Popular Science Monthly/Volume 26/March 1885/A Naturalist's Excursion in Dominica
|A NATURALIST'S EXCURSION IN DOMINICA.|
THE British Island of Dominica, although it forms only an insignificant colony, takes a rank among the first of the West Indies, when considered in regard to the richness of its scenery. Built up of lofty volcanic masses, which interpose almost insurmountable rocky barriers to the entrance of civilization into the interior, it still conceals among its hills and ravines a life of animals and plants rejoicing in the wildest freedom, and which is developed under the moist, tropical climate into extreme luxuriance. If one desires to make himself acquainted in the shortest possible time with the life of the island, he can do no better than make an excursion to the "Boiling Lake," that wonderful hot water-crater in the interior, which is one of the most curious geological phenomena of the earth. The road of about fifteen miles, but which it takes two or three days to traverse, so rugged is it. from the west coast, passes through cultivated and half-tilled lands into a romantic river-valley, and then through the primitive wilderness over hills and mountain-torrents, into an upland valley about three thousand feet above the sea, in the bottom of which lies the boiling lake, surrounded by a grim waste of volcanic rocks.
The starting-place for our excursion is the little town of Roseau, the chief place of the island, and the only one where the traveler will find a boarding-house, situated on the west coast, picturesquely set down at the mouth of a romantic valley, among sugar-cane fields and palm-gardens, and framed by forest-covered mountains. Before starting on our excursion we will make a short study of the shore, a flat beach of sand and gravel, sunny, hot, and dry, but which supports a characteristic vegetation. We are struck, in looking at this beach flora, with the predominance of the creeping plants, by which the most diversified botanical families are represented. Their habit of growth, with the multitude of rooting points it permits, gives them great advantages in keeping their hold on the shifting sands, and access to numerous points at which they may tap the soil for its scanty supplies of moisture. The succulent nature of the organs is another peculiarity of these plants that will strike the Northern observer. Most of them, whether they be creepers or upright, are either provided with fleshy leaves, or consist of amorphous thick stems without expanded foliage. This property, which must be regarded as a provision to diminish transpiration, is, as every one knows, not uncommon in the vegetation of dry places. In the tropics it marks not only the shore plants and the vegetation of the arid plains, but also the epiphytes, which live upon the dry bark of the trees. European species are represented in this growth by the portulaccas. Among the plants is one, Bryophyllum calycinum, which has long been known to gardeners and botanists by the faculty which its leaves possess, when broken off and laid upon the ground, of developing buds on their edges, which finally become independent plants. This, instead of being a merely adventitious peculiarity, marked only under special circumstances, as has been supposed, is really the normal provision of Nature for the propagation of the plant. This species forms in the course of its growth two kinds of leaves; the entire leaves of the young plant, which are shaped like those of the common live-forever, and, at a later stage of growth, cleft leaves. The two kinds of leaves are not equally competent to form buds, but the property is a peculiarity of the cleft ones. When we gently draw the hand over a well-grown plant of Bryophyllum, we will find the feathered leaves falling like ripe fruits to the ground, while the entire leaves remain fixed upon the plant and will not be disturbed by any shaking. Examining the fallen offsets a few days afterward, we will find their upper surfaces crowned with a circle of sprouts around the edges, while to the lower side is attached a tuft of young rootlets. The plantlets live at first on the nourishing matter of the young leaf, but soon become wholly separated and acquire an independent existence, to become in turn parents of a new brood. Some other plants multiply by offsets from the leaves, but the exhibition of a differentiated propagation-leaf is peculiar to this one. Among the trees that attract our attention is the shore-grape (Coccoloba uvifera), with its curious knotted and bushy growth, and its thick, hard leaves, which is found nowhere but in the Antilles. It offers an odd combination of the creeping and upright growths: in the isolated specimens, the lower limbs bend down and run along the pebbly beach, but without taking root; while the upper limbs spread themselves out in the air, at this time hung with whitish flower-spikes, which are later to develop into the dark-blue "grapes of the shore." On the beach a few miles north of Roseau are some plants of the manchineel-tree (Hippomane mancinella), now becoming quite rare, which is fabled to be deadly to all who sleep under it. The thing that is true about this myth is, that the sap contains an acrid poison that causes painful sores on the skin. The botanist Jacquin, who visited the Antilles in the middle of the last century, says that no animal would touch the fruit of the manchineel, though the