Up to the present time, the general character of the habitat, together with the gross appearance of the plant itself, has been thought sufficient to determine the proper position of a plant or a formation in the water-content classification. Such a method is adequate, however, only for plants and formations which bear a distinct impress. For an accurate classification into the three categories, hydrophytes, mesophytes, and xerophytes, it is necessary to make exact determinations of the normal holard and chresard of the habitat, and to supplement this, in some degree at least, by histological studies. Except in the case of saline, acid, and frozen soils, the holard alone will be a fairly accurate index, especially in habitats of similar soil composition. For an exact and comprehensive classification, however, and particularly in comparative work, the chresard must constitute the sole criterion. As the latter has been ascertained for very few formations, and in Nebraska and Colorado alone, the present characterization of many plants and formations as hydrophytic, mesophytic, or xerophytic must be regarded as largely tentative, and the final classification will be possible only after the thorough quantitative investigation of their habitats.
The water-content groups, hydrophytia, mesophytia, and xerophytia, include all formations found upon the globe. The exactness with which this classification applies to vegetation is made somewhat more evident by dividing mesophytia into forest and grassland. This is based primarily upon light association, but it also reflects water-content differences in a large degree. The groups thus constituted represent the fundamental zonation of the vegetative covering with respect to water-content. Ocean, forest, grassland, and desert correspond exactly to hydrophytia, hylophytia, poophytia, and xerophytia. The difference is merely one of terminology: the first series takes into account the physiognomy of the vegetation itself, while the other emphasizes the causative factors.
THE DEVELOPMENT OF THE FORMATION
259.A strict account of development should trace the results of the various activities of vegetation in their proper sequence. This is aggregation, migration, ecesis, reaction, and competition. These functions are so intimately and often so inextricably associated that it is hardly feasible to discuss development by treating each one separately. In consequence, the two fundamental phenomena, invasion and succession, which they produce, are taken as the basis of the discussion. These, moreover, are different only in degree; succession is merely complete, periodic invasion. Nevertheless, the subject gains in clearness by a separate treatment of each.
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 asLemnaceae, 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.
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 presentin 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. Mobilitydenotes 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, andBacillariaceae, or possess motile propagules, such as mostPhycophyta. 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 manyLiliaceaeto 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 ofPeronospora, the spores and gemmae ofMarchantia, the fruits and runners ofFragaria, etc. Special modifications have, as a rule, been developed in direct connection with sporesand 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 ofSempervivumand the tumbling plants ofCycloloma. 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 andProtococcus, the zoogonidia ofUlothrix,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 aresufficiently 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 nanaandRoripa 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. InOstrya,Physalis,Staphylea, the modification is for wind-distribution, while inCarex,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 inAcer,Betula,Rumex, manyUmbelliferae,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 otherLigulifloraeare connected through Senecio andEriophorumwith 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 inRudbeckia,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 inPulsatilla,Sieversia, andClematis.
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 inTribulusandCenchrus.
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 ofSilene, or the fruit is beset with glandular hairs, as inCerastium,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 inProtococcus,Ulothrix,Oedogonium,Ectocarpus, etc., or with plant bodies similarly motile,BacteriaceaeandVolvocaceae.
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 operculateDiscomycetes, and especially in theAscobolaceae, where the asci project above the hymenium, the spores are raised above thesurface 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. InGasteromycetesand in certainHepaticae, 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. InBryophyta, 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 asStipaandAristida, 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 certainCompositae, 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 scaposeLiguliflorae,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 inCompositae, 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 inPilobolusand inDiscomycetes, though in the latter turgescence results rather in placing the spores in a position to be readily carried by the wind.ImpatiensandOxalisfurnish 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, andLotusillustrate 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 orinvolucre 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 migrationis 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 produceconcomitant structural changes. The former explains such various habitat forms of the same species as are found inGalium boreale,Gentiana acuta, etc., and the latter the morphological constancy of plants likeChamaenerium, 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.
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. Inthis 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 barriersare 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 barriersinclude 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 necessaryfactor. 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 froma 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.
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 synonymouswith 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 thefirst 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.