Popular Science Monthly/Volume 19/May 1881/The Eucalyptus in the Roman Campagna
By H. N. DRAPER, F. C. S.
SO much has already been written by way of contribution to our knowledge of the different species of the eucalyptus-tree, that, interesting as the subject is, it may well be considered to have received already a fair share of attention. There is one aspect of it, however, which can not perhaps be dwelt upon too much, and that is the value of this genus of plants as drainers of the soil and purifiers of the atmosphere. This is probably the true reason why so many attempts, more or less successful, have been made to acclimatize the eucalyptus in Southern Europe and even in Great Britain. No doubt, experiments have been stimulated by other causes. The foliage of these trees is, for example, unlike that of any other in our islands. It is pendulous, quivering, and evergreen; and the peculiar whitish appearance of one side of the leaves—due to a fatty or resinous secretion—is very characteristic. Till the tree is from three to five years old, the leaves grow horizontally; but afterward they generally assume a pendent position. Instead of having one of their surfaces toward the sky, and the other toward the earth, they are often placed with their edges in these directions, so that each side is equally exposed to the light. This arrangement may have something to do with the extraordinary quantity of moisture these trees exhale into the atmosphere.
The eucalyptus belongs to the natural order Myrtaceæ, and is indigenous to the temperate parts of Australia (where it goes by the name of stringy-bark, or gum-tree) and Tasmania—that is, where the mean temperature does not exceed a range of from 52° to 72° Fahr. The foliage is leathery, and almost always characterized by a certain metallic aspect. The leaves are as a rule narrow, and have either a very short and twisted petiole or foot-stalk, or none at all. In Australia they commonly attain a height of two hundred feet, and instances are given in which a height of three hundred and fifty feet has been attained. The flowers are usually pinkish or white, and in the latter case superficially resemble those of the myrtle. Unlike these, however, they are devoid of petals. The fruit contains the seeds—seeds so minute, it is said, that from one pound of those of the variety Globulus more than one hundred and sixty thousand plants could be raised.
I have always taken a great interest in the eucalyptus, and have grown it near Dublin for several years with considerable success. I have had at one time as many as twenty fine healthy saplings of the species Globulus, of from ten to sixteen feet high, and one which reached to twenty-five feet, and had a stem of twenty-two inches circumference. These were all five years old. But cold is the deadly enemy of the gum-tree; and, though I had kept mine during four ordinary Irish winters, I lost them all during the almost Arctic winter of 1878-'79, I may say, in passing, that I have not been quite discouraged, and that I have again several healthy plants making good progress. My interest in the subject has received a new stimulus from a recent experience of eucalypt-culture in the wild plain known as the Campagna of Rome.
One lovely morning in last October we left our hotel hard by the Pantheon, and in a few minutes came to the Tiber. If we except the quaint and bright costumes of many classes of the people, and the ever-changing street scenes of Rome, there is nothing in the drive of very much interest until we reach the river. Here, looking back, we see the noble structure which crowns the Capitoline Hill. The fine building on the farther bank of the river is the Hospital of St. Michele. On this side we are passing the small harbor of the steamboats which ply to Ostia. Presently, the Marmorata, or landing-place of the beautiful marble of Carrara, is reached. From here a drive of a few minutes brings us to the cypress-covered slope of the Protestant Cemetery, where, in the shadow of the pyramid of Cestius, lie the graves of Shelley and Keats. Apart from the interest attached to these two lowly tombs and the memories aroused by their touching epitaphs, no Englishman can visit this secluded spot and look without deep feeling upon the last resting-places of his countrymen, who have died so many hundred miles from home and friends. The cemetery is kept in order and neatness, and flowers grow upon nearly all the graves.
Our route next lay along the base of that remarkable enigma the Monte Testaccio, a hill as high as the London Monument or the Vendôme Column at Paris, made entirely of broken Roman pots and tiles, as old perhaps as the time of Nero! Leaving behind this singular heap of earthenware, we thread long avenues of locust-trees, and presently, passing through the gate of St. Paul, reach the magnificent basilica of that name. Nor can I pause here to dwell upon the marvels of this noble temple, or to tell of its glorious aisles and column supported galleries; of its lake-like marble floor, or of the wealth of malachite, of lapis lazuli, of verde antique, of alabaster, and of gold, that has been lavished upon the decoration of its shrine. I must stop, however, to note that nowhere has the presence of the dread malaria made itself so obvious to myself. We had scarcely entered the church, when we became conscious of an odor which recalled at once the retort-house of a gas-works, the bilge-water on board ship, and the atmosphere of a dissecting-room; and we were obliged to make a hasty retreat. There could be little doubt that the gaseous emanations which produced this intolerable odor were equally present in the Campagna outside, but that in the church they were pent up and concentrated.
