Research methods in ecology

by Frederic E. Clements

INVASION

260. By invasion is understood the movement of plants from an area of a certain character into one of a different character, and their colonization in the latter. This movement may concern an individual, a species, or a group of species. From the nature of invasion, which contains the double idea of going into and taking possession of, it usually operates between contiguous formations, but it also takes place between formational zones and patches. More rarely and less noticeably, there may be invasion into a remote vegetation, as a result of long carriage by wind, water, birds, railroads, or vessels. Movement or migration, however, represents but one of the two ideas involved in invasion. Migration merely carries the spore, seed, or propagule into the area to be invaded. In ecesis, the spores or seeds germinate and grow, after more or less adjustment, and in case the latter becomes sufficiently complete, the new plants reproduce and finally become established. With all terrestrial plants, invasion is possible only when migration is followed by ecesis, because of the inherent differences of formations or of areas of the same formation. In the case of surface floating forms, such as Lemnaceae, and of the plancton, ecesis is of much less importance, on account of the uniformity of the medium and the lack of attachment, and migration is often practically synonymous with invasion.

MIGRATION

261. Migration has been sometimes used loosely as a synonym for invasion, but it is here employed in its proper sense of removal or departure, i. e., movement, and is contrasted with ecesis, the making of a home, the two ideas being combined in invasion, which is a moving into and a taking possession of. An analysis of migration reveals the presence of four factors, mobility, agency, proximity, and topography. Not all of these are present in every instance of migration, as for example in the simple elongation of a rootstalk, but in the great majority of cases each plays its proper part. Mobility represents the inherent capacity of a plant for migration, and in its highest expression, motility, is in itself productive of movement. As a general rule, however, modifications for securing mobility are ineffective in the absence of proper agents, and the effective operation of the two will be profoundly influenced by distance and topography.

262. Mobility denotes potentiality of migration as represented by modifications for this purpose. It corresponds, in a sense, to dissemination, though seed production also enters into it. Its most perfect expression is found in those plants which are themselves motile, Bacteriaceae, Oscillatoria, Volvocaceae, and Bacillariaceae, or possess motile propagules, such as most Phycophyta. On the other hand, it is entirely undeveloped in many plants with heavy unspecialized seeds and fruits. Between these two extremes lie by far the greater number of plants, exhibiting the most various degrees of mobility, from the motile though almost immobile offshoots of many Liliaceae to the immotile but very mobile spores of fungi. It is thus seen that motility plays a relatively small part in migration, being practically absent in terrestrial forms, and that it bears a very uncertain relation to mobility. In analyzing the latter, contrivances for dissemination are seen to determine primarily the degree of mobility, while the number of seeds produced will have an important effect in increasing or decreasing it. A third factor of considerable importance is also involved, namely, position with reference to the distributive agent, but any exact knowledge of its importance must await systematic experiment somewhat after the methods of Dingler, but with air-currents, etc., of known velocity and direction. The time is not distant when by such methods it will be possible to establish a coefficient of mobility, derived from terms of position, weight, resistant surface, and trajectory for definite wind velocities or for particular propulsive mechanisms.

263. Organs for dissemination. Plants exhibit considerable diversity with reference to the part or organ modified, or at least utilized, for dissemination. This modification, though usually affecting the particular product of reproduction, may, in fact, operate on any part of the plant, and in certain cases upon the entire plant itself. In the majority of plants characterized by alternation of generations, the same individual may be disseminated in one generation by a reproductive body, and in the other by a propagative one, as is the case in the oogones and conidia of Peronospora, the spores and gemmae of Marchantia, the fruits and runners of Fragaria, etc. Special modifications have, as a rule, been developed in direct connection with spores and seeds, and mobility reaches its highest expression in these. It is, on the other hand, greatly restricted in offshoots and plant bodies, at least in terrestrial forms, though it will now and then attain a marked development in these, as shown by the rosettes of Sempervivum and the tumbling plants of Cycloloma. For the sake of convenience, in analyzing migration, all plants may be arranged in the following groups with reference to the organ or part distributed.

1. Spore-distributed, sporostrotes. This includes all plants possessing structures which go by the name of spore, such as the acinetes of Nostoc and Protococcus, the zoogonidia of Ulothrix, Ectocarpus, etc., the conidia, ascospores, and basidiospores of fungi, the tetraspores of red seaweeds, and the gemmae and spores proper of liverworts, mosses, and ferns. These are almost always without especial contrivances for dissemination, but their extreme minuteness results in great mobility.

2. Seed-distributed, spermatostrotes. This group comprises all flowering plants in which the seed is the part modified or at least disseminated. The mobility of seeds is relatively small, except in the case of minute, winged or comate seeds.

3. Fruit-distributed, carpostrotes. The modifications of the fruit for distribution exceed in number and variety all other modifications of this sort. All achenes, perigynia, utricles, etc., properly belong here.

4. Offshoot-distributed, thallostrotes. To this class are referred those plants, almost exclusively cormophytes, which produce lateral, branch-like propagules, such as root-sprouts, rhizomes, runners, stolons, rosettes, etc. Migration with such plants is extremely slow, but correspondingly effective, since it is almost invariably followed by ecesis.

5. Plant-distributed, phytostrotes. This group includes all plancton and surface forms, whether motile or non-motile, and those terrestrial plants in which the whole plant, or at least the aerial part, is distributed, as in tumbleweeds and in many grasses.

264. Contrivances for dissemination. Any investigation of migration to be exact must confine itself to fixed forms. For these the degree of perfection shown by dissemination contrivances corresponds almost exactly to the degree of mobility. Because of the difficulty of ascertaining the effect of ecesis, it is impossible to determine the actual effectiveness in nature of different modifications, and the best that can be done at present is to regard mobility, together with the occurrence and forcefulness of distributive agents, as an approximate measure of migration. The general accuracy of such a measure will be more or less evident from the following. Of 118 species common to the foot-hill and sand-hill regions of Nebraska, regions which are sufficiently diverse to indicate that these common species must have entered either one by migration from the other, 83 exhibit modifications for dissemination, while 8 others, though without special contrivances, are readily distributed by water, and 4 more are mobile because of minuteness of spore or seed. Some degree of mobility is present in 73 per cent of the species common to these regions, while of the total number of species in which the mode of migration is evident, viz., 95, 66 per cent are wind-distributed, 20 per cent animal-distributed, and 14 per cent are water-distributed. It need hardly be noted that this accords fully with the prevalence and forcefulness of winds in these regions. Of the species peculiar to the foot-hill region, many are doubtless indigenous, though a majority have come from the montane regions to the westward. The number of mobile species is 121, or 60 per cent of the entire number, while the number of wind-distributed ones is 85, or 70 per cent of those that are mobile. Among the 25 species found in the widely separated wooded bluff and foot-hill regions, 2 only, Amorpha nana and Roripa nasturtium, are relatively immobile, but the minute seeds of the latter, however, are readily distributed, and the former is altogether infrequent.

The following groups of plants may be distinguished according to the character of the contrivance by which dissemination is secured:

1. Saccate, saccospores. Here are to be placed a variety of fruits, all of which agree, however, in having a membranous envelope or an impervious, air-containing pericarp. In Ostrya, Physalis, Staphylea, the modification is for wind-distribution, while in Carex, Nymphaea, etc., it is for water-transport.

2. Winged, pterospores. This group includes all winged, margined, and flattened fruits and seeds, such as are found in Acer, Betula, Rumex, many Umbelliferae, Graminaceae, etc.

3. Comate, comospores. To this group belong those fruits and seeds with long silky hairs, Gossypium, Anemone, Asclepias, etc., and those with straight capillary hairs or bristles not confined to one end, Typha, Salix, etc.

