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.

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:

[Illustration: I. Polygenesis II. Polyphylesis III. Monogenesis

(Monophylesis)]

In I, a species A, becomes scattered over a large area in a series of

places, _m_ ... _m^n_, 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.

[Illustration: 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.

SUCCESSION

=288. Concept.= Succession is the phenomenon in which a series of

invasions occurs in the same spot. It is important, however, to

distinguish clearly between succession and invasion, for, while the one

is the direct result of the other, not all invasion produces succession.

The number of invaders must be large enough, or their effect must be

sufficiently modifying or controlling to bring about the gradual

decrease or disappearance of the original occupants, or a succession

will not be established. Partial or temporary invasion can never

initiate a succession unless the reaction of the invaders upon the

habitat is very great. Complete and permanent invasion, on the other

hand, regularly produces successions, except in the rare cases where a

stable formation entirely replaces a less stable one without the

intervention of other stages. Succession depends in the first degree

upon invasion in such quantity and of such character that the reaction

of the invaders upon the habitat will prepare the way for further

invasion. The characteristic presence of stages in a succession, which

normally correspond to formations, is due to the peculiar operation of

invasion with reaction. In the case of a denuded habitat, for example,

migration from adjacent formations is constantly taking place, but only

a small number of migrants, especially adapted to somewhat extreme

conditions, are able to become established in it. These reach a maximum

development in size or number, and in so doing react upon the habitat in

such a way that more and more of the dormant disseminules present, as

well as those constantly coming into it, find the conditions favorable

for germination and growth. The latter, as they in turn attain their

maximum, cause the gradual disappearance of the species of the first

stage, and at the same time prepare the way for the individuals of the

succeeding formation. It is at present impossible to determine to what

degree this substitution is due to the struggle for existence between

the individuals of each species and between the somewhat similar species

of each stage, and to what degree it arises out of the physical

reaction.

It is evident that geological succession is but a larger expression of

the same phenomenon, dealing with infinitely greater periods of time,

and produced by physical changes of such intensity as to give each

geological stage its peculiar stamp. If, however, the geological record

were sufficiently complete, we should find unquestionably that these

great successions merely represent the stable termini of many series of

smaller changes, such as are found everywhere in recent or existing

vegetation.

=289. Kinds of succession.= The fundamental causes of succession are

invasion and reaction, but the initial causes of a particular succession

are to be sought in the physical or biological disturbances of a habitat

or formation. With reference to the initial cause, we may distinguish

_normal succession_, which begins with nudation, and ends in

stabilization, and _anomalous succession_, in which the facies of an

ultimate stage of a normal succession are replaced by other species, or

in which the direction of movement is radically changed. The former is

of universal occurrence and recurrence; the latter operates upon

relatively few ultimate formations. In the origin of normal successions,

nudation may be brought about by the production of new soils or

habitats, or by the destruction of the formation which already occupies

a habitat. In a few cases, the way in which the habitat arises or

becomes denuded is not decisive as to the vegetation that is developed

upon it, but as a rule the cause of nudation plays as important a part

in the development of a succession as does the reaction exerted by the

invaders. The importance of this fact has been insisted upon under

invasion. New soils present extreme conditions for ecesis, possess few

or no dormant disseminules, and in consequence their successions take

place slowly and exhibit many stages. Denuded soils as a rule offer

optimum conditions for ecesis as a result of the action of the previous

succession, dormant seeds and propagules are abundant, and the

revegetation of such habitats takes place rapidly and shows few stages.

The former may be termed _primary succession_, the latter _secondary

succession_.

_PRIMARY SUCCESSIONS_

=290.= These arise on newly formed soils, or upon surfaces exposed for

the first time, which have in consequence never borne vegetation before.

In general they are characteristic of mountain regions, where weathering

is the rule, and of lowlands and shores, where sedimentation or

elevation constantly occur. The principal physical phenomena which bring

about the formation of new soils are: (1) elevation, (2) volcanic

action, (3) weathering, with or without transport.

=291. Succession through elevation.= Elevation was of very frequent

occurrence during the earlier, more plastic conditions of the earth, and

the successions arising as a result of it must have been important

features of the vegetation of geological periods. To-day, elevation is

of much less importance in changing physiography, and its operation is

confined to volcanic islands, coral reefs, and islets, and to rare

movements or displacements in seacoasts, lake beds, shore lines, etc.

