I. ECOLOGICAL FACTORS
MATERIAL.—A number of small flowerpots filled with soils of as many
different kinds as can be found in the neighborhood.
=308. Definition.=—By _ecology_ is meant the relation of plants to
their surroundings, which may be considered under three general
heads: their relations to inanimate nature, to other plants, and
to animals. The subject has been touched upon repeatedly in the
foregoing pages, and, in fact, it is impossible to treat of any
branch of botany without some reference to it. All that was said
about the adjustment of leaves for light and moisture, and their
adaptations for protection and food storage, about the devices for
pollination, and for fruit and seed dispersal, really belong to
ecology.
=309. Symbiosis.=—The relations of plants to animate nature are
_biological factors_, and may act in two ways: (1) through the
destruction of vegetation by hungry animals and by parasitic and
disease-producing organisms; and (2) by associations for mutual
benefit, such as are described in section viii of chapter VII.
Associations of this kind are included under the general term
_symbiosis_, a word which means “living together.” In its broadest
sense symbiosis refers to any sort of dependence or intimate organic
relation between different kinds of individuals, and so may include
the climbing and parasitic habits; but it is usually restricted to
cases where the relation is one of mutual benefit. It may exist
either between plants of one kind with those of another, between
animals with animals, or between plants and animals, as in the case
of the clover and bumblebee, and the yucca and pronuba.
[Illustration: PLATE 13.—Showing the quick response of vegetation to
surroundings. The upper cut shows the appearance of an irrigation
canal in the arid plains region, when first completed; the lower cut,
ten years after completion.]
The occurrence of root tubercles on certain of the leguminosæ (63) is
a clear case of symbiosis, the microscopic organisms in the tubercles
getting their food from the plant and at the same time enabling it to
get food for itself from the air in a way that it could not otherwise
do.
=310. Relations with inanimate nature.=—But it is to the relations of
plants with inanimate nature, and their grouping into societies under
the influence of such conditions, that the term “ecology” is more
strictly applied. The external conditions that lead to the grouping
are called _ecological factors_. The most important of these are
temperature, moisture, soil, light, and air, including the direction
and character of the prevailing winds. Each of these factors is
complicated with the others and with conditions of its own in a way
that often makes it difficult to determine just what effect any one
of them may have in the formation of a given plant society.
=311. Temperature= may be even and steady, like that of most oceanic
regions, or it may be subject to sudden caprices and variations,
like the “heated terms” and “cold snaps” that afflict our Eastern
coast region every few years. It is not the average temperature of
a climate, but its extremes, especially of cold, that limit the
character of vegetation.
Temperature probably has more influence than any other factor upon
the distribution of plants over the globe; but it can have little
or no effect in evolving local differences in vegetation, because
the temperature of any given locality, except on the sides of high
mountains, will ordinarily be the same within a circuit of many miles.
=312. Moisture=, again, may be of all degrees, from the
superabundance of lakes and rivers and standing swamps, to the arid
dryness of the desert, and the water may be still and sluggish,
or in rapid motion. It may exist more or less permanently in the
atmosphere, as in moist climates like those of England and Ireland,
where vegetation is characterized by great verdure; or it may come
irregularly in the form of sudden floods, or at fixed intervals,
causing an alternation of wet and dry seasons. Moreover, the moisture
of the soil or the atmosphere may be impregnated with minerals or
gases, which may affect the vegetation independently of the actual
amount of water absorbed.
[Illustration: FIG. 410.—The effect of cold—a Mt. Katahdin bog.
(_From_ Mo. Botanical Garden Rep’t.)]
Snow is a form of water which may act in two entirely opposite ways:
(1) by keeping the atmospheric precipitation locked up in a solid
state and thus bringing about a condition analogous to drought—for
example, in arctic deserts and Alpine snow fields; (2) by causing
annual floods and overflows when it melts in the spring, as in the
Nile and Mississippi valleys.
In cold temperate regions it also influences vegetation to a
considerable extent by covering the warm earth like a blanket during
winter, and thus protecting tender seeds and shoots that otherwise
would not be able to survive.