ground under the trees was covered with it and inhabited by innumerable crabs. Jacquin denies that there is any danger in sleeping under the trees, because he and his companions rested under one of them for three hours without feeling any inconvenience from it. In his time, manchineel-wood was used for fine cabinet-work, and was obtained without risk from poisoning by building a fire around the tree, by which the greater part of the sap was boiled out, and then cutting it down very carefully with the face veiled. The Capparis cynophallophora attracts notice by its curiously shaped flowers, conspicuous through their numerous long, cream-colored filaments which, drooping when they first come from the buds, gradually erect themselves into an umbel of elastic threads. They are visited by hosts of insects, which, striking against the stamens in their efforts to reach the nectaries, set them into rapid motion and become dusted with the pollen, and are thus constituted bearers of it to other flowers; for the Capparis is proteranderous, and only the pistils 'of flowers that have already cast their pollen are capable of being fertilized. Conspicuous objects are the papilionaceous flowers of the Erythrina corallodendron, which, the tree being leafless at this season, reveal themselves to a vessel approaching the coast in bunches of gorgeous scarlet.
If we continue our excursion till sunset, we are overtaken in returning to the town by the sudden coming on of darkness, for the twilight is very short in this low latitude. But, hardly has the departed sun ceased to gild the crowns of the cocoa-palms, than the moon sheds her soft light through the delicately feathered foliage of the tamarind trees under which we are walking. With every succeeding minute the crowns of these trees grow more transparent and open; for the leaves are putting themselves to sleep, and folding their filaments up against their petioles.
Early in the morning we start for our first night-station, the negro village of Laudat, seven miles from Roseau, in the mountains. We might go on horseback, but prefer a way that will give opportunity for close biological observations; so, having a negro to carry our baggage and botanical books, we start out, armed with umbrella, gun, and opera-glass, with which to scan inaccessible specimens in the tree-tops, on foot. As we pass through the cultivated lands, we admire the areca and cocoa palms, but are disappointed with the banana-trees, whose leaves have been torn to shreds by wind and rain, and find the bread trees at this season presenting but a sorry spectacle. The dark masses of the mango-trees make a better impression, and it is impossible to repress admiration of the calabash-trees (Crescentia cujete), with great pumpkin-fruits hanging from the tips of their slender limbs, and which are devoted to such varied uses: the fruit-pulp to be made into a vegetable viand; the pumpkin-shell into vessels and dishes of every sort; and the outer bark by the West Indian orchid-growers as the ground on which to cultivate their fancifully shaped floral treasures. As we examine the plants by the roadside, many of them stragglers from the sea-shore or from foreign parts, we are struck with the variety of the provisions by which they adapt themselves to resist the heat and aridity of the dry season. We have already mentioned the succulent stems and the condensed surface of the beach plants, and the leafless condition of the coral-tree (Erythrina), which other Leguminosæ also assume during the heats. These and other peculiarities for the same end are exhibited not in the same degree for all of the species, but with numerous individual variations according to the special circumstances of each particular plant, and in such a way as to demonstrate a capacity for individual adaptation. Here are, close together, two specimens of the Bryophyllum calycinum, one standing in the open sunlight, and the other under the shadow of an acacia-tree. The former plant has relatively small, thick leaves, the structure of which is seen under the microscope to be close and made up of palisade like cells; while the other one displays much thinner and more loosely built leaves, exposing many times as much surface to the light as its companion did. Another method of adaptation is shown in the posing of the surface of the leaves parallel to the sun's rays instead of perpendicularly to them. This position in profile sometimes occurs as a peculiarity of the species; is sometimes brought about by the version or folding of the leaf-blades; and is sometimes dependent upon periodical movements of the leaves, which seem to be provided with particular organs for the purpose, according to the intensity of the light. The profile position appears to be fixed in the shore-grapes, which we observed on the beach, in the sapoteas, and in some other species. The faculty of folding the leaves appears rather to be one of individual adaptation. The leaves of the Bryophyllum appear folded in the sun, spread out flat in the shade; and the same phenomenon was observed in a modified form in the very abundant Psidium Guava and some other species. Other plants form close and hard cuticles which restrict evaporation, and some others appear to be furnished with special water vessels in their hypodermic layers. To this class seem to belong the thick-leaved calabash-trees and shore-grapes, and the creeping Commelyaceæ.
