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By Robert Hutchison of
Carlowrie, Kirkliston
[Premium - The Medium Gold Medal]
The rapid expansion of the
commercial and manufacturing activity of the United Kingdom, during the
past quarter of a century, in nearly every part of the country, and the
consequent-erection and enlargement of public works of various
descriptions, —while of paramount importance to the social well-being and
industry of teeming thousands of the population,—are, nevertheless,
inseparably connected with the origin of latent evils which affect in a
greater or less degree, according to circumstances, in the vicinity of
such manufactories, the amenity and salubrity of the district, the growth
and health of the adjacent crops within their influence, and the vitality
of vegetation generally from the tiniest blade of grass to the tree of
giant dimensions.
These pernicious influences
are of various kinds and degrees, affecting the atmosphere, the soil, the
water supply, or, it may be, all these together. For simplification they
may be classed under the following heads:—
1st, Pollution of
water-supply, and food of plants by chemical discharges from public works
into open drains, water courses, or rivers.
2nd, Saturation of the soil
from deleterious matters filtrating into it, and its consequent
destruction for plant-life.
3rd, Atmospheric pollution
by smoke, and other noxious vapours or gaseous discharges, whereby the
pure natural air is vitiated, and rendered injurious to the respiratory
organs of vegetable life, and an artificial atmosphere, as it were, by
long continuance of constant smoke exhalations, is created, inimical to
growth of plants, from its containing in excess large amounts of
sulphurous and acid vapours.
Sometimes the effects so
produced are individually sufficient to constitute a serious sanitary
objection to the continuance of the works which cause them; but in many
instances, where the intelligent practical application of chemical science
and engineering has been called in to aid in mitigating or entirely
removing the evils complained of, success has been achieved, without
injuriously interfering with the industrial pursuits of those engaged in
the particular manufacture; and it is to be hoped that in many other
cases, where no remedial measures have as yet been
attempted,—manufacturers will yet see it to be their duty,—while carrying
on advantageously to themselves their commercial enterprizes,—to use every
effort which science and experience may discover or suggest, to mitigate
evils inseparably connected with their processes, but which are capable of
amelioration in the interests of the amenities of nature and their
neighbourhood generally.
With the clamant evils of
river pollution, the government, last session, endeavoured (unfortunately
without success) to grapple, but the subject is so important, that once
fairly roused, as it has been, it will probably again form matter for
legislation, until successfully settled; and in that way an immense source
of destruction to vegetable life will be put an end to. With the gaseous
vapours and smoke discharges from large public works polluting the air for
miles around, the question is a more difficult one, and a problem of
greater intricacy, and it is with the effects produced by this species of
destruction to trees and shrubs that we have in this paper more
particularly to deal at present.
Without specifying in
detail the various noxious chemical ingredients discharged from large
public works, and which differ in the degree and extent of injury which
they create, according to the nature of their chemical composition, and
vary according to the special description of work or process whence they
emanate, it may be generally stated that the primary and chief destruction
to vegetation in the neighbourhood of such works, or large towns, is
caused by the smoke discharges from them, arising from the combustion of
coal in furnaces, and by the gases and vapours that are produced during
metallurgical processes of the chemical arts and manufactures. Vegetable
life, in general, is very susceptible of impurities existing in the air
around it, and even slight modifications in the proportions of its
constituent parts in their natural forms, more or less injuriously affect
all growths, and chiefly those in shady or sheltered spots where there may
be, to some extent, want of light, or defective circulation of air. The
wonder, therefore, rather should be, that in the vicinity of large cities
and manufacturing centres of industry, and sometimes even in their very
midst, we see trees and shrubs existing as they do, and sustaining life
under such abnormal conditions, with comparative hardihood. But, as will
be afterwards explained, nature placed in circumstances so uncongenial may
be artificially assisted in many ways, and induced, with a little
attention and care to her subjects, to cheer and brighten with her
freshness and greenery many a dismal town square or city walk.
In the immediate
neighbourhood of chemical works, from which, by day and by night in
ceaseless volume, large quantities of poisonous exhalations are poured
forth into the atmosphere, the living functions of vegetation and
tree-life are much more seriously crippled than by the mere smoke from
coal-consumption near towns, and in such situations herbage of all kinds
is stunted and browned, the very grass lingers on a feeble existence,
trees are leafless and withered, and in a very few years cease to live.