[Illustration: FIG. 411.—Dogwood, a tree tolerant of shade, growing
and blooming in a deeply wooded glen.]
=313. Light= may be of all degrees of intensity, from the blazing
sun of the treeless plain to the darkness of caves and cellars where
no green thing can exist. Between these extremes are numberless
intermediate stages: the dark ravines on the northern side of
mountains, the dense shade of beech and hemlock forests, and the
light, lacy shadows of the pines,—each characterized by its peculiar
form of vegetation. Absence of light, too, is usually accompanied by
a lowering of temperature and a reduction of transpiration, factors
which tend to accentuate the difference between sun plants and shade
plants, giving to the latter some of the characteristics of aquatic
vegetation. Generally, the tissues of these are thin and delicate,
and having no need to guard against excessive transpiration, they
wither rapidly when cut or exposed to too great intensity of heat and
light.
[Illustration: FIG. 412.—A red cedar grown in a barren, wind-beaten
situation.]
[Illustration: FIG. 413.—A red cedar grown under normal conditions.]
=314. Winds= affect vegetation, not only as to the manner of
seed distribution and the conveyance of pollen, but directly by
increasing transpiration, and necessitating the development of strong
holdfasts in plants growing upon mountain sides and in other exposed
situations. The nature of the region from which they blow—whether
moist, dry, hot, cold, etc.—is also an important factor. In a
district open to sea breezes, live oaks, which require a salt
atmosphere, may sometimes be found as far as a hundred miles from the
coast.
=315. Soil.=—While water is the most important, soil is perhaps the
most interesting of these factors to the farmer, because it is the
one that he has it most largely in his power to modify. It is to be
viewed under two aspects: first, as to its mechanical properties,
whether soft, hard, compact, porous, light, heavy, etc.; secondly, as
to its chemical composition and the amount of plant food-materials
contained in it. The first can be regulated by tillage and drainage,
the second by a proper use of fertilizers.
EXPERIMENT 92. TO SHOW THE INFLUENCE OF SOIL AS AN ECOLOGICAL
FACTOR.—Fill a number of small earthen pots with all the different
kinds of soil that are to be found in your neighborhood. Keep well
moistened and make a list of the plants that appear spontaneously
in each. Is there any difference in the kinds produced by different
soils? In vigor or abundance of the same or different kinds? Do
more seedlings appear in any of the pots than could live if left
alone? What becomes of a majority of the seedlings that come up in
a state of nature?
After a time, stop watering until all the plants are dead and new
ones cease to appear. Notice the rate at which vegetation dies out
in each and the kind of plants that can live longest without water.
Which of the different soils is capable of sustaining vegetation
longest without a fresh supply of moisture? To what quality of the
soil is this due? (Exp. 53.)
Practical Questions
1. Is the relation between man and the plants cultivated by him a
symbiosis? (309.)
2. Why is it that plants of the same, or closely related species
are found in such different localities as the shores of Lake
Superior, the top of Mt. Washington, and the Black Mountains in
North Carolina? (311, 330.)
3. Which of the five ecological factors mentioned in paragraphs
311-315 has probably most largely influenced their distribution?
4. What is the prevailing character of the soil in your
neighborhood?
5. Is your climate moist or dry? Warm or cold?
6. Can you trace any connection between these factors and the
prevailing types of vegetation?
II. PLANT ASSOCIATIONS
MATERIAL.—The subject is not well suited to laboratory work,
though, if time permits, it is recommended that a detailed study
be made of at least one typical hydrophyte, halophyte, and
xerophyte plant. Some good examples are: (1) Hydrophyte: pond
weed, waterlily, pipewort (_Eriocaulon_), bladderwort, arrowhead
(_Sagittaria_); (2) Halophyte: sea lavender, sea rocket, sea
lettuce, water hyacinth; (3) Xerophyte: cactus, century plant,
pineapple, stonecrop, purslane, lichen.
