Darkness and poison! Does not this sound worse than a plague of Egypt? Yet we town-folk suffer it without much grumbling, and scientists spend as much time in learning what the poison consists of, and in tracing exactly how the injuries come about, as would suffice, one would imagine, to discover a cure. Oddly enough, more poisons are found in fog than even coal-burning altogether accounts for; the exact nature of some of the substances which are present in the atmosphere of foggy weather is a matter about which scientists themselves confess to ignorance.
Still, there is one thing on which all agree, and that is the perfect harmlessness of clean mist. Neither mountain nor country mists do any wrong to plant life, and from the coasts of Kent and Sussex, Essex and Norfolk, come assurances of the innocuous character of sea-fogs.
Of the known impurities of town-fog the following list gives most of those suspected of being inimical to plants. “Suspected” is the scientific way of putting it. Our scientists are wary; they must be, for they know how everybody weighs their words; and besides that, they can never be sure what fresh discoveries will be made to-morrow; the latest are oftentimes upsetting.
The amount of miscellaneous ingredients that enrich a London fog is startling. Our list is taken from an analysis of the deposits left on the glass roofs of plant-houses at Chelsea and Kew during the severe fogs of February, 1891:—
Carbon, hydrocarbons, organic bases, sulphuric acid, hydrochloric acid, ammonia, metallic iron, and magnetic oxide, with other mineral matter, chiefly silica and ferric-oxide. Sulphuric acid, it seems, is the principal cause of injury to trees and shrubs, and sulphurous acid to herbaceous and soft-wooded plants.
The effects of fog are seen sometimes in the breaking-down of the plant, sometimes in its discoloration; leaves gradually turn yellow, progressing from below upwards, and they drop off in the order in which they showed the change of colour. Thus two things have happened: destruction of the green colouring matter, and structural injury at the point where leaf meets stalk. Where is the London flower-grower who has not watched these processes with sad eyes? When an ill wind blows soot-laden fog towards Kew or Chelsea—places where so many of our choicest plants and trees and flowers are cherished—loud are the lamentations because of damage done.
Mr. Watson, assistant curator of the Royal Kew Gardens, says he gathers up bushels of leaves in the palm-houses every morning while a bad fog lasts, and after a long spell of it many hard-wooded as well as the more delicate plants are reduced to an unsightly condition of almost bare stems, blotched and discoloured leaves, and fallen foliage. Among certain groups even the soft stems disarticulate at the nodes.
Mr. W. Thiselton-Dye, Director of Kew Gardens, describes the substance deposited on his glass-houses as a solid brown paint, weighing about twenty-two pounds to the acre, or six tons to the square mile. This makes our fog enemy appear a very real thing indeed; no wonder it breaks plants down, and is the ruin of many fruit and floral industries in the south of London.
Are there any means by which town cultivators may counteract these malign influences? Only by very expensive appliances. The grower wants an air-tight greenhouse, with definite openings where the admitted air can be filtered. Filtering foggy air may counteract or even keep out poison, but even then one has to make up for the darkness. This can only be done by a generous installation of electric light.