There has been no investigation of the development of vegetation on

islands that are rising, or have recently been elevated, probably

because of the slow growth of coral reefs and the rare appearance of

volcanic islands. On coral reefs, the first vegetation is invariably

marine, but as the reef rises higher above the surf line and the tide,

the vegetation passes into a xerophytic terrestrial type adapted to an

impervious rock soil, and ultimately becomes mesophytic. In volcanic

islands, unless they are mere rocks over which the waves rush, the

succession must always begin with a xerophytic rock formation. The best

known example of a rising coast line is found in Norway and Sweden,

where the southeastern coast is rising at the rate of five or six feet a

century. There can be little question that such changes of level will

produce marked changes in vegetation, but the modification will be so

gradual as to be scarcely perceptible in a single generation. It is

probable that the forests of the Atlantic coastal plains are the

ultimate stages of successions initiated at the time of the final

elevation of the sea bottom along the coast line.

[Illustration: Fig. 60. A lichen formation (_Lecanora-Physcia-petrium_),

the first stage of the typical primary succession

(_Lecanora-Picea-sphyrium_) of the Colorado mountains.]

=292. Succession through volcanic action.= The deposition of volcanic

ashes and flows of lava are relatively infrequent at present,

occurring only in the immediate vicinity of active volcanoes, chiefly

in or near the tropics. Successions of this sort are in consequence

not only rare, but they are also relatively inaccessible to

investigators. They have been studied in a few cases, for example,

those of Krakatoa by Treub, but this study has been confined to the

general features of revegetation. Ash fields and lava beds are widely

different in compactness, but they agree in having a low water- and

nutrition-content. The pioneer plants in both will be intense

xerophytes, but the soil differences will determine that these shall

be sand-binders in the former, and rock-weathering plants in the

latter.

=293. Weathering.= Practically all primary successions start on soils

produced by weathering. This is also true of coral or volcanic islets

and of lava beds, for no terrestrial vegetation can secure a foothold

upon them until the surface of the rock has been to some extent

decomposed or disintegrated. Weathering, as is well known, consists of

two processes, disintegration and decomposition, which usually operate

successively, though they are sometimes concomitant. Disintegration

usually precedes, especially in rock masses, and unless it is soon

followed by decomposition, results in dysgeogenous soils. Decomposition

often goes hand in hand with disintegration, or it takes place so

rapidly and perfectly that it alone seems to be present. In either case,

the resulting soil is eugeogenous. The relation of decomposition to

disintegration determines the size and compactness of the soil

particles, and upon the latter depend the porosity, capillarity, and

hygroscopicity of the soil. These control in large degree the character

of the first vegetation to appear on the soil.

Another point of fundamental value in determining revegetation is the

disposition of the weathered rock. If it remains _in situ_, it will

evidently differ in respect to compactness, homogeneity,

nutrition-content, water-content, disseminules, etc., from weathered

material which has been transported. An essential difference also arises

from the fact that a rock may be weathered a long distance from the

place where the decomposed particles are finally deposited, and in the

midst of a vegetation very different from that found in the region of

deposit. The disposition of the weathered material affords in

consequence a satisfactory basis for the arrangement of primary

successions. The following classification is proposed, based upon the

soil groups established by Merrill.[39]

=294. Succession in residuary soils.= Residuary soils are always

sedentary, i. e., they are formed _in situ_. They show certain

differences dependent upon the rock from which they originate, which may

be mixed crystalline shale, sandstone, or limestone, but the

thoroughness of decomposition causes these differences to be

comparatively small. Residuary soils are typically eugeogenous; their

successions in consequence usually begin with mesophytes, and consist of

a few stages. The soluble salt-content is comparatively low, since all

soluble matters are readily leached out. Successions in these soils are

especially characteristic of shale, sandstone, and limestone ledges or

banks. Cumulose deposits, like residuary ones, are sedentary in

character, but as they are produced by the accumulation of organic

matter, they will be considered under reactions of vegetation upon

habitat.

[Illustration: Fig. 61. Talus arising from the disintegration of a

granitic cliff; the rocks are covered with crustose lichens.]