=316. Modes of grouping.=—Plants group themselves in their favorite
habitats, not according to their botanical relationships, but with
regard to the predominance of one or more of the ecological factors
that influence their growth. Sometimes one or two species will
take practical possession of large areas, like the coarse grasses
that spread over certain salt marshes, or the pines that formerly
constituted the sole forest growth over extensive regions in North
Carolina and Maine. Exclusive growths of this kind over limited
areas are sometimes called plant _colonies_, and the individuals
composing them belong, as a general thing, to the hardy, pushing sort
known as “pioneers,” which are among the first to take possession
of new soil and force their way into unoccupied territory. But more
usually we find a great diversity of forms brought together by their
common requirements as to shade, soil, moisture, and other external
conditions.
Any well-defined assemblage of plants, whether of one kind or many,
originating in such a common response to the same influences, is
called a _formation_. These associations are variously classed,
according to the nature of their habitat, as salt water, fresh water,
sand hill, swamp, bog, river bottom, or such other kinds as their
ecological character may indicate. Local conditions in limited
areas may lead to the segregation of smaller and more compact groups
called _societies_. This term, however, is used rather loosely, being
treated in some works as synonymous with formations, in others as
analogous with what have here been defined as colonies.
[Illustration: FIG. 414.—A colony of Alabama primroses (_Œnothera
speciosa_).]
=317. Principles of subdivision.=—The mixed associations described in
the last paragraph are quite independent of botanical relationships,
and any of the factors named in 310, or others of a different kind,
could be made the basis of their classification. They might be
grouped, for instance, according to their economic uses, or according
to origin, whether native or introduced, as best suited the purpose
of the classification in each case. The moisture factor, however, has
been generally agreed upon as the one most convenient for ordinary
purposes. Upon this principle plants are divided into three great
groups:—
=Hydrophytes=, or water plants, those that require abundant moisture.
=Xerophytes=, or drought plants, those that have adapted themselves
to desert or arid conditions.
[Illustration: FIG. 415.—A water plant (_Limnophila_), with water
leaves and air leaves and transitional forms.]
=Mesophytes=, plants that live in conditions intermediate between
excessive drought and excessive moisture. To this class belong
most of our ordinary cultivated plants and the greater part of the
vegetation of the globe.
=Halophytes=, “salt plants,” is a term used to designate a fourth
class, based not directly upon the water factor, but upon the
presence of a particular mineral in the water or the soil which
they can tolerate. They seem to bear a sort of double relation to
hydrophytes on the one hand and to zerophytes on the other.
=318. Hydrophyte societies.=—These embrace a number of forms, from
those inhabiting swamps and wet moors, to the submerged vegetation of
lakes and rivers. An examination of almost any kind of water plant
will show some of the physiological effects of unlimited moisture.
Take a piece of pondweed, or other immersed plant, out of the water
and notice how completely it collapses. This is because, being buoyed
up by the water, it has no need to spend its energies in developing
woody tissue. Floating and swimming plants will generally be found
to have no root system or very small ones, because they absorb their
nourishment through all parts of the epidermis directly from the
medium in which they live. That they may absorb readily, the tissues
are apt to be soft and succulent and the walls of the cells composing
them very thin. In some of the pipeworts (_Eriocaulon_), the ells
are so large as to be easily seen with the unaided eye. If you can
obtain one of these, examine it with a lens and notice how very thin
the walls are. Water plants also contain numerous air cavities, and
often develop bladders and floats, as in the common bladderwort and
many seaweeds. The leaves of submerged plants are usually either
greatly reduced in size or very much cut and divided, while the ones
that rise above water, like those of the water lily, are apt to be
large and entire, to facilitate floating, and have stomata on their
upper surface. Floating plants sometimes form such large colonies as
to be a serious menace to navigation. Well-known instances of this
are the water hyacinths in the St. John’s River, Florida, and the
vast formations of swimming gulfweed from which the Sargasso Sea
takes its name.
[Illustration: FIG. 416.—Seaweed (_sargassum_) with bladderlike
floats.]