4. Parachute, petasospores. The highly developed members of this group, Taraxacum, Lactuca, and other Liguliflorae are connected through Senecio and Eriophorum with the preceding. These represent the highest development of mobility attained by special modification.

5. Chaffy-pappose, carphospores. In this group are placed those achenes with a more or less scaly or chaffy pappus with slight mobility, as in Rudbeckia, Brauneria, Helianthus, etc.

6. Plumed, lophospores. In the fruits of this class, the style is the part usually modified into a long plumose organ, possessing a high degree of mobility, as in Pulsatilla, Sieversia, and Clematis.

7. Awned, ascospores. These are almost exclusively grasses, in which the awns serve for distribution by wind, water, or animals, and even, according to Kerner, by hygroscopic creeping movements. The mobility in many cases is great.

8. Spiny, centrospores. This group contains a few representatives which possess a moderate degree of mobility by attachment, as in Tribulus and Cenchrus.

9. Hooked, oncospores. The members of this group are extremely numerous, and the degree of mobility as a rule is very high. All exhibit in common the development of hooks or barbs, by which they are disseminated in consequence of attachment, though the number, size, and disposition of the hooks vary exceedingly.

10. Viscid, gloeospores. In these, the inflorescence is more or less covered with a viscid substance, as in species of Silene, or the fruit is beset with glandular hairs, as in Cerastium, Salvia, etc.

11. Fleshy, sarcospores. These are intended for dissemination by deglutition, largely by birds; the effectiveness of the modification depends in a large degree upon the resistance of the seed envelope to digestion. The mobility varies greatly, but the area over which migration may be effected is large.

12. Nut-fruited, creatospores. This group includes those plants with nut fruits which are carried away and secreted by animals for food.

13. Flagellate, mastigospores. These are plants with ciliate or flagellate propagative cells, i. e., zoogonidia, as in Protococcus, Ulothrix, Oedogonium, Ectocarpus, etc., or with plant bodies similarly motile, Bacteriaceae and Volvocaceae.

265. Position of disseminule. The position on the plant of the organ to be disseminated, i. e., its exposure to the distributing agent, plays a considerable part in determining the degree of mobility. In the majority of plants, the position of the inflorescence itself results in maximum exposure, but in a large number of forms special modifications have been developed for placing the spores or seeds in a more favorable position. In both cases, there are often present also devices for bringing about the abscission of the seed or fruit. It is, moreover, self-evident that the height of the inflorescence above ground or above the surrounding vegetation is likewise of considerable importance in increasing the trajectory. It is yet too early to make a complete classification of contrivances for placing disseminules in the most favorable exposure, but the following will serve as a basis for future arrangements.

1. In all operculate Discomycetes, and especially in the Ascobolaceae, where the asci project above the hymenium, the spores are raised above the surface by tensions within the apothecium. This might be regarded as dissemination by expulsion, if it were not for the fact that the spores fall back into the cup, unless carried away by the wind.

2. In Gasteromycetes and in certain Hepaticae, the spores are not only elevated slightly above the sporophore by the expanding capillitium or by the mass of elaters, but they are also held apart in such a way that the wind blows them out much more readily.

3. In Bryophyta, the sporophore regularly dehisces by a slit, or is provided with a peristome. Both structures are for the purpose of sifting the spores out into the wind; by reason of their hygroscopicity, they also insure that the spores will not be shaken out in wet weather.

4. In a few grasses, such as Stipa and Aristida, the twisting and intertwining of the awns lift the floret out of the glumes, and at the same time constitute a contrivance readily blown away by the wind or carried by attachment.

5. In certain Compositae, the involucral scales are reflexed at maturity, and at the same time the disk becomes more or less convex, serving to loosen the achenes. This result is also secured in certain species by the drying and spreading of the pappus hairs.

6. The scapose Liguliflorae, Taraxacum, Agoseris, etc., are characterized by the elongation of the scape after anthesis, with the result that the head is raised to a considerable height by the time the achenes are mature.

7. Carpotropic movements, though primarily for another purpose, often serve to bring seeds and fruits into a better position for dissemination.

266. Seed production. The relation of spore or seed-production to mobility is obvious in the case of mobile species; in the case of immobile ones, it is just as evident that it has no effect, though it may still have considerable influence in increasing migration. In the case of two species with equally effective dissemination contrivances, the one with the largest seed-production will be the more mobile. On the basis of the relation of seeds to flower, two groups of plants may be distinguished, one, Polyanthae, in which the flowers are many and the seeds few or single, as in Compositae, and the other, Polyspermatae, Portulaca, Yucca, etc., in which the number of seeds to each flower is large. So far as the actual number of seeds produced is concerned, polyanthous plants may not differ from polyspermatous ones, but, as a rule, they are much more highly specialized for dissemination and are more mobile. The number of fertile seeds is also much greater, a fact which is of great importance in ecesis, and which, taken in connection with mobility, partially explains the supremacy of the composites. Among the fungi and algae, the amount of spore-production in a large degree determines the mobility, since these forms are intrinsically permobile.

267. Agents of migration. In the last analysis, however, the possibility of migration depends upon the action of distributive agents; in the absence of these, even the most perfect contrivance is valueless, while their presence brings about the distribution of the most immobile form. In short, migration depends much more upon such agents than upon mobility, however perfect the latter may be. It is, moreover, evident that the amount and extent of migration will be determined primarily by the permanence and forcefulness of the agent, as indicated by its ability to bring about transportation. Finally, as will be shown later, the direction and rapidity of migration depend directly upon the direction and intensity of the agent.

Migration results when spores, seeds, fruits, offshoots, or plants are moved out of their home by water, wind, animals, man, gravity, glaciers, growth, or mechanical propulsion. Corresponding to these agents, there may be recognized the following groups:

1. Water, hydrochores. These comprise all plants distributed exclusively by water, whether the latter acts as ocean currents, tides, streams, or surface run-off. In the case of streams and run-off, especially, mobility plays little part, provided the disseminules are impervious or little subject to injury by water. Motile plants, or those with motile cells, which belong entirely to this group, may be distinguished as autochores, which correspond closely to mastigospores.

2. Wind, anemochores. This group includes the majority of all permobile terrestrial plants, i. e., those in which modifications for increasing surface have been carried to the extreme, or those which are already permobile by reason of the minuteness of the spore or seed. Saccate, winged, comate, parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits represent the various types of modifications for wind-distribution.

3. Animals, zoochores. Among terrestrial plants, dissemination by attachment represents essentially the same degree of specialization as is found in wind-distributed plants. The three types of contrivances for this purpose are found in spinose, hooked, and glandular fruits. Dissemination by deglutition and by carriage, either intentional or unintentional, though of less value, play a striking part on account of the great distance to which the seeds may be carried. Dissemination by deglutition is characteristic of sarcospores, and distribution by carriage of creatospores.

4. Man, brotochores. Dissemination by man has practically no connection with mobility. It operates through great distances and over immense areas as well as near at hand. It may be intentional, as in the case of cultivated species, or unintentional, as in thousands of native or exotic species. No other disseminating agent is comparable with man in respect to universal and obvious migration.

5. Gravity, clitochores. The members of this group are exclusively colline, montane, and alpine plants, growing on rocks, cliffs, and gravel slides (talus), etc., in which the seeds reach lower positions merely by falling, or more frequently by the breaking away and rolling down of rock or soil masses and particles. Dissemination by this method is relatively insignificant, though it plays an important part in the rock fields and gravel slides of mountain regions, particularly in the case of immobile species.

6. Glaciers, crystallochores. At the present time, transport by glaciers is of slight importance, because of the restriction of the latter to alpine and polar regions, where the flora is poorly developed. In the consideration of migrations during the glacial epoch, however, it plays an important point.