=295. Succession in colluvial soils.= Colluvial deposits owe their

aggregation solely or chiefly to the action of gravity. They are the

immediate result of the disintegration of cliffs, ledges, and mountain

sides, decomposition appearing later as a secondary factor. The masses

and particles arising from disintegration are extremely variable in

size, but they agree as a rule in their angular shape. The typical

example of the colluvial deposit is the talus, which may originate from

any kind of rock, and contains pieces of all sizes. Gravel slides differ

from ordinary talus in being composed of more uniform particles, which

are worn round by slipping down the slope in response to gravity and

surface wash. Boulder fields are to be regarded as talus produced by

weathering under the influence of joints, resulting in huge boulders

which become more and more rounded under the action of water and

gravity. This statement applies to those fields which are in connection

with some cliff that is weathering in this fashion; otherwise, boulder

fields are of aqueous or glacial origin. The character of the

successions in talus will depend upon the kind of rock in the latter. If

the rock is igneous or metamorphic, decomposition will be slow, and the

soil will be dysgeogenous. Successions on such talus consist of many

stages, and the formations are for a long time open and xerophytic. In

talus formed from sedimentary rocks, especially shales, limestones, and

calcareous sandstones, decomposition is much more rapid, and the

successions are simpler and more mesophytic.

=296. Succession in alluvial soils.= Alluvial soils are fluvial when

laid down by streams and rivers, and litoral when washed up by the waves

or tides. They are formed when any obstacle retards the movement of the

water, decreasing its carrying power, and causing the deposit of part or

all of its load. They consist of more or less rounded, finely comminuted

particles, mingled with organic matter and detritus. Alluvial deposits

are especially frequent at the mouth of streams and rivers, on their

terraces and flood plains, and along silting banks as compared with the

erosion banks of meanders. The filling of ponds by the erosion due to

surface drainage, and of lakes by the deposition of the loads of streams

that enter them, results in the formation of new alluvium. A similar

phenomenon occurs along coasts, where bays and inlets are slowly

converted into marshes in consequence of being shallowed by the material

washed in by the waves and tides. Such paludal deposits are invariably

salt water or brackish. Contrasted with these, which are uniformly black

in consequence of the large amount of organic matter present, are the

sandbars and beaches, which, though due to the same agents, are light

grey or white in color, because of the constant leaching by the waves.

Two kinds of alluvial deposits may accordingly be distinguished: (1)

those black with organic matter, and little disturbed by water, and (2)

those of a light color, which are constantly swept by the waves. The

successions corresponding to these are radically different. In the

first, the pioneer vegetation is hydrophytic, consisting largely of

amphibious plants. The pioneer stages retard the movement of the water

more and more, and correspondingly hasten the deposition of its load.

The marsh bed slowly rises in consequence, and finally the marsh begins

to dry out, passing first into a wet meadow, and then into a meadow of

the normal type. A notable exception to this sequence occurs when the

swamp contains organic matter or salts in excess, in which case the

vegetation consists indefinitely of swamp xerophytes, or halophytes. The

first vegetation on fresh water sandbars is xerophytic, or, properly,

dissophytic, unless they remain water-swept, and the ultimate stages of

their successions are mesophytic woodlands composed of water-loving

genera, _Populus_, _Salix_, etc. It seems certain, however, that these

will finally give way to longer-lived hardwoods. Maritime sandbars and

beaches are always saline, and their successions run their short course

of development entirely within the group of halophytes, unless the

retreat of the sea or freshwater floods change the character of the

soil. The chemical action of underground waters also produces new soils,

which might be classed as alluvial. These soils are essentially rock

deposits, travertine, silicious sinter, etc., made by iron and lime

springs and by geysers, and they must be changed by decomposition into

soils proper to be comparable with alluvial soils.

[Illustration: Fig. 62. Talus arising from the decomposition of granite;

the gravel is covered with a formation of foliose lichens

(_Parmelia-chalicium_), the second stage of the primary talus

succession; the herbs are pioneers of the next stage.]

=297. Succession in aeolian soils.= The only wind-borne soils of

geological importance at the present time are those which form dunes,

both inland and coastal. Aeolian deposits consist largely of rounded

sand particles, which are of almost uniform size in any particular dune,

but vary greatly in dunes of different ages. The reaction of the

pioneers on dunes plays an important part in building the latter, but

the immense dunes of inland deserts, which are entirely destitute of

vegetation, seem to indicate that its value has been overestimated. The

first stages in dune successions are dissophytic, i. e., the plants grow

in a soil of medium or high water-content, but in an atmosphere that is

extremely xerophytic. The ultimate stages vary widely in accordance with

the region in which they occur; they may be xerophytic heaths or

mesophytic meadows and forests. Because of their striking character and

economic significance, dunes have received much attention, with the

result that their successions are the most thoroughly known of all.