[Illustration: FIG. 417.—A pioneer swamp colony of cattails. (_From_
a photograph by Harry B. Shaw, U.S. Dept. Agr.)]
=319. Swamp societies.=—These include what may be regarded as the
amphibious portion of the hydrophyte group. They compose the sedge
and cattail bogs, reed jungles, moss and fern thickets, forests of
cypress, magnolia, black gum, pine, tamarack, balsam, and the like.
The sedges and cattails are the pioneers of these societies, which
tend constantly to encroach upon the water and so prepare the way
for the advance of other colonists. Drawing their nourishment from
the loose soil in which they are anchored, and lacking the support
of a liquid medium, they develop roots and vascular stems. The roots
of plants growing in swamps have difficulty in obtaining proper
aëration on account of the water, which shuts off the air from them;
hence they are furnished with large air cavities, and the bases
of the stems are often greatly enlarged, as in the Ogeechee lime
(_Nyssa capitata_) and cypress, to give room for the formation of air
passages. The peculiar hollow projections known as “cypress knees”
are arrangements for aërating the roots of these trees.
[Illustration: FIG. 418.—A Southern cypress swamp, showing on the
left the peculiar enlargements for aëration, known as “cypress
knees.” (_From_ Mo. Botanical Garden Rep’t.)]
=320. Xerophyte societies= are adapted to conditions the reverse of
those affected by hydrophytes. The extreme of these conditions is
presented by regions of perennial drought, like our Western arid
plains and the great deserts of the interior of Asia and Africa.
Under these conditions plants have two problems to solve,—to collect
all the moisture they can and to keep it as long as they can. Hence,
plants of such regions have a diminished evaporating surface, owing
to the absence of foliage and the compacting of their tissues into
the stem, after the manner of the cactus and prickly euphorbia; or
their leaves may become thick and fleshy so as to resist evaporation
and retain large amounts of moisture, as in the case of the yucca and
century plant. They also frequently develop a thick, hard epidermis,
or cover themselves with protective hairs and scales.
The principal types of xerophyte plants are: (1) the lichens, mosses,
and saxifrages found on bald rocks and mountain cliffs; (2) sand
plants, such as cockspur grass, sand spurry, wiregrass, and the
like, inhabiting sea beaches and pine barrens; (3) the sage brush,
greasewood, and switch plants of our Western alkali plains; (4) the
cactus and yuccas of southern California, Arizona, and Mexico; (5)
the acacias, agaves, and hardy “chapparal” thickets of southern
Texas and Mexico. The first class are of importance as the pioneers
and pathfinders of the xerophyte community. In tropical and polar
deserts alike they are the first settlers, and by aiding in the
disintegration of rocks and their gradual conversion into soil, they
pave the way for the coming of the higher plants, and it may be of
man himself.
[Illustration: PLATE 14.—A xerophyte formation of yuccas, cacti,
and switch plants, near Zacatecas, Mexico. (_From_ a photograph by
Professor F. E. Lloyd.)]
=321. Partial xerophytes.=—Plants exposed to periodic and occasional
droughts frequently provide against hard times by laying up stores
of nourishment in bulbs and rootstocks and retiring underground
until the stress is over. This is known as the _geophilous_, or
earth-loving, habit. Others, as some of the lichens, and the little
resurrection fern (_Polypodium incanum_, Figs. 419, 420), so common
on the trunks of oaks and elms in the Southern States, make no
resistance, but wither away completely during dry weather, only to
waken again to vigorous life with the first shower.
[Illustration: FIGS. 419, 420.—A resurrection fern: 419, in dry
weather; 420, after a shower.]
=322. Physiological xerophytes.=—Plants growing in thin or poor
soil, such as that on denuded hillsides, fresh railroad cuts, and
newly graded streets, are apt to take on a more or less xerophytic
character, even though there may be no lack of moisture. Such soils
are called “new” because the mineral elements in them have not
been exposed long enough to have become decomposed and mixed with
humus, and the vegetation that first populates them has to do the
pioneer work of disintegrating and impregnating the substratum with
humus. For similar reasons the vegetation of sandy bogs and sea
beaches, owing to the poverty of the soil in nitrogenous matter,
usually develops xerophyte adaptations, even though there may be a
superabundance of moisture. Plants growing on high mountain tops and
in cold arctic bogs take on the same characteristics (Fig. 410).
