

=-etum= (locative suffix), suffix used to denote a consocies.

=eugeogenous= (εὖ-, well, γῆ, soil), weathering readily to form soil.

=facies=, a dominant species of a formation: a distinct area
controlled by it is a consocies.

=family=, a group of individuals belonging to one species.

=fixity=, the condition characterized by little or no response to
stimuli.

=flavovirens=, yellow green.

=forewold=, equivalent to the German “vorwald,” the thicket zone
bordering a forest.

=-genous= (γένω, to produce), producing.

=geotome= (γῆ, earth, τομή, edge), an instrument for obtaining soil
samples.

=gloeospore= (γλοιός, sticky stuff), a plant with viscid disseminules.

=-graph= (γραφή, a writing), combining term for a recording
instrument.

=gregarious= (gregarius, grouped in herds), used of species in which
the individuals occur in groups.

=habitat=, a definite physical area characterized by a formation;
=habitat form=, the impress given the plant by the habitat.

=harmosis= (ἅρμοσις, an adapting), response to stimuli, comprising
both adjustment and adaptation.

=hedi´um= (ἕδος, a sitting, base), a succession in a residuary soil.

=heliad= (ἥλιος, sun), a heliophyte; =heliophyll=, the leaf of a sun
plant; =heliophyte=, a sun plant; =heliophyti´um=, a sun plant
formation; =heliophilous=, sun-loving.

=heli´um= (ἕλος, marsh), a marsh formation; =helad=, a marsh plant;
=helodi´um= (ἑλώδης, marshy), a swampy open woodland formation;
=helodad=, a marsh plant; =helodrium= (δρίος, thicket), a thicket
formation; =helodrad=, a plant of a marshy thicket; =helohyli´um=
(ὕλη, forest) a marsh forest formation; =helohylad=, a marsh forest
plant.

=hepodoche= (ἕπω, follow), a secondary succession.

=hizometer= (ἵζω, to sink), an instrument for measuring gravitation
water.

=holard= (ὅλος, whole), the total water-content of the soil.

=hydrad= (ὑδρο-, water), a hydrophyte; =hydrochore=, a plant
distributed by water; =hydroharmose=, response to water stimuli;
=hydrophyll=, the leaf of a hydrophyte; =hydrophyte=, a water plant;
=hydrophyti´um=, a water plant formation; =hydrophilous=,
water-loving; =hydrosta´tic= (στατικός, standing), completing the
succession under hydrophytic conditions; =hydrotropic= (τροπικός,
turning), applied to successions which become mesophytic.

=hygrome´tric= (ὑγρίς, wet), measuring or absorbing water;
=hygroscopic= (σκοπέω, look), measurable only by a hygroscope; able
to absorb moisture.

=hyli´um= (ὕλη, forest), a forest formation; =hylad=, a forest plant;
=hylocolum=, dwelling in a forest; =hylodi´um= (ὑλώδης, wooded), a
dry open woodland formation; =hylodad=, a plant of this formation;
=hylophyte=, a forest plant.

=hypsi´um= (ὕψος, elevation), a succession caused by elevation.

=-ile= (locative affix), suffix denoting a society.

=immobile=, without effective devices for migration.

=indigenous= (_indigena_, sprung from the land), native.

=insolation=, exposure to intense heat and light.

=isabellinus=, leather-colored.

=isolation=, separation by barriers.

=isopho´tic= (ἴσος, equal), equally illuminated; =isophotophyll=, a
leaf in which both halves of the chlorenchym are alike, due to equal
illumination.

=-ium= (-εῖον, locative affix), suffix denoting a formation.

=labile=, plastic, easily modified.

=lauri´um= (λαύρα, drain), a drain formation; =laurad=, a drain plant.

=limni´um= (λίμνη, lake), a lake formation; =limnad=, a lake plant;
=limnodium= (λιμνῶδες, marshy ground), a salt marsh formation;
=limnodad=, a plant of a salt marsh.

=lochmi´um= (λόχμη, thicket), a thicket formation; =lochmad=, a
thicket plant.

=lophi´um= (λόφος, crest, hill), a hill formation; =lophad=, a hill
plant; =lophospore=, a plant with plumose disseminules.

=mastigospore= (μάστιξ, ιγος, lash), a plant with ciliate or
flagellate disseminules.

=melleus=, honey-colored.

=meridian=, used chiefly as a synonym for apparent noon; also an
imaginary line of longitude.

=mesad= (μέσος, middle), a mesophyte; =mesophilous=, growing in moist
soils; =mesophyll=, the leaf of a mesophyte; =mesophyte=, a plant of
moist soils; =mesophyti´um=, a mesophytic formation; =mesosta´tic=
(στατικίς, standing), completing the succession under mesophytic
conditions; =mesotro´pic= (τροπικός, turning), applied to
successions which become mesophytic.

=-meter= (μέτρον, measure), combining term for instrument.

=micti´um= (μικτόν, mixture), a mixed formation.

=migrant=, a plant that is migrating or invading.

=migration= (_migratio_, removal), the movement of plants into new
areas; =migration circle=, a circle employed to measure migration.