Prairie and steppe formations are probably to be regarded as the

ultimate stages of successions established on wind-borne loess, and it

is possible that the same is true of sand-hill vegetation in the prairie

province.

=298. Succession in glacial soils.= The formation of glacial deposits is

at present confined to alpine and arctic regions. Recent successions in

such soils are localized in these regions, and are in consequence

relatively unimportant. There can be little question, however, that the

thorough investigation of succession in and near the moraines of

existing glaciers will throw much light upon the successions of the

glacial period. Moraines, drumlins, eskars, and alluvial cones represent

the various kinds of glacial deposits. They agree in being heterogeneous

in composition, and are covered to-day with ultimate stages of

vegetation, except in the immediate vicinity of glaciers.

_SECONDARY SUCCESSIONS_

=299.= Generally speaking, all successions on denuded soils are

secondary. When vegetation is completely removed by excessive erosion,

it is an open question whether the resulting habitat is to be regarded

as new or denuded. Erosion is rarely so extreme and so rapid, however,

as to produce such a condition, even when it results from cultivation or

deforestation. It is, moreover, especially characteristic of newly

formed soils, and in studying succession in eroded habitats, it is

fundamentally important to determine whether erosion has produced

denudation, or has operated upon a new soil. The great majority of

secondary successions owe their origin to floods, animals, or the

activities of man, and they agree in occurring upon decomposed soils of

medium water-content, which contain considerable organic matter, and a

large number of dormant migrants. These successions consist of

relatively few stages, and are rarely of extreme character.

=300. Succession in eroded soils.= Eroded soils show considerable

differences, as they arise in consequence of erosion by water or by

wind, though the initial stages of revegetation derive their character

more from the aggregation of the soil than from the nature of the

erosive agent. Eroded soils are as a rule xerophytic. In the case of

erosion by water, dysgeogenous soils are readily worn away in

consequence of their lack of cohesion, as in sand draws, etc., while

eugeogenous soils are easily eroded only on slopes, as in the case of

ravines, hillsides, etc. In the former, the extreme porosity and slight

capillarity of the sand and gravel result in a low water-content. In the

finer soils, the water-content is also low, on account of the excessive

run-off, due to compactness of the particles and to the slope. The

erosive action of winds upon soils bearing vegetation is not very

general; it is found to some extent in more or less established dunes,

and exists in a marked degree in buttes, mushroom rocks, and blowouts.

The first two are regularly xerophytic, the last as a rule, dissophytic.

The early stages of successions in eroded soils are composed of

xerophytes. In loose soils, these are forms capable of binding the soil

particles together, thus preventing wash, and increasing the

accumulation of fine particles, especially of organic matter. In compact

soils, the effect is much the same; the pioneers not only decrease

erosion, but at the same time also increase the water-content by

retarding the movement of the run-off.

=301. Succession in flooded soils.= The universal response of vegetation

to floods is found in the amphibious plant, which is a plastic form

capable of adjustment to very different water-contents. Floods are

confined largely to river basins and coasts. In hilly and mountainous

regions, where the slope is great, any considerable accumulation of

flood waters is now impossible, although of frequent occurrence when

land forms were more plastic.

In all streams that have become graded, the fall is insufficient to

carry off the surplus water in the spring when snows are melting

rapidly, or at times of unusual precipitation. These waters accumulate,

and, overflowing the banks, spread out over the lowlands, resulting in

the formation of a well-defined flood plain. This is a periodical

occurrence with mature streams, and it occurs more or less regularly

with all that are not torrent-like in character. The effect of the

overflow is to destroy or to place at a disadvantage those plants of the

flood plain that are not hydrophytes. At the same time, a thin layer of

fresh silt is deposited upon the valley floor of sand or alluvium.

