equatorial zone the noonday sun is truly vertical twice every
year, and for several months it passes so near the zenith that the
difference can hardly be detected without careful observation of
the very short shadows of vertical objects. The absence of distinct
horizontal shadows at noon which thus characterises a considerable
part of the year, is itself a striking phenomenon to an inhabitant
of the temperate zones; and equally striking is the changed aspect
of the starry heavens. The grand constellation Orion, passes
vertically overhead, while the Great Bear is only to be seen low
down in the northern heavens, and the Pole star either appears
close to the horizon or has altogether disappeared according as we
are north or south of the equator. Towards the south the Southern
Cross, the Magellanic clouds, and the jet-black “coal sacks” are the
most conspicuous objects invisible in our northern latitudes. The
same cause that brings the sun overhead in its daily march equally
affects the planets, which appear high up towards the zenith far
more frequently than with us, thus affording splendid opportunities
for telescopic observation.
_Intensity of Meteorological Phenomena at the Equator._--The
excessive violence of meteorological phenomena generally supposed to
be characteristic of the tropics is not by any means remarkable in
the equatorial zone. Electrical disturbances are much more frequent,
but not generally more violent than in the temperate regions. The
wind-storms are rarely of excessive violence, as might in fact be
inferred from the extreme steadiness of the barometer, whose daily
range at Batavia rarely exceeds one-eighth of an inch, while the
extreme range during three years was less than one-third of an inch!
The amount of the rainfall is very great, seventy or eighty inches
in a year being a probable average; and as the larger part of this
occurs during three or four months, individual rainfalls are often
exceedingly heavy. The greatest fall recorded at Batavia during
three years was three inches and eight-tenths in one hour,[4] but
this was quite exceptional, and even half this quantity is very
unusual. The greatest rainfall recorded in twenty-four hours is
seven inches and a quarter; but more than four inches in one day
occurs only on two or three occasions in a year. The blue colour
of the sky is probably not so intense as in many parts of the
temperate zone, while the brilliancy of the moon and stars is not
perceptibly greater than that of our clearest frosty nights, and
is undoubtedly much inferior to what is witnessed in many desert
regions, and even in Southern Europe.
[4] On January 10th, 1867, from 1 to 2 A.M.
On the whole, then, we must decide, that uniformity and abundance,
rather than any excessive manifestations, are the prevailing
characteristic of all the climatal phenomena of the equatorial zone.
_Concluding Remarks._--We cannot better conclude our account of the
equatorial climate than by quoting the following vivid description
of the physical phenomena which occur during the early part of the
dry season at Para. It is taken from Mr. Bates’ _Naturalist on
the Amazons_, and clearly exhibits some of the more characteristic
features of a typical equatorial day.
“At that early period of the day (the first two hours after sunrise)
the sky was invariably cloudless, the thermometer marking 72° or
73° Fahr.; the heavy dew or the previous night’s rain, which lay on
the moist foliage, becoming quickly dissipated by the glowing sun,
which, rising straight out of the east, mounted rapidly towards the
zenith. All nature was fresh, new leaf and flower-buds expanding
rapidly. * * * The heat increased hourly, and towards two o’clock
reached 92° to 93° Fahr., by which time every voice of bird and
mammal was hushed. The leaves, which were so moist and fresh in early
morning, now became lax and drooping, and flowers shed their petals.