=mobile=, able to be moved, i. e., modified for migration.

=monochronic= (μόνος, single, χρόνος, time), arising but once;
=monogenesis= (γένεσις, origin), the origin of a new form at a
single place or time; =monophyle´sis= (φῦλον, race), origin from a
single ancestral type; =monoto´pic= (τόπος, place), arising at one
place only.

=motile=, able to move by growth, by means of cilia, etc.

=mutable=, able to produce mutants; =mutant=, a form arising by
mutation; =mutation=, the sudden appearance of new forms.

=namati´um= (νάμα, ατος, brook), a brook formation; =namatad=, a brook
plant.

=nomi´um= (νομός, pasture), a pasture formation; =nomad=, a pasture
plant.

=occupation=, possession of the ground by plants.

=oceani´um= (ὠκεανός, ocean), an ocean formation; =oceanad=, an ocean
plant; =oceanophyte=, an ocean plant; =oceanophilous=,
ocean-dwelling.

=ocheti´um= (ὀχετός, drain), a succession due to artificial drainage.

=ochroleucus=, yellowish white.

=ochthi´um= (ὄχθη, bank), a bank formation; =ochthad=, a bank plant.

=oligope´lic= (ὀλίγος, little, πηλός clay), containing little clay;
=oligopsam´mic= (ψάμμος, sand), containing little sand.

=olisthi´um= (ὄλισθος, slip), a succession in a landslip.

=ombrometer= (ὄμβρος, a rainstorm), a rain gauge.

=-on= (-ών, locative suffix), suffix used to denote a family.

=oncospore= (ὄγκος, hook), a plant with hooked disseminules.

=orgadi´um= (ὀργάς, άδος, meadowland partially wooded), an open
woodland formation; =orgadad=, an open woodland plant.

=orophyti´um= (ὄρος, mountain), a subalpine plant formation.

=oxodi´um= (ὀξώδης, sour), a humus marsh formation; =oxodad=, a plant
of a humus marsh.

=pagi´um= (πάγος, rocky hill, glacier), a succession in a glacial
soil; =pagophyti´um=, a foot-hill plant formation.

=pediophyti´um= (πεδίον, plain), an upland plant formation.

=pelagi´um= (πέλαγος, surface of the sea), a surface sea formation;
=pelagad=, a plant of the sea surface.

=pelochthi´um= (πηλός, mud, ὄχθη, bank), a mud bank form;
=pelogenous=, producing clay; =pelopsammic= (ψάμμος, sand), composed
of mixed clay and sand; =pelopsammogenous=, producing clay and sand.

=permobile=, extremely mobile.

=perquadrat=, a quadrat of 16 square meters or more.

=petasospore= (πέτασος, sunshade), a plant with parachute-like
disseminules.

=petri´um= (πέτρα, rock, stone), a rock formation; =petrad=, a rock
plant; =petrochthi´um= (ὄχθη, bank), a rock bank formation.

=petrodi´um= (πετρώδης, abounding in boulders), a boulder field
formation; =petrodad=, a plant of a boulder field.

=phelli´um= (φελλεύς, stony ground), a rock field formation;
=phellad=, a rock field plant.

=-philous= (φίλος), loving, dwelling in.

=-photic= (φῶς, φωτός, light), pertaining to light; =photoharmose=,
response to light stimuli; =photometer=, an instrument for measuring
light.

=phreti´um= (φρητός, tank), a tank formation; =phretad=, a tank plant.

=phyad= (φυή, form of growth), a vegetation form, e. g., tree, shrub,
etc.

=-phyll= (φύλλον, leaf), combining term for leaf.

=-phyte= (φυτόν, plant), combining term denoting plant; =phyteris=
(ἔρις, strife), plant competition; =-phyti´um= (φυτεῖον, place
covered with plants), combining term for formation; =phytostrote=, a
species migrating by means of the plant body.

=pladobole= (πλάδος, moisture), a plant whose seeds are scattered by
propulsion due to moisture.

=plasticity=, the condition characterized by ready response to
stimuli.

=pnoi´um= (πνοή, blast), a succession in an aeolian soil.

=poi´um= (πόα, meadow), meadow formation; =poad=, a meadow plant;
=poophyte=, a meadow plant.

=polyan´thous= (πολύς, many, ἄνθος, flower), producing many flowers;
=polychro´nic= (χρόνος, time), arising at two or more times;
=polyde´mic= (δῆμος, district), occurring in two or more formations
or natural regions; =polygenesis= (γένεσις, origin), the origin of a
new form at two or more places or times; =polyphyle´sis= (φῦλον,
race), the origin of a form, species, or genus from two or more
ancestral types; =polyspermatous= (σπέρμα, seed), producing many
seeds in each flower; =polyto´pic= (τόπος, place), arising at two or
more distinct places.

=ponti´um= (πόντος, deep sea), a deep sea formation.

=potami´um= (ποταμός, river), a river formation; =potamad=, a river
plant.

=potometer= (ποτόν, drink), an instrument for measuring absorption.

=prevernal=, pertaining to early spring.

=prior=, earlier, used of alpine aspects.