Flooding is most frequent and of longest duration near the banks of the

stream. It extends more or less uniformly over the flood plain, and

disappears gradually or abruptly as the latter rises into the bench

above. Floods destroy vegetation and make a place for secondary

successions by drowning out mesophytic species, by washing away the

aquatic forms of ponds and pools, and by the erosion of banks and

sandbars. They affect the amphibious vegetation of swamp and shore to a

certain extent, but, unless the period of flooding is long, they tend to

emphasize such formations rather than to destroy them. The still-water

formations of many cutoff and oxbow lakes owe their origin to a river

which cuts across a meander in time of flood. This result is more often

attained by the alternate silting and erosion of a meandering river by

which it cuts across a bend in its channel. The usual successions in

flooded lands are short as a rule; amphibious algae, liverworts, and

mosses soon give way to ruderal plants, and these in turn to the

original mesophytes of meadows, or dissophytes of sandbars. In the case

of ponds and pools, the process of washing-out or silting up merely

removes or destroys the vegetation, without effectively modifying the

habitat, and the secondary successions that follow are extremely short.

=302. Succession by subsidence.= Subsidence is a factor of the most

profound importance in changing vegetation. It operates over vast areas

through immense periods of time. For these reasons, the changes are so

slow as to be almost imperceptible, and the resulting successions can be

studied only in the geological record. Extensive subsidence is confined

to-day to coastal plains, as in Greenland, the south Atlantic coast, and

the region of the Mississippi delta, where its effects are merged with

the paludation of tidal rivers, and the wave and tide erosion of the sea

shore. Such successions are unique, inasmuch as the denuding force

operates very slowly instead of quickly, and the first pioneers of the

new vegetation appear before the original formation has been destroyed.

In all cases, the succession is from mesophytic or halophytic formations

to paludose, and, finally, marine vegetation. In small areas of

subsidence, such as shore slips along lakes and streams, sink holes, and

sunken bogs, the succession is usually both short and simple, mesophytes

giving place to amphibious and ultimately to aquatic forms.

=303. Successions in landslips.= Landslips occur only in montane and

hilly regions, and here they are merely of local importance. In many

respects, they are not unlike talus; they show essential differences,

however, in that they are not sorted by gravity, and in that they

destroy vegetation almost instantly. The succession arises as a rule,

not upon the original soil, but upon that of the landslip, and, as

pointed out elsewhere, might well be regarded as primary.

=304. Succession in drained, or dried soils.= In geological times, the

subsidence of barriers must often have produced drainage and drying-out,

just as elevation frequently resulted in flooding and lake formation. At

the present time, the drying-out of lakes and ponds is the result of

artificial drainage, or of climatic changes. The former will be

considered under successions brought about by the agency of man.

Climatic changes when general operate so slowly that the stages of such

successions are perceptible only when recorded in strata. More locally,

climate swings back and forth through a period of years, with the result

that in dry years the swamps and ponds of wetter seasons are dried out,

and the vegetation destroyed or changed. If the process be gradual, the

succession passes from hydrophytic through amphibious to mesophytic,

and, in dry regions, xerophytic conditions. When the process of

drying-out occurs rapidly, as in a single summer, the original formation

is destroyed, and the new vegetation consists largely of ruderal plants.

A peculiar effect of climate occurs in regions with poor drainage, where

the result of intense evaporation is to produce alkaline basins and salt

lakes, in which the succession becomes more and more open, and is

finally represented by a few stabilized halophytes, or disappears

completely.

[Illustration: Fig. 63. A typical gravel slide (talus) of the Rocky

mountains, before invasion.]

=305. Succession by animal agency.= Successions of this class are

altogether of secondary importance, the instances in which animals

produce denudation being relatively few. Such are the heaps of dirt

thrown up by prairie dogs and other burrowing animals, upon which

ruderal plants are first established, to be finally crowded out by the

species of the original formation. Buffalo wallows furnish examples of

similar successions in which the initial stages are subruderal, while

overstocking and overgrazing frequently produce the same result with

ruderal plants.

=306. Succession by human agency.= The activities of man in changing the

surface of the earth are so diverse that it is impossible to fit the

resulting successions in a natural system. While man does not exactly

make new soils, he exposes soils in various operations: mining,

irrigation, railroad building, etc. He destroys vegetation by fires,

lumbering, cultivation, and drainage, and if he can not control climate,

he at least modifies its natural effects by irrigation and the

conservation of moisture. The operations of man extend from seacoasts

and swampy lowlands through mesophytic forests and prairies to the

driest uplands and inlands. Since the adjacent formations determine in

large degree the course and constitution of a succession, it will be

seen that the effects of any particular activity upon vegetation will

differ greatly in different regions. For convenience, all classes of

successions arising from the presence and activity of man will be

considered in this place, though, as indicated above, some might well be

regarded as producing primary successions, while others produce

anomalous ones.