On most days in June and July a heavy shower would fall some time
in the afternoon, producing a most welcome coolness. The approach
of the rain-clouds was after a uniform fashion very interesting
to observe. First, the cool sea-breeze which had commenced to
blow about ten o’clock, and which had increased in force with the
increasing power of the sun, would flag, and finally die away. The
heat and electric tension of the atmosphere would then become almost
insupportable. Languor and uneasiness would seize on every one, even
the denizens of the forest betraying it by their motions. White
clouds would appear in the east and gather into cumuli, with an
increasing blackness along their lower portions. The whole eastern
horizon would become almost suddenly black, and this would spread
upwards, the sun at length becoming obscured. Then the rush of a
mighty wind is heard through the forest, swaying the tree-tops; a
vivid flash of lightning bursts forth, then a crash of thunder,
and down streams the deluging rain. Such storms soon cease, leaving
bluish-black motionless clouds in the sky until night. Meantime
all nature is refreshed; but heaps of flower-petals and fallen
leaves are seen under the trees. Towards evening life revives
again, and the ringing uproar is resumed from bush and tree. The
following morning the sun again rises in a cloudless sky; and so the
cycle is completed; spring, summer, and autumn, as it were in one
tropical day. The days are more or less like this throughout the
year. A little difference exists between the dry and wet seasons;
but generally, the dry season, which lasts from July to December,
is varied with showers, and the wet, from January to June, with
sunny days. It results from this,--that the periodical phenomena of
plants and animals do not take place at about the same time in all
species, or in the individuals of any given species, as they do in
temperate countries. In Europe, a woodland scene has its spring,
its summer, its autumnal, and its winter aspects. In the equatorial
forests the aspect is the same or nearly so every day in the year:
budding, flowering, fruiting, and leaf-shedding are always going
on in one species or other. It is never either spring, summer, or
autumn, but each day is a combination of all three. With the day
and night always of equal length, the atmospheric disturbances of
each day neutralising themselves before each succeeding morn; with
the sun in its course proceeding midway across the sky, and the
daily temperature almost the same throughout the year--how grand
in its perfect equilibrium and simplicity is the march of Nature
under the equator!”
II.
EQUATORIAL VEGETATION.
The Equatorial Forest-Belt and its Causes--General features of
the Equatorial Forests--Low-growth Forest-trees--Flowery trunks
and their probable cause--Uses of Equatorial Forest-trees--The
Climbing Plants of the Equatorial Forests--Palms--Uses of
Palm-trees and their Products--Ferns--Ginger-worts and wild
Bananas--Arums--Screw-pines--Orchids--Bamboos--Uses of the
Bamboo--Mangroves--Sensitive-plants--Comparative scarcity of
Flowers--Concluding Remarks on Tropical Vegetation.
In the following sketch of the characteristics of vegetable life
in the equatorial zone, it is not intended to enter into any
scientific details or to treat the subject in the slightest degree
from a botanical point of view; but merely to describe those general
features of vegetation which are almost or quite peculiar to this
region of the globe, and which are so general as to be characteristic
of the greater part of it rather than of any particular country or
continent within its limits.
_The Equatorial Forest-Belt and its Causes._--With but few and
unimportant exceptions a great forest band from a thousand to fifteen
hundred miles in width girdles the earth at the equator, clothing
hill, plain, and mountain with an evergreen mantle. Lofty peaks and
precipitous ridges are sometimes bare, but often the woody covering
continues to a height of eight or ten thousand feet, as in some
of the volcanic mountains of Java and on portions of the Eastern
Andes. Beyond the forests both to the north and south, we meet first
with woody and then open country, soon changing into arid plains or
even deserts which form an almost continuous band in the vicinity
of the two tropics. On the line of the tropic of Cancer we have,
in America the deserts and dry plains of New Mexico; in Africa the
Sahara; and in Asia, the Arabian deserts, those of Beloochistan and
Western India, and further east the dry plains of North China and
Mongolia. On the tropic of Capricorn we have, in America the Grand
Chaco desert and the Pampas; in Africa the Kalahari desert and the
dry plains north of the Limpopo; while the deserts and waterless
plains of Central Australia complete the arid zone. These great
contrasts of verdure and barrenness occurring in parallel bands all
round the globe, must evidently depend on the general laws which
determine the distribution of moisture over the earth, more or
less modified by local causes. Without going into meteorological
details, some of which have been given in the preceding chapter, the