The same remarks apply to plants and trees in close proximity to the
calcining hearths of ironstone pits and blast furnaces. The discharge from
these of smoke strongly impregnated with the sulphate of alumina is highly
deleterious to all life. Blown by a strong and steadily prevailing wind
across any district, however fertile and highly farmed and cultivated it
may be, the pernicious effects to cereals and green crops is most
apparent, and cause great deterioration and damages annually to many
farmers. In many parts of Lanarkshire, Fife, and Stirlingshire, the losses
caused by these effects are severely felt, and although compensation by
pecuniary payment be made in most cases where the damage has been proved,
the indirect deterioration to the farmer, from lack of straw, and head,
and bulb, from their more stinted growth, caused by the agency of this
nuisance, is not compensated for by any pecuniary allowance. Again, in
some districts in Ayrshire and West Lothian, as well as in Lanarkshire,
the damage caused to dairy farms from the destruction to their produce by
the deposit of soot-flakes, from these and other works, such as the shale
oil manufactories, is very great; and it can easily be understood that the
permanent injury to the perennial occupants of the ground, such as trees
and shrubs, must be also very marked. If it were not for the presence of
the chemicals already referred to, in the smoke issuing from these and
kindred works, the mischief would not be nearly so serious, as the
winter's snow and rain bleach the merely smoke-begrimmed trunk and
branches, and thus clear out the plugged-up pores to a great extent
periodically.
Probably the most fatal
enemy to plant-life is an acid vapour termed hydrochloric acid (a mixture
of hydrogen and chlorine). It is evolved among the dense white fumes
issuing from most chemical works, and in a very short time causes absolute
sterility to all land within its influence. Its poisonous effects upon a
tree or shrub are first observed by the shrivelling up and drooping of the
leaves, which in a few days are shed. The young wood buds lose their
plump, healthy aspect, becoming-scaly and falling off; the twigs then
present a dead appearance, which soon spreads itself down the branches,
the bark cracks and shrivels off, here and there on the stem, a young bud
may in spring be seen struggling in the vain effort to develop itself into
a new branch to sustain the failing vitality, but very soon these last
attempts at self-resuscitation cease, and the tree dies. The effects
produced upon tree and shrub life by smoke from public works are similar
in operation to those now described, and when the functions of the leaves
in vegetable economy are considered, it will be easily understood how the
health of plants is interfered with by these effects.
As the function mainly of
the foliage of a tree or shrub, is to expose the secretions of the plant
to light and air, and so to assist in the formation of wood, if the leaves
are coated over by dense deposits of carbon and other substances, a film
is formed sufficiently opaque to prevent the free action of light and air
upon the leaves, and hence the secretions necessary for the formation of
wood-buds are impeded in their development. Hence we see how stunted in
stature, and in thickness of trunk, are trees in the vicinity of towns as
contrasted with others of similar age planted in the open country, or in
rural districts with an uncontaminated atmosphere.
Again, leaves have another
most important function to perform in the healthy economy of trees and
shrubs. They daily give off into the atmosphere large quantities of watery
fluid by the process which they perform, called in technical terms
exhalation, The importance of this function in relation to smoke deposit
on the leaves, must be at once apparent. The moisture on the surface of
the leaf, created by its healthy action, at once attracts the minute
particles of carbon and other injurious ingredients which thereby adhere
to the leaves, and, coating them over, at once arrest the healthy process
of exhalation over the whole system of the plant; for as the leaf surface
cannot find vent for the secretions pumped up from the root action, this
latter function is next impaired, and a drag, so to speak, is put upon its
healthy flow. The humidity of air which trees promote in their immediate
proximity, by the process of exhalation, is very materially lessened by
its cessation, and the moist and cool condition of soil, which is so
essential to their welfare, is in its turn affected, and the direct action
of the sun's rays has more power and opportunity to cause undue
evaporation. And with a decreased healthy action of foliage, or with a
scant crop of leaves induced by the effects of such substances as have
been referred to, another very important agency in sanitary economy is
interfered with; for while leaves, doubtless, in the first instance, by
the healthy action of their various functions, act for their own benefit;
they, at the same time materially influence and conduce to the purity of
the atmosphere itself, by their absorption of carbonic acid gas, which in
excess is so injurious and fatal to animal life. Under the influence of
light, leaves in a healthy state, and young green twigs and shoots of
shrubs and trees, decompose this deleterious gas, and so contribute
largely to the preservation of the purity of the air for man's own
breathing.