7. Growth, blastochores. The mobility of species disseminated by offshoots is extremely slight, and the annual movement relatively insignificant. The certainty of migration and of ecesis, is, however, so great, and the presence of offshoots so generally the rule in terrestrial plants that growth plays an important part in migration, especially within formations.

8. Propulsion, bolochores. Like growth, dissemination by mechanical propulsion, though operating through insignificant distances, exerts an important effect in consequence of its cumulative action. The number of plants, however, with contrivances for propulsion is very much smaller than the number of blastochores. All bolochorous species agree in having modifications by means of which a tension is established. At maturity, this tension suddenly overcomes the resistance of sporangium or fruit, and throws the enclosed spores or seeds to some distance from the parent plant. In accordance with the manner in which the tension is produced, sling-fruits may be classified as follows:

(a) Hygroscopicity, pladoboles. These include the ferns with annulate sporangia, in which the expansion of the annulus by the absorption of moisture bursts the sporangium more or less suddenly, though the actual propulsion of the spores seems to come later as a result of dessication.

(b) Turgescence, edoboles. Dissemination by turgescence is highly developed in Pilobolus and in Discomycetes, though in the latter turgescence results rather in placing the spores in a position to be readily carried by the wind. Impatiens and Oxalis furnish familiar examples of fruits which dehisce in consequence of increased turgidity.

(c) Dessication, xerioboles. The number of fruits which dehisce upon drying is very large, but only a small portion of these expel their seeds forcibly. Geranium, Viola, Erysimum, and Lotus illustrate the different ways in which dessication effects the sudden splitting of fruits.

(d) Resilience, tonoboles. In some plants, especially composites, labiates, and borages, the achenes or nutlets are so placed in the persistent calyx or involucre that the latter serves as a sort of mortar for projection, when the stem of the plant is bent to one side by any force, such as the wind or an animal. It will be noticed that two separate agents are actually concerned in dissemination of this sort.

Frequently, two or more agents will act upon the same disseminule, usually in succession. The possibility of such combinations in nature is large, but actual cases seem to be infrequent, except where the activities of man enter into the question. Some parts, moreover, such as awned inflorescences, are carried almost equally well by wind or animals, and may often be disseminated by the cooperation of these two agents. The wind also often blows seeds and fruits into streams by which they are carried away, but here again, parts adapted to wind-dissemination are injured as a rule by immersion in water, and the number of plants capable of being scattered by the successive action of wind and water is small.

In the present state of our knowledge of migration, it is impossible to establish any definite correspondence between dissemination-contrivance, agent, and habitat. As a general rule, plants growing in or near the water, in so far as they are modified for this purpose at all, are adapted to water-carriage. Species which grow in exposed grassy or barren habitats are for the most part anemochores, while those that are found in the shelter of forests and thickets are usually zoochorous, though the taller trees and shrubs, being exposed to the upper air currents, are generally wind-distributed. There is then a fair degree of correspondence, inasmuch as most hydrophytes are hydrochorous, most hylophytes, zoochorous, and the majority of poophytes and xerophytes, anemochorous. Definite conclusions can be reached, however, only by the statistical study of representative formations.

With respect to their activity, agents may be distinguished as constant, as in the case of currents, streams, winds, slope, growth, and propulsion, or intermittent, animals and man. In the former, the direction is more or less determinate, and migration takes place year by year, i. e., it is continuous, while in the latter dissemination is largely an accidental affair, indeterminate in direction, and recurring only at indefinite intervals. The effective conversion of migration into invasion is greatest when the movement is continuous, and least when it is discontinuous, since, in the latter, species are usually carried not only out of their particular habitat but even far beyond their geographical area, and the migration, instead of being an annual one with the possibility of gradual adjustment, may not recur for several years, or may, indeed, never take place again. The rapidity of migration is greatest in the case of intermittent agents, while the distance of migration is variable, being great chiefly in the case of man, ocean currents, and wind, and slight when the movement is due to slope, growth, or propulsion. Disregarding the great distances over which artificial transport may operate, seeds may be carried half way across the continent in a week by strong-flying birds, while the possibilities of migration by growth or expulsion are limited to a few inches, or at most to a few feet per year. This slowness, however, is more than counterbalanced by the enormously greater number of disseminules, and their much greater chance of becoming established.

268. The direction of migration is determinate, except in the case of those distributive agents which act constantly in the same direction. The general tendency is, of course, forward, the lines of movement radiating in all directions from the parent area. This is well illustrated by the operation of winds which blow from any quarter. In the case of the constant winds, migration takes a more or less definite direction, the latter being determined to a large degree by the fruiting period of any particular species. In this connection, it must be kept clearly in mind that the position of new areas with reference to the original home of a species does not necessarily indicate the direction of migration, as the disseminules may have been carried to numerous other places in which ecesis was impossible. The local distribution of zoochorous species is of necessity indeterminate, though distant migration follows the pathways of migratory birds and animals. In so far as dissemination by man takes place along great commercial routes, or along highways, it is determinate. In ponds, lakes, and other bodies of standing water, migration may occur in all directions, but in ocean currents, streams, etc., the movement is determinate, except in the case of motile species. The dissemination of plants by slopes, glaciers, etc., is local and definite, while propulsion is in the highest degree indeterminate. Migration by growth is equally indefinite, with the exception that hydrotropism and chemotropism result in a radiate movement away from the mass, while propulsion throws seeds indifferently into or away from the species-mass. From the above it will be seen that distant migration may take place by means of water, wind, animals or man, and, since all these agents act in a more or less definite direction over great distances, that it will be in some degree determinate. On the other hand, local migration will as regularly be indeterminate, except in the case of streams and slopes. The direction of migration, then, is controlled by these distributive agents, and the limit of migration is determined by the intensity and duration of the agent, as well as by the character of the space through which the latter operates.

ECESIS

269. Concept. By the term ecesis is designated the series of phenomena exhibited by an invading disseminule from the time it enters a new formation until it becomes thoroughly established there. In a word, ecesis is the adjustment of a plant to a new habitat. It comprises the whole process covered more or less incompletely by acclimatization, naturalization, accommodation, etc. It is the decisive factor in invasion, inasmuch as migration is entirely ineffective without it, and is of great value in indicating the presence and direction of migration in a great number of species where the disseminule is too minute to be detected or too little specialized to be recognizable.

The relation of migration to ecesis is a most intimate one: the latter depends in a large measure upon the time, direction, rapidity, distance, and amount of migration. In addition, there is an essential alternation between the two, inasmuch as migration is followed by ecesis, and the latter then establishes a new center from which further migration is possible, and so on. The time of year in which fruits mature and distributive agents act has a marked influence upon the establishment of a species. Disseminules designed to pass through a resting period are often brought into conditions where they germinate at once, and in which they perish because of unfavorable physical factors, or because competing species are too far advanced. On the other hand, spores and propagules designed for immediate germination may be scattered abroad at a time when conditions make growth impossible. The direction of movement is decisive in that the seed or spore is carried into a habitat sufficiently like that of the parent to secure establishment, or into one so dissimilar that germination is impossible, or at least is not followed by growth and reproduction. The rapidity and distance of migration have little influence, except upon the less resistant disseminules, conidia, gemmae, etc. Finally, the amount of migration, i. e., the number of migrants, is of the very greatest importance, affecting directly the chances that vigorous disseminules will be carried into places where ecesis is possible.