Such situations are said to be “physiologically dry,” because the
moisture they have is not in a condition to be readily absorbed. The
vegetation of arctic regions suffers more from physiological drought
than from cold.
[Illustration: FIG. 421.—A halophyte swamp of mangroves. Notice the
tangle of adventitious prop roots assisting in the work of absorption
from the brackish marsh soil. (_From_ Mo. Botanical Garden Rep’t.)]
=323. Halophytes= include plants growing by the seashore and
the vegetation around salt springs and lakes and that of alkali
deserts. Seaweeds are in a sense halophytes, since they live in salt
water, but as they are true aquatic plants and exhibit many of the
peculiarities of hydrophytes in their mechanical structure, they are
classed with them. The name _halophyte_ applies more particularly to
land plants that have adapted themselves to the presence in the soil
or in the atmospheric vapor, of certain minerals, popularly known as
salts, which cause them to take on many xerophyte characteristics.
The reason for this, as was shown in Exp. 39, is because the mixture
of salt in the water of the soil increases its density so that it
is difficult for the plant to absorb as much as it needs, and thus
halophytes are living under “physiologically” xerophyte conditions.
If you have ever spent any time at the seashore, you cannot fail
to have observed the thick and fleshy habit exhibited by many
of the plants growing there, such as the samphire, sea purslane
(_Sesuvium_), and sea rocket (_Cakile_). A form of goldenrod found by
the seashore has thick, fleshy leaves, and is as hard to dry as some
of the fleshy xerophytes.
Another characteristic of desert plants that is common also to
seaside vegetation is the frequent occurrence of a thick, hard
epidermis, as in the sea lavender and saw grass. The live oaks, trees
that love the salt air and never flourish well beyond reach of the
sea breezes, have small, thick, hard leaves, very like those of the
stunted oaks that grow on the dry hills of California. The presence
of spines and hairs, it will be observed, is also very common;
_e.g._ the salsola, the sea oxeye, and the low primrose (_Œnothera
humifusa_). In other cases the leaf blades are so strongly involute
or revolute (202) as to make them appear cylindrical. All these, it
will be observed, are xerophyte adaptations, and the object in both
cases is the same—the conservation of moisture.
=324. Mesophytes.=—These embrace the great body of plants growing
under the ordinary conditions of temperate regions, which may vary
from the liberal water supply of low meadows and shady forests to
the almost desert barrenness of dusty lanes and gullied, treeless
hillsides. The forms and conditions they present are so varied that
it would be impracticable to consider them all in a work like this,
but they may be summed up under the two general heads of (1) _open
ground_ and (2) _woodland_. Under the first are included: (_a_) all
cultivated grounds—fields, meadows, lawns, pastures, and roadsides,
with their characteristic shrubs, flowers, and grasses; (_b_) heaths
and plains of northern or alpine regions, with their low, stunted
perennials and bright, but fugacious, flowers. Under the second are
classed all woods, thickets, and copses, with the shrubs and herbs
that form their undergrowth. These may be grouped in three main
divisions: (_c_) mixed forests of maple, ash, oak, hickory, birch,
sweet gum, etc.; (_d_) pure forests of pine, balsam, fir, cypress,
and the like; and finally (_e_), the perennial splendors of the
tropical forest, where the vegetation of the globe reaches its climax
in luxuriance and variety of growth.
Practical Questions
1. Why do florists cultivate cactus plants in poor soil? (320.)
2. What would be the effect on such a plant of copious watering and
fertilizing?
3. Why must an asparagus bed be sprinkled occasionally with salt?
(323.)
4. If a gardener wished to develop or increase a fleshy habit in a
plant, to what conditions of soil and moisture would he subject it?
(320, 323.)