But not only upon the
leaves of trees and shrubs are the effects of smoke and other deleterious
substances apparent, nor is it through their medium alone that the life of
the plant is destroyed. The lateral air-vessels which penetrate the bark
and stems of trees horizontally for the purpose of "oxygenating," as it
may be called, the juices and secretions, form another channel by which
the injurious and subtle poisons are conveyed into the system of the
plant. The plugging-up of the mouths of these surface-absorbents prevents
the inhalation by the bark of the moisture of the atmosphere, and in this
way, acts prejudicially upon the growth and development of the alburnum or
sap-wood, as well as upon the expansion of the new buds, and consequently
upon the ultimate growth of the tree itself. A familiar instance may be
given of the injury which accrues to a tree by the adherence of a foreign
substance to the surface of the bark, if we notice the effects produced by
smearing the stem with coal-tar, oil, or paint, so as to completely stop
up the spiracula or pores of the bark for some considerable length up the
trunk from the ground. The result will be found to correspond precisely
with what has been stated with regard to smoke clogging the bark from
atmospheric causes. The new buds will first be destroyed, and finally the
tree itself.
Upon the various
descriptions of trees and shrubs the effects of smoke or a polluted
atmosphere are not always equally severe or rapid. Doubtless plants, like
animals, may individually vary in regard to the delicacy of their
constitution, and while some are much more susceptible than others of the
alterations in the temperature and humidity of the atmosphere, it is quite
possible that some plants may be injuriously, if not even fatally,
affected by the presence of foreign inimical matter in the air, or soil,
or food, from which others of the same species, of a hardier and more
robust constitution, would scarcely suffer. But independently of this,
there are species of trees and shrubs which are found to thrive better
than other species do when placed by the force of circumstances in
situations inimical to their nature, or under conditions which injuriously
affect plant life generally.
Coniferous trees and shrubs
will not thrive—the phrase must be taken as comparative—in any polluted
atmosphere; for as we have shown that trees and shrubs breathe through
both the bark and the leaf, such species as exude or secrete gum or resin
in any quantity are peculiarly liable to suffer from polluted air. In the
case of such trees, soot and such-like substances cling to them, and when
their resinous exudations become coated with carbon, the rain has no
effect in removing it, but rather the reverse, as it washes down aqueous
volumes of soot-impregnated water upon their branches and bark. In like
manner, hard-wooded trees, possessing gummy or sticky buds and leaf
sheaths, suffer in a similar way; and should not be planted in the near
proximity of large public works, or in the open spaces in the midst of
densely populated towns.
The varieties of trees and
shrubs which will be found to thrive best (again using the phrase in a
comparative sense) in smoky atmospheres, are deciduous hardwoods with
smooth leaf-surfaces and scaly barks ; as for example, trees like the
plane or the maple. From the smooth upper surface of the leaf, much of the
coating deposit which settles there is easily washed off by rain, while
the constantly peeling off habit of the scales of the outer bark keeps up
a recuperative process in the cells and pores communicating with the
alburnum and inner bark surface for the oxygenation of the sap.
Many evergreens also
possess remarkable vitality and power of resisting the baneful influences
of atmospheric pollution. As a rule, such plants as possess a thick
leathery leaf with smooth surface endure the effects of smoke with
greatest impunity; such, for example, as the common ivy and Aucuba
japonica, whose glossy leaves are easily cleansed, and consequently the
epidermis is kept in a more healthy condition from the action of the rain,
than is the case with those plants and bushes with a downy or hairy
foliage. But although some evergreens may succeed well in smoky
atmospheres, there seems no doubt that, owing to the complete renewal
annually of their foliage, deciduous trees and shrubs have a marked
advantage in those situations over the evergreen species; and where it is
intended to plant under such trying circumstances, deciduous trees and
shrubs should be preferred, and such varieties of them as we have
indicated as possessing special qualifications for withstanding the
injurious effects of an impure atmosphere should be selected.
The following list
comprises such trees and shrubs as seem most adapted for culture in these
situations:—Platanus occidentalis, Acer eriocarpum, Populus balsamifera,
Populus fastigiata, Populus alba, Quercus ilex, Tilia europrea, Fraxinus,
Robinia, Cytisus Laburnum, Syringa, Ulmus, Ligustrum, Vinca, Viburnum
Tinus, Philadelphus, Crataegus, Ampelopsis hederacea, Clematis, Aucuba
japonica, Ailanthus glandulosa, Ficus Carica, Cydonia japonica, Hedera
helix, Jasminum officinale, Rhamnus alaternus, Ribes sanguinea, Sophora
japonica, Ilex aquifolium, Sambucus, and Leycesteria formosa.