Normally, ecesis consists of three essential processes, germination, growth, and reproduction. This is the rule among terrestrial plants, in which migration regularly takes place by means of a resting part. In free aquatic forms, however, the growing plant or part is usually disseminated, and ecesis consists merely in being able to continue growth and to insure reproduction. Here establishment is practically certain, on account of the slight differences in aquatic habitats, excepting of course the extremes, fresh water and salt water. The ease indeed with which migration and ecesis are effected in the water often makes it impossible to speak properly of invasion in this connection, since aquatics are to such a large extent cosmopolitan. In dissemination by offshoots, the conditions are somewhat similar. Here, also, ecesis comprises the sequence of growth and reproduction, and invasion, in the sense of passing from one habitat to another, is of rare occurrence, as the offshoot grows regularly under the same conditions as the parent plant. The adjustment of growing plants and parts is so slight, and their establishment so certain on account of their inability to migrate into very remote or different habitats, that they may be ignored in the following discussion.

In accordance with the above, it would be possible to distinguish three groups of terrestrial plants: (1) those migrants which germinate and disappear, (2) those which germinate and grow but never reproduce, (3) those which reproduce, either by propagation or generation, or both. Such a classification has little value, however, since the same species may behave in all three fashions, depending upon the habitat to which it has migrated, and since invasion does not occur unless the plant actually takes possession, i. e., reproduces. From the latter statement, it follows that invasion occurs only when a species migrates to a new place, in which it germinates, matures, and reproduces. Maintenance by annual invasion simply, in which the plants of each year disappear completely, can not then be regarded as invasion proper. On the other hand, though such instances are rare, it is not necessary that the invaders produce fruit, provided they are able to maintain themselves, or to increase by propagation. Furthermore, if a plant germinate, grow, and reproduce, it is relatively immaterial whether it persist for a few years or for many, since, as we shall see under Succession, the plants of one invasion are displaced by those of the next, the interval between invasions increasing with the stabilization.

270. Germination of the seed. The germination of seed or spore is determined by its viability and by the nature of the habitat. Viability depends upon the structural characters of fruit, seed-coat, and endosperm, and to a degree upon the nature of the protoplasm or embryo. The first three affect the last directly, by protecting the embryo against dryness, against injury due to carriage by water, or by deglutition, and probably in some cases against excessive heat or cold. Marloth[^[33]] has investigated the structure of seed coats, establishing the following groups, which are summarized here somewhat fully because of their bearing upon ecesis: (1) seed coats without protective elements, endosperm absent or rudimentary, Epilobium, Impatiens, Parnassia, Sagittaria, etc.; (2) protective elements lacking or few, endosperm highly developed with thick-walled cells, Liliaceae, Primulaceae, Rubiaceae, etc.; (3) protective cells present in the seed coats, endosperm little or none, Boraginaceae, Crassulaceae, Cruciferae, Labiatae, Papilionaceae, etc.; (4) protective elements present, Asclepias, Campanula, Gentiana, Silene, Saxifraga, etc.; (5) protective cells present, endosperm thick-walled, Euonymus, Helianthemum, Ribes. The protective cells are of various kinds: (1) epidermal cells strongly cuticularized, Caryophyllaceae, Crassulaceae, Fumariaceae, Saxifragaceae; (2) parenchyma thick-walled, several-layered, Aesculus, Castanea, Fagus; (3) parenchyma cells with the inner or radial walls thickened, Campanula, Erythraea, Gentiana; (4) epidermal cells cup-shaped, thick-walled, Cruciferae, Ribes, Vaccinium; (5) parenchyma with thickened, cellulose walls, Geranium, Viburnum; (6) a single row of stone-cells, Labiatae; (7) tissue of stone-cells, Hippuris, Naias, Potamogeton; (8) elongate stone-cells, Coniferae, Cupuliferae, Euphorbia, Linum, Malva, Viola; (9) short, columnar, thick-walled branched cells, Cucurbitaceae, Datura, Hypericum; (10) prosenchyma with cellulose walls, Clematis; (11) prosenchyma with lignified walls, Fraxinus, Rhamnus, Ranunculus. The seed coats have a certain influence in determining germination at the proper time, inasmuch as they make it difficult for the seed to germinate under the stimulus of a quantity of warmth and moisture insufficient to support the seedling. The effect of the endosperm, as well as that of other food supply in the seed, upon germination and the establishment of the seedling is obvious.

The behavior of seed or spore with respect to germination depends in a large degree upon the character of the protoplasm or embryo, though in just what way is at present a matter of conjecture. It is evident that many seeds are not viable because fertilization has not been effected, and in consequence no embryo has developed. This is the usual explanation of the low germinating power of the seeds of some species, especially polyspermatous ones. But even in viable seeds the behavior is always more or less irregular. The seeds of some species will grow immediately after ripening, while others germinate only after a resting period of uncertain duration. The same is true of spores. Even in the case of seeds from the same parent, under apparently similar conditions, while the majority will germinate the first year, some will lie dormant for one or more years. The precise reason why many seeds and spores germinate more readily after being frozen is equally obscure. The period of time for which disseminules may remain viable is extremely diverse, though, as would be expected, it is much longer as a rule for seeds than for spores. The greater vitality of seeds in the case of ruderal plants suggests that this diversity may be due simply to variation in the vigor of the embryos. It would seem that under proper conditions seeds may retain their viability for an indefinite period.

The influence of habitat upon germination is of primary importance, though the manner in which its influence is exerted is by no means as evident as might be supposed. In the case of seeds sown in the planthouse, it is almost universally the case that germination is less than in nature, notwithstanding the fact that temperature and moisture appear to be optimum. In nature, the seeds of the species may be carried into a number of different formations, any one or all of which may present conditions unfavorable to germination. With respect to probability of germination, habitats are of two sorts: those which are denuded and those which bear vegetation. It is impossible to lay down general propositions with respect to either group, since germination will vary with the character of the invading species, the annual distribution of heat and moisture in the habitat, etc. In a general way, however, it may be stated that the chances for germination are greater in vegetation than in denuded areas, chiefly because the latter are usually xerophytic. On the other hand, the lack of competition in the denuded area tends to make ultimate establishment much more certain. Here, as elsewhere when exact statistical results are desired, the use of the quadrat, and especially of the permanent quadrat, is necessary to determine the comparative germination of the invading species in relation to denudation and vegetation.

271. Adjustment to the habitat. The seedling once established by germination, the probability of its growing and maturing will depend upon its habitat form, plasticity, and vegetation form. Even though it may germinate under opposite conditions, a typical hylophyte, such as Impatiens for example, will not thrive in an open meadow, nor will characteristic poophytes, such as most grasses, grow in deep shade. In the same way, xerophytes do not adapt themselves to hydrophytic habitats, nor hydrophytes to xerophytic conditions. Many mesophytes, however, possess to a certain degree the ability to adjust themselves to somewhat xerophytic or hydrophytic situations, while woodland plants often invade either forest or meadow. This capability for adjustment, i. e., plasticity, is greatest in intermediate species, those that grow in habitats not characterized by great excess or deficiency of some factor, and it is least in forms highly specialized in respect to water-content, shade, etc. It may then be established as a fundamental rule that ecesis is determined very largely by the essential physical similarity of the old and the new habitat, except in the case of plastic forms, which admit of a wider range of accommodation. The plasticity of a plant is not necessarily indicated by structural modification, though such adjustment is usually typical of plastic species, but it may sometimes arise from a functional adaptation, which for some reason does not produce concomitant structural changes. The former explains such various habitat forms of the same species as are found in Galium boreale, Gentiana acuta, etc., and the latter the morphological constancy of plants like Chamaenerium, which grow in very diverse habitats.