From admiring a number of highly colored flowers, our attention was drawn to the modest sensitive-plant (Mimosa pudica), which. was here growing in masses as a common weed alongside of the cultivated fields. A goat was feeding along the hedge-side, and had stretched out his tongue toward the delicate mimosa-leaves, but had not reached them, when he suddenly drew his head back in astonishment at the strange sight of an array of sharp thorns, forbidding closer approach, where he had only an instant before anticipated the taste of a mouthful of delicious foliage. The mimosa thus protects itself against the unwelcome feeder upon it in the same manner as the hedgehog escapes his enemies by rolling himself up into a prickly ball. Now was explained to us the observation we had made before in the country, of islands of mimosa-plants rising untouched from the pastures in which all the other plants around them had been closely eaten away. The same property of withdrawing itself from unfriendly contact operates to protect the mimosa against injury from wind and rain.
As we go up the mountain-walled valley of the Roseau, in the intervals of which cultivation still presses hard upon the primitive vegetation, we admire the variety and brilliancy of the extra-floral display by which some of the species are made conspicuous, and which is one of the marked features of the West Indian flora. Here is a begonia, with rose-red peduncles; there are some bromelias, with brightly colored bracts attached to their flower-stocks. The Heliconia, or wild-banana, is marked from afar off not more by its enormous leaves than by the brilliant purple spathe that surrounds its unobtrusive inflorescence; and the Euphorhia heterophylla is equally distinguishable by the patches of crimson on the whorl of leaves nearest to its flowers; while many other plants have their real leaves variegated with stripes or spots of color. Of most graceful and noble bearing are a group of tree-ferns, the unapproachable delicacy of whose leaf-carving, the remarkable harmonizing of the green of their foliage with the dark brown of their stems, and the perfect symmetry and pose of their crowns, are worthy of and receive the highest admiration. As we continue the ascent, the wood becomes largely composed of the Bursera gummifera, a tree of the terebinth family, the magnificent stems of which are supported by wide-spreading pillar-roots and varnished with the white balsam that has exuded from their bark. Moss-like plants nestle under the shelter of the root-pillars, lianas climb around the trunks, and multitudes of epiphytes flourish in their airy crowns, among which we are surprised to see the Clusias, themselves trees, enthroned high upon the topmost limbs. From the height of more than a hundred feet these parasites, called in the country "Scotch attorneys," and "cursed fig-trees," send their rope-like, tufted air-roots clear to the ground, to draw up water and food to their lofty abode, while they establish their mechanical security on the stem of the mother-tree by a close network of holding-roots. Sometimes the Bursera dies in the embrace of the strangler, and its trunk molders away without crumbling up, within its tight envelope, and finally falls, if it is not held up by the vines, bringing its destroyer down with it.
At Laudat, where we are to spend the night, two thousand feet above the sea, we find a better opportunity than we have ever before enjoyed to become acquainted with the structure and habits of the epiphytic phanerogams. At all other places on the island these plants live in the tree-tops, and we have to content ourselves with looking at them through the glass, or to rely for more careful examination upon such specimens as we can bring down with the gun. Here, where the forest has been cleared away for several acres, these plants have come down with the trees, and, finding enough light near the ground, live upon the bushes. They have been quite fully described by A. W. F. Schimper, in the "Botanisches Centralblatt," and we check our own observations by his account. Most of the epiphytes of Landat belong to the families of the orchids, aroids, bromelias, and ferns, while many other families are represented by individual forms.