Of course, in naming these
it must be quite understood that their success in such situations is
merely comparative, and such of them as may be classed as flowering shrubs
cannot be expected to blossom at all freely under the disadvantageous
circumstances of their situation.
But it may be said, are
there no remedies for the pollution of the atmosphere by public works and
smoke-creating nuisances? This is a very delicate question, but one which
will stand discussion, inasmuch as the Public Health Acts, the Smoke
Consumption Acts, the Alkali Acts (in England), all seem more or less to
have been framed for the suppression of such noxious discharges as poison
air and destroy the amenity of whole neighbourhoods. A more stringent
application of their enactments and enforcement of their provisions would
doubtless in many places tend greatly to the mitigation, if not the
abolition, of the evils complained of, where the manufacturers themselves
(the pollutors) are disposed to act fairly and liberally. But in many
other instances, such, for example, as calcining of ironstone, it is next
to impossible, without absolutely stopping the process, to cure the evil.
No doubt, when the injury to crops became very serious the operation of
calcining might be prohibited, excepting during the winter season or
non-growing period of the year, when there would be no risk of damage to
crops, whether cereal or otherwise. Indeed this remedy is practised in
some districts where the ironstone is wrought in connection with coal, and
is found in too thin a state, or in such small quantities that the damages
to be paid for smoke pollution to crops would outrun the value of the
calcined mineral. The stone is accordingly raised with the coal and
"binged" on the hearth till the harvest is carried, when the fires are lit
and the process completed.
Much, however, might be
done by artificial means to aid nature in many instances when placed in
situations so uncongenial to her well-being, as in the midst of towns or
in their immediate vicinity. A very few adventitious appliances of no
costly description would tend greatly to compensate to many town gardens
and shrubberies for the conditions inseparable from their lot. These may
be briefly summed up in the following recommendations:—(1) Give more free
space around each individual tree; (2) Prune early, regularly, and
judiciously; (3) Soak well during drought the entire garden; and (4)
Top-dress with fresh soil over the roots and shrubberies occasionally. The
first is rendered absolutely necessary by the confined circulation of air
in town situations and vitiated conditions of the atmosphere, combined
with the frequent absence of light in such localities ; and which is so
essential an element in tree-growth. The second suggestion is proposed,
with a view to the proper balance of head and due development of upright
stem, and to prevent a flat-headed habit being acquired, to which trees in
such sites are extremely prone—a habit which results in one or more of the
stronger-growing side branches outrunning their neighbours and interfering
with the upright form of the tree. Branch and stem pruning, and
foreshortening the upper tiers of branches in the head, will be found most
beneficial in trees so situated, when about from twelve to twenty feet in
height; although gentle hand pruning should be commenced at a much earlier
stage in their growth. The third recommendation we have made is a very
important one to all trees which suffer from the smoke about towns, and
although the moisture of our climate and frequency of rains mitigate to
some extent the evils produced by the sooty deposits, nevertheless much
may be done by judicious watering, especially to shrubs. A short supply of
water at the roots, causing the premature shedding of the leaves before
they have fulfilled their functions—a process which they require longer
time to perform, owing to their vitiated and debilitated condition from
the smoke and soot—is the principal cause of disease and stunted growth of
town trees. Drains, both surface and underground, and hard paved streets,
and walks impervious to water in the vicinity of tree roots,—as must be
the case more or less in all large towns,—draw off the rainfall which in
natural circumstances would go to supply the root moisture, and
consequently in most situations of the nature referred to, only a very
limited quantity ever reaches the rootlets and spongeoles at all. Thus
summer watering and a thorough soaking, in dry, dusty weather, which could
be quite easily accomplished in any town public gardens by the aid of the
fire brigade engines, would be of immense value in restoring the proper
equilibrium of moisture and health to the plants in such places, and an
experiment which would repay itself in the enhanced amenity of the walks
and gardens. The fourth and last suggestion made for the amelioration of
tree and shrub culture in smoky localities, namely,—top-dressing the roots
occasionally with fresh loam, is rendered necessary in such situations, by
the loss which the soil sustains every season by the removal of the fallen
leaves which in ordinary circumstances in the forest or park, would go to
nourish and refresh as manure the surface of the ground, whence the
decomposed ingredients are carried down by the winter's rainfall and
action of frost into the soil, forming food for those young rootlets near
the surface, which, ramifying amongst the otherwise exhausted earth
acquire a stunted growth, and will produce impaired root action unless
Nature's own and recuperative process, when removed by the force of
circumstances, is replaced by artificial means. |