The vegetation form of the invading species is often of the greatest importance in determining whether it will become established. The vegetation form represents those modifications which, produced in the original home by competition, i. e., the struggle for existence, are primarily of value in securing and maintaining a foothold. These comprise all structures by means of which the plant occupies a definite space in the air, through which the necessary light and heat reach it, and in the soil, from which it draws its food supply. These structures are all organs of duration or of perennation, such as root, rootstalk, bulb, tuber, woody stem, etc., which find their greatest development among trees and shrubs, and their least among annual herbs. But while the invaders are aided in securing possession by the proper vegetation form, the occupation of the plant already in possession is increased by the same means, and the outcome is then largely determined by other factors. To avoid repetition, the bearing of occupation upon invasion will be considered under succession.

BARRIERS

272. Concept. DeCandolle[^[34]] seems to have been the first to use the term barrier and to distinguish the various kinds, though Hedenberg[^[35]] clearly saw that stations of one kind were insurmountable obstacles to plants belonging to a very different type. De Candolle pointed out that the natural barriers to continuous invasion (“transport de proche en proche”) are: (1) seas, which decrease invasion almost in inverse proportion to their extent; (2) deserts; (3) mountain ranges, which are less absolute on account of passes, valleys, etc.; (4) vegetation, marshes being barriers to dry land plants, forests to those that fear the shade, etc. Grisebach[^[36]], in discussing the effect of barriers upon the constitution of vegetation, laid down the fundamental rule that: “The supreme law which serves as the basis of the permanent establishment of natural floras is to be recognized in the barriers which have hindered or completely prevented invasion.”

Any feature of the topography, whether physical or biological, that restricts or prevents invasion, is a barrier. Such features are usually permanent and produce permanent barriers, though the latter may often be temporary, existing for a few years only, or even for a single season. In this last case, however, they are as a rule recurrent. Barriers may furthermore be distinguished as complete or incomplete with respect to the thoroughness with which they limit invasion. Finally, the consideration of this subject gains clearness if it be recognized that there are barriers to migration as well as to ecesis, and if we distinguish barriers as physical or biological with reference to the character of the feature concerned.

273. Physical barriers are those in which limitation is produced by some marked physiographic feature, such as the ocean or some other large body of water, large rivers, mountain ranges and deserts (including ice and snow fields). All of these are effective by virtue of their dominant physical factors; hence they are barriers to the ecesis of species coming from very different habitats, but they act as conductors for species from similar vegetation, especially in the case of water currents. A body of water, representing maximum water-content, is a barrier to mesophytic and xerophytic species, but a conductor for hydrophytic ones; deserts set a limit to the spread of mesophytic and hydrophytic plants, while they offer conditions favorable to the invasion of xerophytes; and a high mountain range, because of the reduction of temperature, restricts the extension of macrothermal and mesothermal plants. A mountain range, unlike other physical barriers, is also an obstacle to migration, inasmuch as natural distributive agents rarely act through it or over it.

274. Biological barriers include vegetation, man and animals, and plant parasites. The limiting effect of vegetation is exhibited in two ways. In the first place, a formation acts as a barrier to the ecesis of species invading it from the formations of another type, on account of the physical differences of the habitats. Whether such a barrier be complete or partial will depend upon the degree of dissimilarity existing between the formations. Hylophytes are unable to invade a prairie, though open thicket plants may do so to a certain degree. In the same way, a forest formation on account of its diffuse light is a barrier to poophytes; and a swamp, because of the amount and character of the water-content, sets a limit to both hylophytes and poophytes. Formations, such as forests, thickets, etc., sometimes act also as direct obstacles to migration, as in the case of tumbleweeds and other anemochores, clitochores, etc. A marked effect of vegetation in decreasing invasion arises from the closed association typical of stable formations and of social exclusive species. In these, the occupation is so thorough and the struggle for existence so intense that the invaders, though fitted to grow under the physical factors present, are unable to compete with the species in possession for the requisite amount of some necessary factor. Closed associations usually act as complete barriers, while open ones restrict invasion in direct proportion to the degree of occupation. To this fact may be traced a fundamental law of succession, viz., the number of stages in a succession is determined largely by the increasing difficulty of invasion as the habitat becomes stabilized. Man and animals affect migration directly, though not obviously, by the destruction of disseminules. They operate as a pronounced barrier to ecesis wherever they alter conditions in such a way as to make them unfavorable to invading species, or when, by direct action upon the latter, such as grazing, tramping, parasitism, etc., they turn the scale in the struggle for existence. The absence of insects adapted to insure fertilization is sometimes a serious barrier to the establishment of adventitious or introduced plants. The presence of parasitic fungi, in so far as they destroy the seeds of plants, acts as an obstacle to migration, and restricts or prevents ecesis in so far as the fungi destroy the invaders, or place them at a disadvantage in the struggle for existence.

275. Influence of barriers. Physical barriers are typically permanent in character, while biological ones are either permanent or temporary, depending upon the permanence of the formation and the constancy of the physical factors which determine it. A stable formation, such as a forest or meadow, which acts as a decided barrier to invasion from adjacent vegetation, may disappear completely, as a result of a landslide, flood, or burn, or through the activity of man, and may leave an area into which invaders crowd from every point. Often, without undergoing marked change, a formation which has presented conditions unfavorable to the ecesis of species of mesophytic character may, by reason of a temporary change in climate, become sufficiently modified to permit the invasion of mesophytes. On the other hand, a meadow ceases to be a barrier to prairie xerophytes during a period of unusually dry years. A peculiar example of the modification of a barrier is afforded by the defoliation of aspen forests in the mountains as a result of which poophytes have been enabled to invade them. Nearly all xerophytic stretches of sand and gravel, dunes, blowouts, gravel slides, etc., and even prairies to a certain degree, exhibit a recurrent seasonal change in spring, as a result of which the hot, dry surface becomes sufficiently moist to permit the germination and growth of invaders, which are entirely barred out during the remainder of the year. In an absolute sense, no barrier is complete, since the coldest as well as the driest portions of the earth’s surface are capable, at times at least, of supporting the lowest types of vegetation. Relatively, however, in connection with the natural spread of terrestrial plants, it is possible to distinguish partial barriers from complete ones. Such a distinction is of importance in the consideration of invasions from a definite region, as it is only in this restricted sense that complete barriers have produced endemism.

Distance, though hardly to be considered a barrier in the strict sense of the word, unquestionably plays an important part in determining the amount of invasion. The effect of distance is best seen in the case of migration, as it influences ecesis only in those rare cases where viability is affected. The importance of distance, or take the converse, of proximity, is readily ascertained by the study of any succession from denudation. It has been established that the contiguous vegetation furnishes 75–90 per cent of the constituent species of the initial formation, and in mountainous regions, where ruderal plants are extremely rare, the percentage is even higher. The reason for this is to be found not only in the fact that the adjacent species have a much shorter distance to go, and hence will be carried in much greater quantity, but also in that the species of the formations beyond must pass through or over the adjacent ones. In the latter case, the number of disseminules is relatively small on account of the distance, while invasion through the intermediate vegetation, if not entirely impossible, is extremely slow, so that plants coming in by this route reach the denuded area only to find it already occupied. It is as yet impossible to give a definite numerical value to proximity in the various invasions that mark any particular succession. This will not be feasible until a satisfactory method has been found for determining a coefficient of mobility, but, this once done, it will be a relatively simple matter, not merely to trace the exact evolution of any succession of formations, but actually to ascertain from the adjacent vegetation the probable constitution of a particular future stage.

From what has been said, it follows that the primary effect of barriers upon vegetation is obstruction. Where the barrier is in the pathway of migration, however, it causes deflection of the migrant as a rule, and sets up migration in a new direction. This is often the case when the strong winds of the plains carry disseminules towards the mountains and, being unable to cross the range, drop them at the base, or, being deflected, carry them away at right angles to the original direction. The same thing happens when resistant fruits and seeds borne by the wind fall into streams of water or into ocean currents. The direction of migration is changed, and what is normally a barrier serves as an agent of dissemination.