Even did space admit, this is not the place to enter into any prolonged dissertation on the history or causes of this terrible scourge of the Roman Campagna, the fever-producing malaria. The name expresses the unquestionable truth, that it is a gaseous emanation from the soil; and all that is certainly known about it may be summed up in a very few lines. The vast undulating plain known as the Campagna was ages ago overflowed by the sea, and owes it present aspect to volcanic agency. Of this the whole soil affords ample evidence. Not only are lava, peperino, and the volcanic puzzuolana abundant, but in many places—as at Bracciano and Baccano—are to be seen the remains of ancient craters. When the Campagna was in the earliest phase of its history, it was one fertile garden, interspersed with thriving towns and villages. It was also the theatre of events which terminated in making Rome the mistress of the world. This very supremacy was the final cause of its ruin and of its present desolation. While the land remained in the possession of small holders every acre was assiduously tilled and drained; but when it passed into the hands of large landed proprietors, who held it from the mere lust of possession, it became uncared for and uncultivated.
Filtering into a soil loaded with easily decomposed sulphur compounds, the decomposing vegetable matter finds no exit through the underlying rock. The consequences may be imagined, but, to those who have not experienced them, are not easily described. This once fertile land is now a horrid waste, untouched, except at rare intervals, by the hand of the farmer, and untenanted save by the herdsman. Even he, during the months of summer, when the malaria is at its worst, is compelled, if he will avoid the fever, to go with his flocks to the mountains. It may be mentioned, in passing, that the malaria fever, or "Roman fever" as it has been called, has been the subject of recent investigation by Professor Tommassi-Crudelli, of Rome, who attributes it to the presence of an organism, to which the specific name of Bacillus malariæ has been given.
Leaving St. Paul's, we pursued for a short time the Ostian road; and at a poor osteria, where chestnuts, coarse bread, and wine, were the only obtainable refreshments, our route turned to the left, along a road powdered with the reddish dust of the pozzuolana—the mineral which forms the basis of the original "Roman cement" large masses of which rock form the roadside fences. After a drive of perhaps half an hour, we found ourselves at the Monastery of Tre Fontane (three fountains). The Abbey of the Tre Fontane comprises within its precincts three churches, of which the earliest dates from the ninth century. One of these, San Paolo alle Tre Fontane, gives its name to the monastery. A monk, wearing the brown robe and sandals of the Trappist order, met us at the gate. The contrast now presented between the sterile semi-volcanic country around and the smiling oasis which faces us, is striking. Here are fields which have borne good grass; some sloping hills covered with vines; and, directly in the foreground, almost a forest of eucalypt-trees.
We have come to learn about eucalypts; and our guide takes quite kindly to the role of informant. What follows is derived from his viva voce teaching, from my own observation on the spot, and from a very interesting pamphlet, printed at Rome in 1879, and entitled "Culture de l'Eucalyptus aux Trois Fontanes," by M. Auguste Vallée.
Before the year 1868, the abbey was entirely deserted. It is true that a haggard-looking monk was to be found there, who acted as cicerone to visitors to the churches; but even he was obliged to sleep each night in Rome. The place attained so evil a reputation that it was locally known as "The Tomb." There are now twenty-nine Brothers attached to the monastery, all of whom sleep there each night. This remarkable result, though no doubt to a great extent due to the drainage and alteration of the character of the soil by cultivation, is unquestionably mainly owing to the planting of the eucalyptus. It would take long to tell of the heroic perseverance of these monks; of the frequent discouragements, of the labor interrupted by sickness, of the gaps made in their number by the fatal malaria, and the undaunted courage in overcoming obstacles which has culminated in the result now achieved. Let us pass to the consideration of the actual means by which so happy a change in their immediate surroundings has been brought about. At Tre Fontane are cultivated at least eleven varieties of eucalyptus. Some of these, as E. viminalis and E. botryoïdes, flourish best where the ground is naturally humid; E. resinifera and E. meliodora love best a drier soil. The variety Globulus (blue gum-tree) possesses a happy adaptability to nearly any possible condition of growth. At the monastery, as in most elevated parts of the Campagna, the soil is of volcanic origin, and there is not much even of that; often only eight, and rarely more than sixteen inches overlying the compact tufa. But, with the aid of very simple machinery, the Trappists bore into the subsoil, blast it with dynamite, and find, in the admixture of its débris with the arable earth, the most suitable soil for the reception of the young plants.