The peculiar conditions under which these plants live require peculiar adaptations. One of their most general characteristics, and frequently a very prominent one, is the succulent or leathery constituency of their leaves, which, operating to impede transpiration, well adapts them to the dry conditions of their dwelling-place. Some of them are protected by a clothing of hairs. Many epiphytes are characterized by superficial extensions of their organs at relatively small heights above their substratum; quite usual are the arrangement in rosettes of the leaves at the base of the stem, thickenings of the stem into knots, and a creeping or climbing habit—all peculiarities denoting adaptation to the absorption of water and food, and to the gaining of a secure footing on the substratum. In the way of special adaptations, we may, with Schimper, distinguish among the epiphytes four groups, according to the manner in which they take up their food.
Those of one group simply derive their nourishment from the bark to which they are attached, and are in this respect analogous to the ground-plants. Those of the second group send down roots to the ground, besides those by which they adhere to the tree, and thus put themselves as to nourishment in almost precisely the condition of ground-plants. Of these are the Clusia, which we have described, and two plants which were conspicuous at Laudat, by their handsome flowers and the luster of their leaves. In some epiphytal orchids, aroids, and ferns, the roots weave themselves on their bark support into something like birds' nests, in which are gradually accumulated dead leaves and other organic detritus, to form a humus. The fourth class, to which the bromelias belong, is distinguished from all the others by the fact that water and food are taken up by the leaves, while the roots are either not developed, or are reduced to mere organs of attachment. The Tillandsia usneoides ("Spanish moss"), which, having no roots, hangs from the limbs, is clothed with a silver-gray hair, having shield-like processes which represent water-absorbing organs. Other epiphytic bromelias have similar absorptive vessels, and special provisions in the dish-like arrangement of the leaf-rosettes for storing rain-water and dew and more solid food for a considerable time. One may be convinced in a very instructive manner of the presence of water in these leaf-basins, by bending down a limb covered with epiphytes, when, unless he proceeds very carefully, he will receive a quart or more of water on his head. We learn from these considerations that these epiphytes are not real parasites, but only tenant forms, which, fixing their homes on other plants, derive their food support from the atmosphere and from dead matter. There are, however, besides these, real parasites at Laudat, which prey upon the living wood of the trees.
Among the forms of animal life at Laudat are three hummingbirds, one of which is so tame that the children catch it in their hands, and another is hardly two inches long; and, in sharp contrast with them, the largest of all insects. This is a beetle, which entomologists have named, in recognition of its gigantic size and great strength, Dynastes Hercules. The male is armed, like our stag-beetles, with two immense tusk-like processes on the head, the physiological significance of which is unknown. The female is unarmed, and of much more slender constitution.
So absorbed were we in the contemplation of the new forms of life around us that we would have been unmindful that the afternoon was passing away were it not that a bird called out to inform us that the sun would set in half an hour, and ten minutes later it would be dark. The sunset-bird, as the American Ober, who discovered it in this island, has named it in his "Camps in the Caribees," utters its peculiar cry only twice during the day—half an hour before sunrise, and as long before sunset—and keeps complete silence for the rest of the day. For a very brief interval after sunset the air is perfectly clear and transparent, and the light-effects are most picturesque; then, as if some signal had been given, begins the concert of the tree-frogs and locusts, and finally darkness settles over the landscape, to be broken up shortly by the rising of the moon, whose light gives a new series of picturesque effects.