=307. Succession in burned areas.= It will suffice merely to point out

that “burns” may arise naturally through lightning, volcanic cinders,

lava flows, etc., but the chances are so slight that these causes may be

ignored. The causes of fires are legion, and as they have little or no

effect upon results, they need not be considered. From their nature,

fires are of little significance in open vegetation, deserts, polar

barrens, alpine fields, etc., since the area of the burn can never be

large. In closed formations, the extent of fires is limited only by the

area of the vegetation, and the effect of wind, rain, and other forces.

Forest fires usually occur during the resting period, except in the case

of coniferous forests. In grassland, the living parts are underground

during autumn and winter, when prairie fires commonly occur. As a

consequence, the repeated annual burning of meadow or prairie does not

result in denudation and subsequent succession. On the contrary, it acts

in part as a stabilizing agent, inasmuch as it injures the typical

vegetation forms of grassland much less than it does the woody invaders.

All formations with perennial parts above ground, viz., thicket, open

woodland, and forest, are seriously injured by fire. A severe general

fire destroys the vegetation completely; a local fire destroys the

formation in restricted areas; while a slight or superficial burn

removes the undergrowth and hastens the disappearance of the weaker

trees. In the latter case, while the primary layer of the forest remains

the same, succession takes place in the herbaceous and shrubby layers.

These successions are peculiar in that they are composed almost wholly

of the proper species of the forest, and that they are very short,

showing only a few poorly defined stages. A local fire initiates a

succession in which the pioneers are derived largely from the original

formation, particularly when the latter encloses the burned area more or

less completely. The constitution of the intermediate and ultimate

stages will depend in a larger degree still upon the size and position

of the burn. When a particular formation is destroyed wholly or in large

part, the first stages of the new vegetation are made up by invaders

from the adjacent formations. In the most perfect types of succession,

this dissimilarity between the new and the old vegetation continues to

the last stage, in which the reappearance of the facies precedes that of

the subordinate layers. In many forest successions, however, the general

physical similarity of the ultimate stages permits the early

reappearance of the herbaceous and shrubby species, and the final stages

affect the facies alone. Successions in burned areas operate usually

within the water-content groups. The reconstruction of a mesophytic

forest takes place by means of mesophytes; of the rarer xerophytic and

hydrophytic forests, through xerophytes and hydrophytes respectively.

This is due to the fact that the alteration of the soil is slight,

except where the burning of the vegetation permits the entrance of

erosion, as on mountain slopes.

[Illustration: Fig. 64. Gravel slide formation

(_Pseudocymopterus-Mentzelia-chalicium_), stage III of the talus

succession.]

=308. Succession in lumbered areas.= Commercial lumbering, especially

where practiced for wood-pulp as well as for timber, results in complete

or nearly complete destruction of the vegetation by removal and the

change from diffuse light to sunlight, or by the action of erosion upon

the exposed surface. In the first place, short mesophytic successions

will result; in the second, the successions will be long and complex,

passing through decreasingly xerophytic conditions to a stable

mesophytic forest. Where a forest is cut over for certain species alone,

the undisturbed trees soon take full possession, though the causes

effective in the beginning will ultimately restore the original facies

in many instances. Such successions are anomalous, and will be treated

under that head.

=309. Succession by cultivation.= The clearing of forests and the

“breaking” of grassland for cultivation destroy the original vegetation;