main facts may be explained by the mode in which the great aerial
currents are distributed. The trade winds passing over the ocean
from north-east to south-west with an oblique tendency towards the
equator, become saturated with vapour, and are ready to give out
moisture whenever they are forced upwards or in any other way have
their temperature lowered. The entire equatorial zone becomes thus
charged with vapour-laden air which is the primary necessity of
a luxuriant vegetation. The surplus air (produced by the meeting
of the two trade winds) which is ever rising in the equatorial
belt and giving up its store of vapour, flows off north and south
as dry, cool air, and descends to the earth in the vicinity of
the tropics. Here it sucks up whatever moisture it meets with and
thus tends to keep this zone in an arid condition. The trades
themselves are believed to be supplied by descending currents
from the temperate zones, and these are at first equally dry and
only become vapour-laden when they have passed over some extent
of moist surface. At the solstices the sun passes vertically over
the vicinity of the tropics for several weeks, and this further
aggravates the aridity; and wherever the soil is sandy and there are
no lofty mountain-chains to supply ample irrigation the result is
a more or less perfect desert. Analogous causes, which a study of
aerial currents will render intelligible, have produced other great
forest-belts in the northern and southern parts of the temperate
zones; but owing to the paucity of land in the southern hemisphere
these are best seen in North America and Northern Euro-Asia, where
they form the great northern forests of deciduous trees and of
Coniferæ. These being comparatively well known to us, will form
the standard by a reference to which we shall endeavour to point
out and render intelligible the distinctive characteristics of the
equatorial forest vegetation.
_General Features of the Equatorial Forests._--It is not easy to fix
upon the most distinctive features of these virgin forests, which
nevertheless impress themselves upon the beholder as something quite
unlike those of temperate lands, and as possessing a grandeur and
sublimity altogether their own. Amid the countless modifications in
detail which these forests present, we shall endeavour to point out
the chief peculiarities as well as the more interesting phenomena
which generally characterise them.
The observer new to the scene would perhaps be first struck by
the varied yet symmetrical trunks, which rise up with perfect
straightness to a great height without a branch, and which, being
placed at a considerable average distance apart, give an impression
similar to that produced by the columns of some enormous building.
Overhead, at a height, perhaps, of a hundred feet, is an almost
unbroken canopy of foliage formed by the meeting together of these
great trees and their interlacing branches; and this canopy is
usually so dense that but an indistinct glimmer of the sky is to be
seen, and even the intense tropical sunlight only penetrates to the
ground subdued and broken up into scattered fragments. There is a
weird gloom and a solemn silence, which combine to produce a sense
of the vast--the primeval--almost of the infinite. It is a world
in which man seems an intruder, and where he feels overwhelmed by
the contemplation of the ever-acting forces, which, from the simple
elements of the atmosphere, build up the great mass of vegetation
which overshadows, and almost seems to oppress the earth.
_Characteristics of the Larger Forest-trees._--Passing from the
general impression to the elements of which the scene is composed,
the observer is struck by the great diversity of the details amid
the general uniformity. Instead of endless repetitions of the same
forms of trunk such as are to be seen in our pine, or oak, or beech
woods, the eye wanders from one tree to another and rarely detects
two of the same species. All are tall and upright columns, but they
differ from each other more than do the columns of Gothic, Greek,
and Egyptian temples. Some are almost cylindrical, rising up out of
the ground as if their bases were concealed by accumulations of the
soil; others get much thicker near the ground like our spreading
oaks; others again, and these are very characteristic, send out
towards the base flat and wing-like projections. These projections
are thin slabs radiating from the main trunk, from which they
stand out like the buttresses of a Gothic cathedral. They rise to
various heights on the tree, from five or six, to twenty or thirty
feet; they often divide as they approach the ground, and sometimes
twist and curve along the surface for a considerable distance,
forming elevated and greatly compressed roots. These buttresses
are sometimes so large that the spaces between them if roofed over
would form huts capable of containing several persons. Their use
is evidently to give the tree an extended base, and so assist the
subterranean roots in maintaining in an erect position so lofty a
column crowned by a broad and massive head of branches and foliage.
