« PreviousContinue »
ON THE CAUSES WHICH INTERFERE WITH THE NATURAL CONDITIONS OF PLANTS IN LARGE TOWNS.
AMONG the causes tending to depress vegetation in large towns may be mentioned deficiency of light, the dryness of the atmosphere, the fuliginous matter with which the air of large towns is always more or less loaded, and the evolution of noxious gases from manufactories.
When we consider the all-important agency of light in the functions of the vegetable system, that upon it depend the nutrition of the plant, the formation of its secretions, &c., we shall not err when we attribute a portion of the depressing effects upon some plants to deficiency of light; but that this cannot be the sole cause is clear from the impossibility of growing such plants as ferns and mosses, which can, in any part of London, obtain as much light as they require.
With respect to the dryness of the atmosphere, my friend Mr. W. H. Campbell, late secretary to the Edinburgh Botanical Society, thus writes. "It occurs to me, that the want of moisture in a town-atmosphere is the
greatest enemy with which vegetation has to contend; and it seems obvious, that the larger the space occupied by a town, the greater must this want of moisture be. Stone houses, walls, and streets, are all the ready absorbents or reflectors of heat; and whatever rain falls is speedily drained off their surface, and carried away far from the town. Of course, the more fires, manufactories, and steam-engines there are, the dryness of the atmosphere will be the greater, and the power of vegetation be correspondingly reduced. Away from towns we find these circumstances exactly reversed. When rain falls the soil parts with no moisture until it can absorb no more. What is superfluous is then received in pools, ditches, marshes, lakes and streams, from all of which, and from the saturated soil, are exhaled those refreshing dews and vapours which so rarely visit the sickly vegetation of the town. In Edinburgh we have some instances of the power of moisture in obviating the disadvantages of proximity of smoke from manufactories. In some large pieces of garden-ground attached to houses at the south back of the Canongate, near Salisbury Craigs, it is almost impossible to rear herbaceous plants, or to preserve through a sickly existence any but the hardiest trees and shrubs, on account of their proximity to the heat and smoke of glass-works, breweries, &c. &c.; whereas, at the distance of 20 or 30 feet from these gardens, there is an extensive irrigated meadow, which, kept constantly moist, produces (despite of smoke and every other detrimental agent) ten or twelve crops in a season of the most luxuriant vegetation, the exhalations and irrigation in the driest summer constantly affording the requisite supply of moisture.
"In the same manner are we to account for the oases
in the deserts of Africa. Under the most unfavourable* circumstances, in the midst of arid sand, Nature supplies a spring of water, and immediately a luxuriant vegetation ensues as far as its influence extends. So, in Wellclose Square, I have no doubt, if a perennial fountain could be made to meander through your court-yard, and sun and light were freely admitted, that vegetation would even there assume some of its loveliest forms, despite of smoke and the other concomitants of a Life in London."
I believe, with Mr. Campbell, that a constant supply of moisture would materially benefit vegetation in large towns, but, from the result of my own experiments, and from what every one may see in London itself, I cannot imagine that "the dryness of the atmosphere is the greatest enemy to which the vegetation in London is subjected." Long before I contemplated the growth of plants in closed cases I made an unsuccessful attempt to cultivate fernst and various other plants in the open air, as detailed at the commencement of the ensuing chapter. If we examine old walls in London, which, from the leakage of cisterns or pipes above them, are constantly moist, we shall find a vegetation certainly, but not of a healthy kind. The conditions for mosses will be so far fulfilled as to allow of the development of their leaves; and we shall everywhere see, on such walls, the silvery
* Mr. C. is here under a mistake; light and heat are abundant, and moisture alone is wanting.
†Those who are desirous of acquiring an intimate acquaintance with our native species of this beautiful and interesting order of plants, and wish to cultivate them in the open air, cannot do better than consult Mr. Newman's History of British Ferns.'
tips (when not obscured by soot) of Bryum argenteum ; but we must go two or three miles out of London before we can find it in fructification. It is true that we may, even in London, find the Funaria hygrometrica* in fruit, but this is an exception to the general rule. It is likewise well known that, cæteris paribus, plants with smooth leaves suffer less in London than those which are hairy, or covered with viscid or resinous secretions. Hence the miserable appearance of most of the Conifere in London, although these are plants, many species of which flourish in the driest sands.
We must therefore look for another and more efficient cause of depression, and this I believe is to be found in the sooty particles diffused through the air. In my letter to Sir W. J. Hooker, published in the 'Companion to the Botanical Magazine' for May, 1836, I expressed my opinion that the depressing influence of the air of large towns upon vegetation, depends almost entirely upon the fuliginous matter with which such an atmo
*The Funaria hygrometrica is a remarkable moss, differing widely in its powers of adaptation, and consequently its greater geographical range, from its congeners. It appears to delight as much in heat as other mosses do in cold. There is nothing in its structure to lead us to infer such a difference of constitution. Most mosses are restricted within certain limits, and will not fructify but under certain conditions. The Funaria is found in fruit not only in London, but in every brick-field around it, in my own fern-houses, and likewise in Loddiges' Orchideous house, where the temperature often rises to 120o; and I have specimens in my herbarium from all parts of the world; from Egypt, the Cape of Good Hope, the East and West Indies, New Zealand, New Holland, &c. The peristome of this moss is one of our most beautiful microscopic objects.
sphere is impregnated, and which produces similar effects upon the leaves of plants as upon the lungs of animals. This opinion has been questioned by the late Mr. Ellis, in an admirable paper published in the 'Gardener's Magazine' for September, 1839; and, as the subject is one of great importance -it being impossible to apply remedies without knowing the nature of the disease I shall discuss it at some length. Mr. Ellis says that "the real mode in which such an atmosphere proves injurious to vegetation, was first shown by the experiments of Drs. Turner and Christison, which were published in the 93rd number of the Edinburgh Medical and Surgical Journal.' They ascertained that it is not simply to the diffusion of fuliginous matter through the air, but to the presence of sulphurous acid gas, generated in the combustion of coal, that the mischief is to be ascribed. When added to common air in the proportion of good or rooo part, that gas sensibly affected the leaves of growing plants in ten or twelve hours, and killed them in forty-eight hours or less. The effects of hydrochloric or muriatic acid gas were still more powerful, it being found that the tenth part of a cubic inch, in 20,000 volumes of air, manifested its action in a few hours, and entirely destroyed the plant in two days. Both these gases acted on the leaves, affecting more or less their color, and withering or crisping their texture, so that a gentle touch caused their separation from the footstalk; and both exerted this injurious operation, when present in such minute proportions as to be wholly inappreciable by the animal senses.
"After having suffered much injury from these acid gases, the plants, if removed in time, will recover, but