ENDEMISM

276. Concept. Since its first use by DeCandolle, the term endemic has been employed quite consistently by phytogeographers with the meaning of “peculiar to a certain region.” Some confusion, however, has arisen from the fact that a few authors have made it more or less synonymous with indigenous and autochthonous, while others have regarded it as an antonym of exotic. In its proper sense, endemic refers to distribution, and not to origin. Its exact opposite will be found then in Fenzl’s term polydemic, dwelling in several regions. Indigenous (autochthonous) and exotic, on the contrary, denote origin, and are antonyms, indigenous signifying native, and exotic foreign. As Drude has shown, endemic plants may be either indigenous, as in the case of those species that have never moved out of the original habitat, or exotic, as in the much rarer instances where a polydemic species has disappeared from its original home and from all regions into which it has migrated except one. It is understood that not all indigenous or exotic species are endemic. The proportion of endemic to polydemic species is a variable and somewhat artificial one, depending upon the size of the divisions employed.

277. Causes. The primary causes of endemism are two, lack of migration and presence of barriers. Since distributive agents are practically universal, lack of migration corresponds essentially to immobility, a fact which decreases the difficulty of ascertaining the immediate causes of endemism in any particular species. Either immobility or a barrier may produce endemism; extremely immobile plants, for example, liliaceous species propagating almost wholly by underground parts, are as a rule endemic, while alpine plants and those of oceanic islands are endemic in the highest degree, regardless of their mobility. When the two conditions act concomitantly upon a species, endemism is almost inevitable. It can not be supposed, however, that immobility or natural barriers alone, or the concomitance of the two, must invariably give rise to endemic species; the most immobile plant may be carried into another region by unusual or accidental agencies, or the most formidable barrier to migration may be overcome by the intensity of an agent or through the action of man. Endemism is also brought about by the modification of species; new or nascent species are as a rule endemic. Whether they will remain endemic or not will depend upon the perfection of their contrivances for dissemination and upon the presence of barriers to migration or ecesis. Finally, as Drude was the first to point out, the disappearance of a polydemic species in all regions but one, owing to the struggle for existence or to changed physical conditions, will result in endemism.

278. Significance. Endemism is readily recognized by methods of distributional statistics, applied to areas limited by natural barriers to migration or ecesis. For political areas, it has no significance whatever, unless the boundaries of these coincide with barriers. It determines in the first degree the validity of regions, though the latter are often recognized also by the presence of barriers and by the character of the vegetation. Endemism may occur in areas of vegetation of any rank from a formation to a zone. When the term is not qualified, however, it should be used of species with reference to formations alone. Comparisons to be of value, however, can be instituted only between areas of the same order, i. e., between two or more formations, two or more regions, provinces, etc. In the same way, taxonomic groups of the same rank should be used in such comparisons, i. e., species should be contrasted with species, genera with genera, and families with families, except when it is desired to obtain some measure of the age of the vegetation by the differentiation of the endemic phyla within it. There will be seen to exist a fundamental correspondence between the rank of the floral division and the taxonomic group, though the apparent exceptions to this are still too numerous to warrant its expression in a general law. As a rule, however, formations most frequently show endemic habitat forms and species, more rarely endemic genera; regions and provinces commonly exhibit endemic species and genera, rarely endemic families; while zones and hemispheres contain endemic orders as well as families. This correspondence is readily seen to depend primarily upon the fact that increased differentiation in the taxonomic sense is a concomitant of the increased invasion of endemic species, measured in terms of distance and difference in habitat.

It is too early to decide satisfactorily whether it is proper to speak of formations as endemic. At first thought it would seem that all formations, with the exception of ruderal ones, were endemic, but a study of almost any transition area between regions would seem to point to the opposite conclusion, viz., that no formations are properly endemic. It is equally impossible at present to distinguish different types of endemics, such as relictae, etc., as any such classification must await the elaboration of a method for determining the phylogeny of a natural group of species by an investigation of their comparative differentiation in connection with their migration in all directions from the vegetation center into new habitats. In short, it will not be possible to make a thorough study of endemism and to postulate its laws until modern methods of research have been extended to a much larger portion of the vegetation of the globe. The final task of phytogeography is the division of the earth’s vegetation into natural areas. It will be at once evident that most plants can not properly be called endemic until the natural regions in which they are found have been accurately defined, a work which has barely begun. In the much simpler matter of distribution, upon which the accuracy of statistical methods depends directly, there are few regions sufficiently well known at the present time to yield anything like permanent results.

POLYPHYLESIS AND POLYGENESIS

279. Concept. The idea of polyphylesis, as advanced by Engler, contains two distinct concepts: (1) that a species may arise in two different places or at two different times from the same species, and (2) that a genus or higher group may arise at different places or times by the convergence of two or more lines of origin. It is here proposed to restrict polyphylesis, as its meaning would indicate, to the second concept, and to employ for the first the term polygenesis,[^[37]] first suggested by Huxley in the sense of polyphylesis. The term polyphylesis is extended, however, to cover the origin of those species which arise at different places or times from the convergence of two or more different species, a logical extension of the idea underlying polyphyletic genera, though it may seem at first thought to be absurd. Polygenesis may be formally defined as the origin of one species from another species at two or more distinct places on the earth’s surface, at the same time or at different times, or its origin in the same place at different times. Polyphylesis, on the contrary, is the origin of one species from two or more different species at different places, at the same time or at different times. It is evident that what is true of species in this connection will hold equally well of genera and higher groups. Opposed to polygenesis is monogenesis, in which a species arises but once from another species; with polyphylesis is to be contrasted monophylesis, in which the species arises from a single other species. It will be noticed at once that these two concepts are closely related. The following diagrams will serve to make the above distinctions more evident:

I. Polygenesis II. Polyphylesis III. Monogenesis (Monophylesis)

In I, a species A, becomes scattered over a large area in a series of places, m ... mⁿ, with the same physical factors, in any or all of which may arise the new species a. In II, a species with xerophytic tendency, A, and one with mesophytic tendency, B, in the course of migration find themselves respectively in a more mesophytic habitat, m, and a more xerophytic one, x, in which either may give rise to the new form, c, which is more or less intermediate between A and B. In III, the method of origin is of the simplest type, in which a species is modified directly into another one, or is split up into several.

280. Proofs of polygenesis. In affirming the probability of a polygenetic origin of species, there is no intention of asserting that all species originate in this way. It seems evident that a very large number of species of restricted range are certainly monogenetic, at least as far as origin in space is concerned. It is possible that any species may arise at two or more distinct times. Polygenesis can occur readily only in species of more or less extensive area, in which recur instances of the same or similar habitat. The relative frequence and importance of the two methods can hardly be conjectured as yet, but origin by monogenesis would seem to be the rule.