The buttressed trees belong to a variety of distinct groups. Thus,
many of the Bombaceæ or silk-cotton trees, several of the Leguminosæ,
and perhaps many trees belonging to other natural orders, possess
these appendages.
There is another form of tree, hardly less curious, in which the
trunk, though generally straight and cylindrical, is deeply furrowed
and indented, appearing as if made up of a number of small trees
grown together at the centre. Sometimes the junction of what seem to
be the component parts, is so imperfect, that gaps or holes are left
by which you can see through the trunk in various places. At first
one is disposed to think this is caused by accident or decay, but
repeated examination shows it be due to the natural growth of the
tree. The accompanying outline sections of one of these trees that
was cut down, exhibits its character. It was a noble forest-tree,
more than 200 feet high, but rather slender in proportion, and it
was by no means an extreme example of its class. This peculiar
form is probably produced by the downward growth of aerial roots,
like some New Zealand trees whose growth has been traced, and of
whose different stages drawings may be seen at the Library of the
Linnean Society. These commence their existence as parasitical
climbers which take root in the fork of some forest-tree and send
down aerial roots which clasp round the stem that upholds them. As
these roots increase in size and grow together laterally they cause
the death of their foster-parent. The climber then grows rapidly,
sending out large branches above and spreading roots below, and
as the supporting tree decays away the aerial roots grow together
and form a new trunk, more or less furrowed and buttressed, but
exhibiting no other marks of its exceptional origin. Aerial-rooted
forest-trees--like that figured in my _Malay Archipelago_ (vol. i.
p. 131)--and the equally remarkable fig-trees of various species,
whose trunks are formed by a miniature forest of aerial roots,
sometimes separate, sometimes matted together, are characteristic
of the Eastern tropics, but appear to be rare or altogether unknown
in America, and can therefore hardly be included among the general
characteristics of the equatorial zone.
[Illustration: Sections of trunk of a Bornean Forest-tree.
1. Section at seven feet from the ground.
2. 3. Sections much higher up.]
Besides the varieties of form, however, the tree-trunks of these
forests present many peculiarities of colour and texture. The
majority are rather smooth-barked, and many are of peculiar whitish,
green, yellowish, or brown colours, or occasionally nearly black.
Some are perfectly smooth, others deeply cracked and furrowed, while
in a considerable number the bark splits off in flakes or hangs
down in long fibrous ribands. Spined or prickly trunks (except of
palms) are rare in the damp equatorial forests. Turning our gaze
upwards from the stems to the foliage, we find two types of leaf
not common in the temperate zone, although the great mass of the
trees offer nothing very remarkable in this respect. First, we
have many trees with large, thick, and glossy leaves, like those
of the cherry-laurel or the magnolia, but even larger, smoother,
and more symmetrical. The leaves of the Asiatic caoutchouc-tree
(_Ficus elastica_), so often cultivated in houses, is a type of this
class, which has a very fine effect among the more ordinary-looking
foliage. Contrasted with this is the fine pinnate foliage of some
of the largest forest-trees which, seen far aloft against the sky,
looks as delicate as that of the sensitive mimosa.
_Forest-trees of Low Growth._--The great trees we have hitherto been
describing form, however, but a portion of the forest. Beneath their
lofty canopy there often exists a second forest of moderate-sized
trees, whose crowns, perhaps forty or fifty feet high, do not touch
the lowermost branches of those above them. These are of course
shade-loving trees, and their presence effectually prevents the
growth of any young trees of the larger kinds, until, overcome by
age and storms, some monarch of the forest falls down, and, carrying
destruction in its fall, opens up a considerable space, into which
sun and air can penetrate. Then comes a race for existence among
the seedlings of the surrounding trees, in which a few ultimately
prevail and fill up the space vacated by their predecessor. Yet
beneath this second set of medium-sized forest-trees there is often
a third undergrowth of small trees, from six to ten feet high,
of dwarf palms, of tree-ferns, and of gigantic herbaceous ferns.
Coming to the surface of the ground itself we find much variety.