5. What difference do you notice between blackberries and
dewberries grown by the water and on a dry hillside?
6. Are there corresponding differences in the root, stem, and
leaves of plants growing in the two situations, and if so account
for them?
7. When a tract of dry land is permanently overflowed by the
building of a dam or levee, why does all the original vegetation
die, or take on a sickly appearance? (319.)
8. Should plants with densely hairy leaves be given much water, as
a general thing? (202, 320.)
9. A farmer planted a grove of pecan trees on a high, dry hilltop;
had he paid much attention to ecology? Give a reason for your
answer.
10. Why do the branches of trees often die, or fail to develop, on
the windward side? (314.)
11. Why do trees grown in dry soil have harder wood than the same
kind grown in wet soil? (123, 318.)
III. ZONES OF VEGETATION
[Illustration: FIG. 422.—A pioneer colony of sumac growing on a
railroad cutting. (_From_ a photograph by J. M. Coulter.)]
=325. The origin of vegetable zones.=—The terms “zone” and “zonation”
are used to express a general tendency of plant societies and
formations to distribute themselves in more or less regular belts
or strata, relatively to the varying intensity of the prevalent
ecological factor of their habitat. In almost every locality there
exists some special feature—a pond, a brook, a small ravine, an
isolated hilltop, a deserted quarry, a gravel pit, or a railroad
cut,—sufficiently distinct from the general surroundings to exercise
a perceptible control over the vegetation in its immediate vicinity,
and thus to become the starting point of a series of plant zones
that mark the decreasing influence of the factor concerned, by
their change of character as they recede from its point of greatest
intensity. Starting from a barren, exposed hilltop, for example,
with a covering of dry broom sedge (_Andropogon_) and fleabane, we
encounter next an almost desert zone of washed and gullied slopes
in whose hard, sunbaked soil nothing but a few scrub pines and
brambles can gain a foothold. This will, perhaps, be succeeded, by a
straggling belt of sassafras, sumac, and buckthorn, mixed with cat
brier and blackberry canes, beyond which, at the foot of the hill,
begins a stretch of meadow, or a bit of woodland crossed by a brook,
or hollowed into a boggy depression. From this new factor originates
a second series of zonations, passing through all the stages of bog,
swamp, shade, and sun plants, back to the prevailing type of the
region. Moisture is really the controlling factor in both cases, its
influence in the first being negative,—that is, inversely,—and in the
other, positive, or directly proportioned to the quantity present.
=326. Direction of zonation.=—When the direction in which the
controlling factor changes is horizontal, as with soil and water,
the zonation will be _horizontal_; when, as in the case of light,
it is vertical, the zonation or stratification will be _vertical_.
Examples of this can be observed in the growth of almost any forest
area, the natural order of succession being: (1) a ground layer of
mosses and fungi; (2) low, creeping vines,—partridge berry, trailing
arbutus, twinflower (_Linnæa_); (3) small ferns and low flowering
herbs—pyrola, clintonia, trillium; (4) a zone of tall herbs and low
bushes—royal fern, cohosh (_Actæa_), blueberries; (5) tall herbs and
shrubs, small trees, and climbing vines—kalmia, dogwood, farkleberry,
smilax, Virginia creeper; (6) tall treetops towering up into full
sunlight.
When the physical cause of intensity is a central area, such as a
pond or a hilltop, the zonation will be _concentric_; that is, the
different belts will succeed each other in widening circles more
or less complete. Where the controlling cause extends in a line,
as a river, or a chain of mountains, the zones run in parallel
belts on each side of it, and the zonation is _bilateral_. In any
case, however, it is seldom regular, being frequently broken and
interrupted through the intervention of other factors. Nor must
precisely the same kind of plants be always looked for in similar
situations, though their place is usually occupied by kindred species
and genera. The common pitch pine, for instance, of the Northern sand
barrens is represented in sandy districts farther south by the tall,
long-leaved pine, a kindred species.