Early in the morning we are awakened for a bath at the junction of two brooks, one bringing warm water from the mountains and the other cold water from springs, where we may take our choice between the temperatures, or of a mixture of the two, or between a douche and a plunge-bath, and for our final start to the boiling lake. There are two bodies of standing water on the island: one is called the freshwater lake, and is cold; while, though lying at a considerable height above the sea, and probably occupying an extinct crater, it is less remarkable for its geological features than for the beauty of its surroundings. The other, the "Boiling Lake," is the object of our excursion. Soon we entered upon a dark wood of painful grandeur. The trees were so large and tall that we were not able with the naked eye to distinguish the forms of the leaves, the flowers, or the fruit upon a single one of them. Even our guns could not reach the atmospheric vegetation, and we had to content ourselves with the examination of casual fallen specimens, or with pulling at the ropy air-roots of the clusias, when the pouring of water into our faces would inform us that there were tillandsias and brocchinias above. Of animal life, we observed a curious rodent occasionally dashing quickly across the way, but no large mammalia, and two large brightly colored parrots peculiar to the island, of which we did not succeed in getting any specimens. It is a remarkable fact that most of the birds of Dominica are found nowhere else. The ornithologist Ober, who visited Dominica in 1880-'81 and studied its birds, was surprised to remark that a very considerable proportion of them were of perfectly new species. Another species that must not be forgotten is one of large land-crabs which run over the ground, and of which Ober records the habit of going every year in the same month to lay their eggs in the saltwater, where they may be met by thousands and thousands. In a short time we reach a mountain-river of clearest water, which is called the Breakfast River, because excursionists to the boiling lake, reaching it at about ten o'clock in the morning, are accustomed to stop and rest awhile and take their breakfast.
On the other side of this stream we have to climb a steep, bush clad rock-wall, till in an hour we reach the top of the mountain and look on a panorama of astonishing magnificence. Behind us in the west lies the forest we have traversed, and the narrow green valley of the Breakfast River. Before us in the east stretches a bare, ravine cut waste, strewed with volcanic stones and yellow sulphur-beds, and seething with hot springs, streams, fumeroles, and solfataras, covered with the remains of destroyed woods, and crowned with a pillar of vapor reaching to the clouds. Beyond a turn of the valley at our feet sounded a dull rumbling, which with the vapor indicated to us the direction in which the boiling lake lay. We scramble over the steep cliff into the valley through a wood of trees burned to a cinder, but yet standing. This desolation was occasioned by an eruption of the lake, which took place in 1880, by which immense masses of glowing ashes and hot mud were thrown over the wooded valley-wall. Between the grim trunks, which are so brittle that they crumble at the slightest push, and which offer not the least of the obstacles to our descent, the ground is covered with ejected matters and pebbles, with here and there some plant growing in the interstices. In the bottom of the valley flows a warm, steaming stream, which is fed by little brooks rushing down on every side, foaming between the blocks of stone. Most of these affluents run with colored water—one blue, another yellow, a third milk-white, a fourth maroon, etc., according to the mineral constituents which it holds in suspension or solution. At many places aqueous and sulphurous vapors issue from the ground as if from the valves of a steam-engine, and here and there is a steaming basin from which escape tumultuous blasts of gas. The bed of the stream is beset with great bowlders, over which we have to find our way with much difficulty and some danger by springing from one to another. Finally we reach the edge of the boiling lake in a state of extreme exhaustion.
A glance into the infernal caldron that lies before us informs us that we are standing here at the mouth of a still active volcano. The basin of the lake lies in the midst of a deep, steeply descending cup, the crater, to which two streams come from the north. One of the streams brings cold chalybeate water, and runs by the basin to unite with its warm effluent; the other, bringing warm water, empties into the boiling lake. On the south side of the crater gaps an opening in the wall which constitutes the outlet of the lake. It is of quite recent origin, for it dates only from the great catastrophe of 1880, in which the valley-forest was destroyed. Previous to this time the area of the lake was about three times as great as it is now, when its diameter is only about forty-five paces. In the center of the basin is a geyser issuing from a mound of black mud, which, when we observed it, spouted to a height of some fifteen or twenty feet. Other observers have given it a height of from sixty to a hundred feet. In the interior of the mud-heap of the geyser we remarked, whenever the wind blew the steam away, a kind of tufaceous structure, of which we were not able to learn anything more exactly. Great masses of sulphurous gas escape over the whole surface of the basin from the black, muddy fluid, and keep up a loud roaring and humming, which only heightens the dismal aspect of the whole place.
This was the end of our excursion into the interior of Dominica.—Translated for the Popular Science Monthly from Kosmos.