Sometimes it is completely bare, a mass of decaying leaves and
twigs and fallen fruits. More frequently it is covered with a dense
carpet of selaginella or other lycopodiaceæ, and these sometimes
give place to a variety of herbaceous plants, sometimes with pretty,
but rarely with very conspicuous flowers.
_Flowering Trunks and their Probable Cause._--Among the minor but
not unimportant peculiarities that characterise these lofty forests,
is the curious way in which many of the smaller trees have their
flowers situated on the main trunk or larger branches instead of on
the upper part of the tree. The cacao-tree is a well-known example
of this peculiarity, which is not uncommon in tropical forests;
and some of the smaller trunks are occasionally almost hidden by
the quantity of fruit produced on them. One of the most beautiful
examples of this mode of flowering is a small tree of the genus
_Polyalthea_, belonging to the family of the custard-apples, not
uncommon in the forests of North-western Borneo. Its slender trunk,
about fifteen or twenty feet high, was completely covered with
star-shaped flowers, three inches across and of a rich orange-red
colour, making the trees look as if they had been artificially
decorated with brilliant garlands. The recent discoveries as to the
important part played by insects in the fertilization of flowers
offers a very probable explanation of this peculiarity. Bees and
butterflies are the greatest flower-haunters. The former love the
sun and frequent open grounds or the flowery tops of the lofty
forest-trees fully exposed to the sun and air. The forest shades are
frequented by thousands of butterflies, but these mostly keep near
the ground, where they have a free passage among the tree-trunks and
visit the flowering shrubs and herbaceous plants. To attract these
it is necessary that flowers should be low down and conspicuous.
If they grew in the usual way on the tops of these smaller trees
overshadowed by the dense canopy above them they would be out of
sight of both groups of insects, but being placed openly on the
stems, and in the greatest profusion, they cannot fail to attract
the attention of the wandering butterflies.
_Uses of Equatorial Forest-trees._--Amid this immense variety of
trees, the natives have found out such as are best adapted to
certain purposes. The wood of some is light and soft, and is used
for floats or for carving out rude images, stools, and ornaments for
boats and houses. The flat slabs of the buttresses are often used to
make paddles. Some of the trees with furrowed stems are exceedingly
strong and durable, serving as posts for houses or as piles on which
the water-villages are built. Canoes, formed from a trunk hollowed
out and spread open under the action of heat, require one kind of
wood, those built up with planks another; and, as the species of
trees in these forests are so much more numerous than the wants of
a semi-civilized population, there are probably a large number of
kinds of timber which will some day be found to be well adapted to
the special requirements of the arts and sciences. The products of
the trees of the equatorial forests, notwithstanding our imperfect
knowledge of them, are already more useful to civilized man than to
the indigenous inhabitants. To mention only a few of those whose
names are tolerably familiar to us, we have such valuable woods
as mahogany, teak, ebony, lignum-vitæ, purple-heart, iron-wood,
sandal-wood, and satin-wood; such useful gums as india-rubber,
gutta-percha, tragacanth, copal, lac, and dammar; such dyes as
are yielded by log-wood, brazil-wood, and sappan-wood; such drugs
as the balsams of Capivi and Tolu, camphor, benzoin, catechu or
terra-japonica, cajuput oil, gamboge, quinine, Angostura bark,
quassia, and the urari and upas poisons; of spices we have cloves,
cinnamon, and nutmegs; and of fruits, brazil-nuts, tamarinds, guavas,
and the valuable cacao; while residents in our tropical colonies
enjoy the bread-fruit, avocado-pear, custard-apple, durian, mango,
mangosteen, soursop, papaw, and many others. This list of useful
products from the exogenous trees alone of the equatorial forests,
excluding those from the palms, shrubs, herbs, and creepers, might
have been multiplied many times over by the introduction of articles
whose names would be known only to those interested in special
arts or sciences; but imperfect as it is, it will serve to afford
a notion of the value of this vast treasure-house which is as yet
but very partially explored.