the temporary or permanent abandonment of cultivated fields then permits

the entrance of ruderal species, which are the pioneers of new

successions. This phenomenon takes place annually in fields after

harvest, resulting in the secondary formations of Warming, in which

practically the same species reappear year after year. In fields that

lie fallow for several years, or are permanently abandoned, the first

ruderal plants are displaced by newcomers, or certain of them become

dominant at the expense of others. In a few years, these are crowded out

by invaders from the adjacent formations, and the field is ultimately

reclaimed by the original vegetation, unless this has entirely

disappeared from the region. The number of stages depends chiefly upon

whether the final formation is to be grassland or woodland. Other

activities of man, such as the construction of buildings, roads,

railways, canals, etc., remove the native vegetation, and make room for

the rapid development of ruderal formations. In and about cities, where

the original formations have entirely disappeared, the chance for

succession is remote, and the initial ruderal stages become more or less

stabilized. Elsewhere the usual successions are established, and the

ruderal formation finally gives way to the dominant type. In mountain

and desert regions, where ruderal plants are rare or lacking, their

place is taken by subruderal forms, species of the native vegetation

capable of rapid movement in them. These, like ruderal plants, are

gradually replaced by other native species of less mobility, but of

greater persistence, resulting in a short succession operating often

within a single formation. From the nature of cultivated plants,

succession after cultivation generally operates within the mesophytic

series.

=310. Succession by drainage.= Successions of this kind show much the

same stages as are found in those due to flooding. They proceed from

aquatic or swamp formations to mesophytic termini, either grassland or

woodland. When drainage takes place rapidly and completely, the pioneer

stages are usually xerophytic; cases of this sort, however, are

infrequent.

=311. Succession by irrigation.= Irrigation produces short successions

of peculiar stamp along the courses of irrigating canals and ditches,

and in the vicinity of reservoirs. These are recent, as a rule, and are

usually found in the midst of cultivated lands, so that their complete

history is still a matter of conjecture. The original xerophytes are

forced out not only by the disturbance of the soil, but also by its

increased water-content. A few of them often thrive under the new

conditions, and, together with the usual ruderal plants and a large

number of lowland mesophytes and amphibious forms derived from the banks

of the parent stream, constitute a heterogeneous association. This is

doubtless to be regarded as an initial stage of a succession, but it is

an open question whether the succession will early be stabilized as a

new formation, or whether the original vegetation will sooner or later

be reestablished under somewhat mesophytic conditions. From the number

of mesophytes and from the behavior of valleys, it seems certain that

the banks of such canals will ultimately be occupied by a formation more

mesophytic than hydrophytic, into which some of the surrounding

xerophytes of plastic nature have been adopted.

=312. Anomalous successions= are those in which the physical change in

the habitat is relatively slight, resulting in a displacement of the

ultimate stage, or the disturbance of the usual sequence, merely,

instead of the destruction and reconstruction of a formation, or the

gradual development of a new series of stages on new soil. In nature,

the ultimate grass or forest stage of a normal succession is often

replaced by a similar formation, especially if the facies be few or

single. It is evident that certain trees naturally replace others in the

last stages of a forest succession, without making the latter anomalous.

The last occurs only when a normal stage is replaced by one belonging

properly to an entirely different succession, as when a coniferous

forest replaces a deciduous one in a hardwood region. The presence and

development of such successions can be determined only after the normal

types are known. The interpolation of a foreign stage in a natural

succession, or a change of direction, by which a succession that is

mesotropic again becomes hydrophytic, is easily explained when it is the

result of artificial agents, as is often the case. In nature, anomalous

successions are commonly the result of a slow backward and forward swing

of climatic conditions.

=313. Perfect and imperfect successions.= A normal succession will

regularly be perfect; it passes in the usual sequence from initial to

ultimate conditions without interruption or omission. Imperfect

succession results when one or more of the ordinary stages is omitted

anywhere in the course, and a later stage appears before its turn. It

will occur at any time when a new or denuded habitat becomes so

surrounded by other vegetation that the formations which usually furnish

the next invaders are unable to do so, or when the abundance and

mobility of certain species enable them to take possession before their

proper turn, and to the exclusion of the regular stage. Incomplete

successions are of great significance, inasmuch as they indicate that

the stages of a succession are often due more to biological than to

physical causes, the proximity and mobility of the adjacent species

being more determinative than the physical factors. Subalpine gravel

slides regularly pass through the rosette, mat, turf, thicket, woodland,

and forest stages; occasionally, however, they pass immediately from the

rosette, or mat condition, to an aspen thicket which represents the next

to the last stage. Such successions are by no means infrequent in hilly

and montane regions; in regions physiographically more mature or stable,

perfect successions are almost invariably the rule.

[Illustration: Fig. 65. Half gravel slide formation

(_Elymus-Muhlenbergia-chalicium_), stage IV of the talus succession.]