The arguments adduced by Engler in support of polygenesis are in themselves conclusive, but the investigations of the past decade have brought to light additional proofs, especially from the experimental side. In determining the physical factors of prairie and mountain formations, and especially by methods of experimental ecology, the author has found that habitats are much less complex than they are ordinarily thought to be, since water-content and humidity, and to a less degree light, constitute the only factors which produce direct modification. In addition, it has been ascertained that the minimum difference of water-content, humidity, or light, necessary to produce a distinguishable morphological adjustment is much greater than the unit differences recorded by the instruments. In short, the differences of habitats, as ascertained by thermograph, psychrometer and photometer, are much greater than their efficient differences, and, with respect to their ability to produce modification, habitats fall into relatively few categories. A striking illustration of this is seen in the superficially very different habitats, desert, strand, alkali plain, alpine moor, and arctic tundra, all of which are capable of producing the same type of xerophyte. It follows from this that many more or less plastic species of extensive geographical area will find themselves in similar or identical situations, measured in terms of efficient differences, and will be modified in the same way in two or more of these. In mountain regions, where interruption of the surface and consequent alternation are great, the mutual invasion of contiguous formations is of frequent occurrence, often resulting in habitat forms. The spots in which these nascent species, such as Galium boreale hylocolum, Aster levis lochmocolus, etc., are found, are often so related to the area of the parent species as to demonstrate conclusively that these forms are the result of polygenesis and not of migration. Naturally, what is true of a small area will hold equally well of a large region, and the recurrence of the same habitat form may be accepted as conclusive proof of polygenesis. The most convincing evidences of multiple origin, however, are to be found in what De Vries has called “mutations.” It makes little difference whether we accept mutations in the exact sense of this author, or regard them as forms characterized by latent variability. The evidence is conclusive that the same form may arise in nature or in cultivation, in Holland or in America, not merely once, but several or many times. In the presence of such confirmation, it is unnecessary to accumulate proofs. Polygenesis throws a new light upon many difficult problems of invasion and distribution, and, as a working principle, admits of repeated tests in the field. It obviates, moreover, the almost insuperable difficulties in the way of explaining the distribution of many polygenetic species on the basis of migration alone.

281. Origin by polyphylesis. In 1898, the author first advanced a tentative hypothesis to the effect that a species homogeneous morphologically may arise from two distinct though related species. During subsequent years of formational study, the conviction has grown in regard to the probability of such a method of origin. Since the appearance of Engler’s work, a polyphyletic origin for certain genera has been very generally accepted by botanists, but all have ignored the fact that the polyphylesis of genera carries with it the admission of such origin for species, since the former are merely groups of the latter. I can not, however, agree with Engler, that polyphyletic genera, and hence species also, are necessarily unnatural. If the convergence of the lines of polyphylesis has been great, resulting in essential morphological harmony, the genus is a natural one, even though the ancestral phyla may be recognizable. If, on the other hand, the convergence is more or less imperfect, resulting in subgroups of species more nearly related within the groups than between them, the genus can hardly be termed natural. This condition may, however, prevail in a monophyletic genus with manifest divergence and still not be an indication that it is artificial.

Darwin[^[38]], in speaking of convergence, has said: “If two species, belonging to two distinct though allied genera, had both produced a large number of new and divergent forms, it is conceivable that these might approach each other so closely that they would have all to be classified under the same genus; and thus the descendants of two distinct genera would converge into one.” The application of this statement to species would at once show the possibility of polyphylesis in the latter, and a further examination of the matter will demonstrate its probability. It is perfectly evident that a species may be split into two or more forms by varying the conditions, let us say of water-content, and that the descendants of these forms may again be changed into the parent type by reversing the process. This has, in fact, been done experimentally. Since it is admittedly impossible to draw any absolute line between forms, varieties, and species, it is at once clear that two distinct though related species, especially if they are plastic, may be caused to converge in such a way that the variants may constitute a new and homogeneous species. This may be illustrated by a concrete case at present under investigation. Kuhnistera purpurea differs from K. candida in being smaller, in having fewer, smaller, and more narrow leaflets, and a globoid spike of purple flowers in place of an elongated one of white flowers; in a word, it is more xerophytic. This conclusion is completely corroborated by its occurrence. On dozens of slopes examined, Kuhnistera purpurea has never been found mingling with K. candida on lower slopes, except where an accident of the surface has resulted in a local decrease of water-content. The experiment as conducted is a simple one, consisting merely in sowing seed of each in the zone of the other, and in growing K. purpurea under controlled mesophytic conditions, and K. candida under similarly measured xerophytic conditions in the planthouse.

While the polyphyletic origin of species is in a fair way to be decided by experiment, it receives support from several well-known phenomena. The striking similarity in the plant body of families taxonomically so distinct as the Cactaceae, Stapeliaceae, and Euphorbiaceae, or Cyperaceae and Juncaceae, indicates that a vegetation form may be polyphyletic. On the other hand, the local appearance of zygomorphy, of symphysis, and of aphanisis in the floral types of phylogeneticallv distinct families is a proof of the operation of convergence in reproductive characters. To be sure, the convergence is never so great as to produce more than superficial similarity, but this is because the groups are markedly different in so many fundamental characters. The same tendency in closely related species would easily result in identity. As in the case of polygenesis, the relatively small number of typically distinct habitats makes it clear that two different species of wide distribution, bearing to each other the relations of xerophyte to mesophyte, of hydrophyte to mesophyte, or of poophyte to hylophyte, might often find themselves in reciprocal situations, with the result that they would give rise to the same new form. The final proof of the polyphylesis of species is afforded by the experiments of De Vries in mutation. De Vries found that Oenothera nanella arose from O. Lamarckiana, O. laevifolia, and O. scintillans; Oenothera scintillans arose from O. lata and O. Lamarckiana; Oenothera rubrinervis from O. Lamarckiana, O. laevifolia, O. lata, O. oblonga, O. nanella, and O. scintillans, etc. Whatever may be the rank assigned to these mutations, whether form, variety, or species, there can be no question of their polyphyletic origin, nor, in consequence of the connection of mutations with variations through such inconstant forms as O. scintillans, O. elliptica, and O. sublinearis, of the possibility of polyphylesis in any two distinct though related species or genera.

KINDS OF INVASION

282. Continuous and intermittent invasion. With respect to the frequency of migration, we may distinguish invasion as continuous, or intermittent. Continuous invasion, which is indeed usually mutual, occurs between contiguous formations of more or less similar character, in which there is an annual movement from one into the other, and at the same time a forward movement through each, resulting from the invaders established the preceding year. By far the greater amount of invasion is of this sort, as may readily be seen from the fact that migration varies inversely as the distance, and ecesis may decrease even more rapidly than the distance increases. The significant feature of continuous invasion is that an outpost may be reinforced every year, thus making probable the establishment of new outposts from this as a center, and the ultimate extension of the species over a wide area. The comparatively short distance and the regular alternation of migration and ecesis render invasion of this sort very effective. An excellent illustration of this is seen in transition areas and regions, which are due directly to continuous and usually to mutual invasion. Intermittent invasion results commonly from distant carriage, though it may occur very rarely between dissimilar adjacent formations, when a temporary swing in the physical factors makes ecesis possible for a time. It is characterized by the fact that the succession of factors which have brought about the invasion is more or less accidental and may never recur. Intermittent invasion is relatively rare, and from the small number of disseminules affected, it is of little importance in modifying vegetation quantitatively. On the other hand, since a species may often be carried far from its geographical area, it is frequently of great significance in distribution.

283. Complete and partial invasion. When the movement of invaders into a formation is so great that the original occupants are finally driven out, the invasion may be termed complete. Such invasion is found regularly in the case of many ruderal formations, and is typical of the later stages of many successions. It is ordinarily the result of continuous invasion. If the number of invaders is sufficiently small that they may be adopted into the formation without radically changing the latter, the invasion is partial. This is doubtless true of the greater number of invasions, though these are regularly much less striking and important than instances of complete invasion.

Fig. 59. Continuous invasion into a new area; mats of Arenaria sajanensis. Silene acaulis and Sieversia turbinata invading an alpine gravel slide.