=327. Succession.=—Zonation is a regular succession of different
kinds of plants in space; there is also an analogous succession in
time, as, when the vegetation of a locality is killed off by fire or
other cause, plants of an entirely different character will nearly
always spring up to occupy its place. A forest of pine, for instance,
is rarely followed by conifers, but by a growth of hardwood trees,
and _vice versa_—nature thus giving an impressive example as to the
effectiveness of a rotation of crops.
[Illustration: FIG. 423.—A thicket of pines that has succeeded a
mixed growth of hard wood trees.]
Succession may be influenced by a variety of causes. Two of the most
efficient are: (1) the exhaustion of the soil by the long-continued
growth of one formation (60), thus causing a deficiency of mineral
material suited for the support of plants of that kind; (2) the
migration of new species into the denuded territory where those which
have different requirements as to mineral nutrients from the former
inhabitants will, other things being equal, have the best chance to
succeed.
[Illustration: FIG. 424.—A successful invasion—Japanese honeysuckle
covering the banks of a ravine and climbing over shrubs and tree
tops.]
=328. Invasion.=—A rapid and widespread occupation of any territory
by a new species is called an _invasion_. Notable examples of
invaders are those of the Russian thistle in the northwestern states
of the Union, and the “bitterweed” (_Helenium tenuifolium_) that
has almost driven out the hardy dog fennel (_Anthemis cotula_)
which formerly held undisputed possession of arid places throughout
the South Atlantic states. A still more remarkable instance is
the invasion of the Japanese honeysuckle (_Lonicera Japonica_),
originally introduced for ornament, but which has naturalized itself
within the last thirty years and overrun waste places everywhere,
from the Gulf to the Potomac, with a vigor and luxuriance equaled by
few of our native species. As its beauty and fragrance are even more
conspicuous in a state of nature than under cultivation, and as it
can, moreover, be made very useful in stopping gullies and washes,
its phenomenally rapid occupation of so large a territory has caused
no alarm and consequently attracted little attention.
=329. Climatic zones.=—These are more general groupings than those
we have been considering. They follow in a rough way the parallels
of latitude, and are classed accordingly as: (1) tropical; (2)
subtropical; (3) temperate; (4) boreal or (on mountains) subalpine;
(5) arctic or (on high mountains) alpine. Taking the cultivated
plants of our own country by way of illustration, we have the
subtropical zone, embracing Florida and the southern portion of the
Gulf states, where sugar cane, rice, and tropical fruits are the
staple crops. Then comes the temperate zone, with three agricultural
subdivisions: (_a_) the great cotton belt, with Indian corn, sweet
potatoes, and the peach, melon, and fig as secondary products.
Farther north, in the Central and Middle Atlantic states, we find
(_b_) the region of maize, hemp, and tobacco, with grapes, apples,
pears, cherries, and a great variety of garden vegetables as side
crops. Finally comes (_c_) the great wheat-growing region of the
North, with buckwheat, hay, and Irish potatoes as subsidiary crops.
Technically, the distribution of the natural zones of vegetation from
south to north is classed under the three general heads of Forest,
Grass Land, and Arctic Desert, with numerous subdivisions in each.
=330. Boundaries of the zones.=—While the broad continental zones
of vegetation follow, in a general way, the climatic zones outlined
above, they are not sharply defined, but run into each other and
overlap in various degrees, so that a map depicting the range of
vegetation in any wide area would show a marked deviation from those
of latitude. Various other geographical factors, such as mountain
ranges and bodies of water, influence the direction and character of
the prevailing winds and rains, and through them the moisture and
temperature, to so great an extent that they become the controlling
factors over wide areas. In countries bordering on the sea, the coast
line always marks a belt of its own, and on the sides of a mountain
range, all the climatic zones from the equator to the pole may be
repeated during an ascent of a few miles.
In our own country, where the mountain chains and coast lines run
approximately north and south, the great continental zones have been
superseded, for all practical purposes, by four regional divisions
running almost at right angles to them. These are, disregarding minor
subdivisions:—
(1) The Forest region, occupying the eastern and south central
portion of the Union. In classifying this territory as forest, it is
not meant to imply that it is now, or ever was, one unbroken jungle,
like parts of central Africa, but that it combines the conditions
most favorable to a vigorous and varied forest growth.