The seeds are sown in autumn, in a mixture of ordinary garden earth, the soil of the country, and a little thoroughly decomposed manure. This is done in wooden boxes, which, with the object of keeping the seeds damp, are lightly covered until germination has taken place. When the young plants have attained to about two inches, they are transferred to very small flower-pots, where they remain until the time arrives for their final transplantation. The best time for this operation is in spring, because the seedlings have then quite eight months in which to gather strength against the winter cold. One precaution taken in planting is worth notice. Each plant is placed in a hole of like depth and diameter. In this way, no individual rootlet is more favored than its fellow, and, as each absorbs its soil-nutriment equally, the regularity of growth and of the final form of the tree is assured. A space of three feet is left between each seedling; but so rapid is the growth, that in the following year it is found necessary to uproot nearly one half of the plants, which finally find themselves at a distance from each other of about five feet. From this time, much care is required in weeding and particularly in sheltering from the wind, for the stem of the eucalyptus is particularly fragile, and violent storms sometimes rage in the Campagna. The other great enemy of the tree is cold, and this offers an almost insurmountable obstacle to its successful culture in Great Britain. It seems to be well proved that most of the species will survive a winter in which the temperature does not fall lower than 23° Fahr. How fortunately circumstanced is the culture of the tree at Rome, may be learned from the fact that the mean lowest temperature registered at the observatory of the Roman College during the years 1863-'74 was 23·48. Once only in those years a cold of 20° was registered, and even that does not seem to have injured the plants; but when, in 1875, the minimum temperature fell to 16°, the result was the loss in a single night of nearly half the plantation of the year.
But when, as at Tre Fontane, the conditions of growth are on the whole favorable, the rapidity of that growth approaches the marvelous. The mean height, for example, of three trees chosen for measurement by M. Vallée in 1879, was twenty-six feet, and the mean circumference twenty-eight inches. These trees had been planted in 1875, or in other words were little more than four years old. Other trees of eight years' growth were fifty feet high and nearly three feet in circumference at their largest part. These figures refer to Eucalyptus globulus, which certainly grows faster than the other species; and it must be remembered that in warmer climates the growth is even still more rapid. I have seen, for example, trees of Eucalyptus resinifera at Blidah in Algeria which at only five years old were already quite sixty feet high.
The question of how and why the eucalypts exercise sanitary changes so important as those which have been effected at this little oasis in the Campagna, may be best answered when two remarkable properties which characterize many of the species have been shortly considered. The first of these is the enormous quantity of water which the plant can absorb from the soil. It has been demonstrated that a square metre—which may roughly be taken as equal to a square yard—of the leaves of Eucalyptus globulus will exhale into the atmosphere, during twelve hours, four pints of water. Now, as this square metre of leaves—of course, the calculation includes both surfaces—weighs two and three quarter pounds, it will be easily seen that any given weight of eucalyptus-leaves can transfer from the soil to the atmosphere nearly twice that weight of water. M. Vallée does not hesitate to say that under the full breeze and sunshine—which could necessarily form no factor in such accurate experiments as those conducted by him—the evaporation of water would be equal to four or five times the weight of the leaves. One ceases to wonder at these figures, on learning that it has been found possible to count, on a square millimetre of the under surface of a single leaf of Eucalyptus globulus, no less than three hundred and fifty stomata or breathing-pores. And it now begins to be intelligible that, if such an enormous quantity of water can be transferred from earth to air, it may be possible that an atmosphere, which without such aid would be laden with malarious exhalations, may be rendered pure by this process of leaf distillation: the putrescible constituents of the stagnant water are absorbed by the roots, and become part of the vegetable tissue of the tree.
But this is not all. Like those of the pine, the leaves of all species of eucalyptus secrete large quantities of an aromatic essential oil. It has recently been shown—and the statement has been very impressively put by Mr. Kingzett—that, under the combined action of air and moisture, oils of the turpentine class are rapidly oxidized, and that, as a result of this oxidation, large quantities of peroxide of hydrogen are produced. Now, peroxide of hydrogen is—being itself one of the most potent oxidizers known—a very active disinfectant; and, as the leaves of some species of eucalyptus contain in each hundred pounds from three to six pounds of essential oil, we can hardly avoid the conclusion that the oxygen-carrying property of the oil is an important element in the malaria-destroying power of the genus. Moreover, the oxidation of the oil is attended by the formation of large quantities of substances analogous in their properties to camphor, and the reputation of camphor as an hygienic agent seems sufficiently well founded to allow us to admit at least the possibility of these bodies playing some part in so beneficent a scheme.
Before closing this paper, it may be well to note that the Trappist monks of the Tre Fontane attach much importance to the regular use of an infusion of eucalyptus-leaves as a daily beverage. The tincture of eucalyptus is said to be useful in intermittent fevers, though of course inferior to quinine. As we threaded the coast-line via Civita Vecchia to Leghorn, we could not help being struck by the fact that the precincts of all the railway-stations were thickly planted with eucalypts. Since our return, I learn with much gratification that the Italian Government have given a grant of land to the Trappists, and have also afforded them the aid of convict-labor to a considerable extent for the establishment of a new plantation. And looking back not only at what has been actually accomplished during the past ten years, but to the important fund of information which has been accumulated, one can only look forward hopefully and with encouragement to the future of the eucalyptus in the Roman Campagna.—Chambers's Journal.