284. Permanent and temporary invasion. The permanence of invasion depends upon the success attending ecesis, and upon the stability of the formation. It has already been noticed that under certain conditions plants may germinate and grow, and if they are perennials, even become established, and still ecesis be so imperfect that reproduction is impossible. Others may find the conditions sufficiently favorable for propagation, but unfavorable for the formation of flowers and fruits. Finally there are plants which seem to be perfectly established for a few years, only to disappear completely. The latter are examples of temporary invasion. It is necessary to draw clearly the line between complete and partial invasion in this connection. The former is temporary in the initial or intermediate stages of nearly all successions, as compared with the ultimate stages, though it is in a large degree permanent in comparison with the partial invasion of species which are able to maintain themselves for a few years. In a sense, there is a real distinction between the two, inasmuch as a particular stage of succession is permanent as long as the habitat remains essentially the same. A critical study of the species of such stages shows, however, that they manifest very different degrees of permanence. Species which invade stable vegetation temporarily have been termed adventive by A. DeCandolle. Permanent invasion occurs when a species becomes permanently established in a more or less stable formation. It is characteristic of the great majority of invaders found in the grassland and forest stages of successions.

Plants which have arisen within a formation or have been a constituent part of it since its origin are indigenous. Contrasted with these are the species which have invaded the formation since it received its distinctive impress: these are derived. The determination of the indigenous and derived species of a formation or larger division is of the utmost importance, as it enables us to retrace the steps by which the formation has reached its present structure, and to reconstruct formations long since disappeared. To render it less difficult, it is necessary to scrutinize the derived elements closely, first, because it is easiest to recognize the indigenous species by eliminating the derived, and second, because this analysis will show that not all derived species have entered the formation at the same time and from the same sources. Derived species may be termed vicine, when they are fully established invaders from adjacent formations or regions, and adventitious, when they have come from distant formations and have succeeded in establishing themselves. Finally, those derived species which are unable to establish themselves permanently are adventive.

MANNER OF INVASION

285. Entrance into the habitat. Since the ecesis of invaders depends in large measure upon the occupation of the plants in possession, the method and degree of invasion will be determined by the presence or absence of vegetation. Areas without vegetation are either originally naked or denuded, while vegetation with respect to the degree of occupation is open (sporadophytia), or closed (pycnophytia). Each type of area presents different conditions to invaders, largely with respect to the factors determining ecesis. Naked habitats, rocks, talus, gravel slides, and dunes, while they offer ample opportunity for invasion on account of the lack of occupation, are really invaded with the greatest difficulty, not only because they contain originally few or no disseminules, but also because of their xerophytic character and the difficulty of obtaining a foothold, on account of the extreme density or instability of the soil. Denuded habitats, blowouts, sand draws, ponds, flood plains, wastes, fields, and burns, usually afford maximum opportunity for invasion. They invariably contain a large number of disseminules ready to spring up as soon as the original vegetation is destroyed. The surface, moreover, is usually such as to catch disseminules and to offer them optimum conditions of moisture and nutrition. Open formations are readily invaded, though the increased occupation renders entrance more difficult than it is in denuded areas. Closed formations, on the other hand, are characterized by a minimum of invasion, partly because invaders from different formations find unfavorable conditions in them, but chiefly because the occupation of the inhabitants is so complete that invaders are unable to establish themselves.

Invasion takes place by the penetration of single individuals or groups of individuals. This will depend in the first place upon the character of the disseminule. It is evident that, no matter how numerous the achenes may be, the invasion of those anemochorous species with comate or winged seeds or one-seeded fruits will be of the first type, while all species in which the disseminule is a several or many-seeded fruit or plant, as in hooked fruits, tumbleweeds, etc., will tend to produce a group of invaders. Occasionally of course, the accidents of migration will bring together a few one-seeded disseminules into a group, or will scatter the seeds of a many-seeded fruit, but these constitute relatively rare exceptions. This distinction in the matter of invasion is of value in studying the relative rapidity of the latter, and the establishment of new centers, but it is of greatest importance in explaining the historical arrangement of species in a formation, and hence has a direct bearing upon alternation. It is entirely independent of the number of invaders, which, as we have seen, depends upon seed-production, mobility, distance, occupation, etc., but is based solely upon mode of arrangement, and will be found to underlie the primary types of abundance, copious, and gregarious. In this connection, it should also be noted that the contingencies of migration, especially the concomitant action in the same direction of two or more distributive agencies, often results in the penetration of a group of individuals belonging to two or more species. This may well be termed mass invasion; it is characteristic of transition areas or regions, and along valleys or other natural routes for migration it gives rise to species guilds. The movement of species guilds constitutes one of the most complex and interesting problems in the whole field of invasion, the solution of which can be attempted only after the thorough analysis of the simpler invasions between formations. A better understanding of the meaning of invasion by species guilds is imperative for the natural limitation of regions, as at present such groups constitute alien associations in many regions otherwise homogeneous.

286. Influence of levels. The invasion of a formation may occur at three different levels: (1) at the level of the facies, (2) below the facies, (3) above the facies, depending directly upon the relative height of invaders and occupants. The invasion level is an extremely simple matter to determine, except in the case of woody plants, such as shrubs and trees, which attain their average height only after many years. Its importance is fundamental. The level at which invasion occurs not only determines the immediate constitution of the formation, whether its impress shall still be given by the occupants or by the invaders or by both together, but it also decides the whole future of the formation, i. e., whether the invaders or occupants shall persist unmodified or modified. The problem is an extremely complex one, but the careful analysis of invasion at each level throws a flood of light upon it. The entrance of invaders of the same general height as the facies of a formation results regularly in mixed formations. This is well illustrated by the structure of the transition areas between two formations of the same category, i. e., forests, meadows, etc. It is seldom, however, that the facies and invaders are so equally matched in height and other qualities that they remain in equilibrium for a long period. One or the other has a slight advantage in height, or the one suffers shading or crowding better than the other, is longer-lived or faster-growing, with the result that invader yields to occupant, or occupant to invader. It is a well-known fact that many mixed formations represent intermediate stages of development.

Invasion at a level different from that of the facies is inevitably followed by modification. If the invasion takes place below the facies, the invaders will be exterminated gradually, or slowly assimilated. In either case, there is little structural change in the formation, and its stability is affected slightly or not at all. If the invaders overtop the facies in any considerable number, the entire formation undergoes partial or complete modification, or in extreme cases it disappears, as is typically the case in succession. A peculiar variation of invasion at a level above the facies is seen where woody plants invade grassland, when the trees or shrubs become more or less uniformly scattered in an open woodland or open thicket. Here the grassland takes on an altogether different appearance superficially, though it is usually unchanged, except beneath and about the invaders, where either adaptation or extermination results. Finally, it should be borne in mind that the invasion of a particular formation, especially in the case of layered thickets and forests, often takes place at two levels, at the height of the facies and below the facies.

INVESTIGATION OF INVASION

287. The methods to be used in the study of invasion are those already described elsewhere. The migration circle is of the first importance because it makes it possible to secure an accurate record of actual movement. Quadrat and transect are valuable, but from their nature they are more serviceable for ecesis than for migration. All of these should be of the permanent type, in order that the fate of invaders may be followed for several years at least. Permanent areas furnish evidence of the changes wrought in the actual vegetation, while denuded ones can serve only to show the potential migration and ecesis of the constituent species. Transition zones and areas are special seats of invasion; they are best studied by means of the belt transect and the ecotone chart. The movement of a line of invaders or of scattered outposts is traced by the use of labeled stakes at the points concerned. It is clear that this method will yield conclusive data in regard to the great invasions between regions, such as the movement of species guilds, the advance of the forest frontier, etc. When invasion is scattered, factor instruments can not be used to advantage, but where the invading line is well marked, or where extra-formational areas occur, a knowledge of the physical factors is a great aid.

An invasion that has been completed can not be studied in the manner indicated. A method of comparison must be used, in order to determine the original home of the invaders. For this an exact knowledge of the contiguous formations and of the abundance of the species common to all is a prerequisite. With this as a basis, it is usually a simple task to refer all the species of the formation concerned to their proper place in the groups, indigenous, derived, and adventitious.