(2) The Plains region, extending from the very irregular western
boundary of the forest region to the Rocky Mountains.
(3) The Rocky Mountain region, including the Rockies and the Sierra
Nevadas with the desert area between them.
(4) The Pacific Slope, a narrow strip between the Sierras and the
Pacific Ocean.
[Illustration: PLATE 15.—This giant tulip tree is a relic of the
primitive forest. It is twenty-seven feet in circumference, at a
distance of four feet from the ground. Notice the sharp elbows of
the large boughs, a mode of branching characteristic of this kind of
tree.]
The boundaries of these regions, like those of the great continental
zones, overlap in various ways, the plants of one region often
appearing in another, like an arm of the sea projecting into the
land. But the district where any class of plants reaches its highest
development is its proper habitat, and as a general thing the one
where its cultivation pays best. It would be a waste of time and
money to try to raise cotton in Maine, or cranberries in Georgia.
Practical Questions
1. Does the native wild growth of a region furnish any indication
of the kind of crops which could be successfully grown there? (325,
326.)
2. Can you give a reason why the zones of cultivation may, in some
cases, be more extensive than the natural range of wild plants in
the same region? (262, 265.)
3. Can you give reasons why the reverse may sometimes be true?
(261, 284.)
4. What crops are raised in different parts of your own state?
5. Name some of the native plants characteristic of different parts
of your state. What are its principal plant formations?
Field Work
1. Ecology offers the most attractive subject for field work of all
the departments of botany. It can be studied anywhere that a blade
of vegetation is to be found. In riding along the railroad, there
is an endless fascination in watching the different plant societies
succeed one another and noting the variations they undergo with
every change of soil or climate.
2. Students in cities can find interesting subjects for study in
the vegetation that springs up on vacant lots, around doorsteps
and area railings, and even between the paving stones of the more
retired streets. On a vacant lot near the public library in Boston,
over thirty different kinds of weeds and herbs were found, and in
the heart of Washington, D.C., on a vacant space of about twelve by
twenty feet, nineteen different species were counted. Just where
such things come from, how they get into such positions, and why
they stay there, will be interesting questions for city students to
solve.
3. But the country always has been and always will be the happy
hunting ground of the botanist. All the factors considered in the
two preceding sections can hardly be found in any one locality, but
by selecting areas traversed by brooks, or by gullies and ravines,
very marked changes in the character of vegetation may often be
observed. Barren, sandy, or rocky soils, the sunbaked clay of naked
hillsides, and the borders of treeless, dusty roads will offer
close approximations to xerophyte conditions.
4. If there are any bodies of water in your neighborhood, examine
their vegetation and see of what it consists. Notice the difference
in the shape and size of floating and immersed leaves and account
for it. Note the general absence of free-swimming plants in
running water, and account for it. Note the difference between
the swamp and border plants and those growing in the water, and
what trees or shrubs grow in or near it. Compare the vegetation of
different bogs and pools in your neighborhood, and account for any
differences you may observe. Compare the water plants with those
growing in the dryest and barrenest places in your vicinity, note
their differences of structure, and try to find out what special
adaptations have taken place in each case. Make a list of those in
each location examined that you would class as pioneers.
5. Draw a map of the vegetation of some locality in your
neighborhood that presents a variety of conditions, such as a
steep hillside, a field or meadow traversed by a brook, the slopes
and borders of a ravine, or the change from cultivated ground to
uncultivated moor or woodland. Represent the different zones and
formations by different colored inks or crayons, or by different
degrees of shading with the pencil.
6. Draw a map of your state showing the different agricultural
regions, as indicated by the character of the cultivated plants in
each; use different colors, or light and dark shading, to define
the boundaries. Notice any irregularities of outline and account
for them—whether due to soil, moisture, geological formation,
winds, or temperature. What is the controlling factor of each
region?