Among diseases which are undoubtedly caused by floating matter in the air must be reckoned the well-known malady "hay fever," which is a veritable scourge during the summer months to a certain percentage of persons, who have, probably, a peculiarly sensitive organization to begin with, and are, in a scientific sense, "irritable."
This disease has been most thoroughly and laboriously investigated by Mr. Charles Blackley, of Manchester, who, being himself a martyr to hay fever, spent ten years in investigating the subject, and published the result in 1873, in a small work entitled "Experimental Researches on the Causes and Nature of Catarrhus æstivus (hay fever or hay asthma)."
Mr. Blackley had little difficulty in determining that the cause of his trouble was the pollen of grasses and flowers, and his investigations showed that the pollen of some plants was far more irritating than the pollen of others. The pollen of rye, for example, produced very severe symptoms of catarrh and asthma, when inhaled by the nose or mouth. Mr. Blackley came to the conclusion that the action of the pollen was partly chemical and partly mechanical, and that the full effect was not produced until the outer envelope burst and allowed of the escape of the granular contents.
Having satisfied himself that pollen was capable of producing all the symptoms of hay fever, Mr. Blackley next sought to determine, by a series of experiments, the quantity of pollen found floating in the atmosphere during the prevalence of hay fever, and its relation to the intensity of the symptoms. The amount of pollen was determined by exposing slips of glass, each having an area of a square centimeter, and coated with a sticky mixture of glycerine, water, proof spirit, and a little carbolic acid. Mr. Blackley gives two tables, showing the average number of pollen grains collected in twenty-four hours on one square of glass, between May 28 and August 21, in both a rural and an urban position. The maximum both in town and country was reached on June 28, when in the town 105 pollen grains were deposited, and in the country 880 grains. The number of grains deposited was found to vary much, falling almost to zero during heavy rain and rising to a maximum if the rain were followed by bright sunshine. Mr. Blackley found that the severity of his own symptoms closely corresponded to the number of pollen grains deposited on his glasses. Mr. Blackley devised some very ingenious experiments to determine the number of grains floating in the air at different altitudes. The experiments were conducted by means of a kite, to which the slips of glass were attached, fixed in an ingenious apparatus, by means of which the surface of the glass was kept covered until a considerable altitude had been reached. Mr. Blackley's first experiment gave as a result that 104 pollen grains were deposited in the glass attached to the kite, while only 10 were deposited on a glass near the ground. This experiment was repeated. Again and again, and always with the same result, there was more pollen in the upper strata of the air than in the lower.
A very interesting experiment was performed at Filey, in June, 1870. A breeze was blowing from the sea, and had been blowing for 12 or 15 hours. Mr. Blackley flew his kite to an elevation of 1,000 feet. The glass attached to the kite was exposed for three hours, and on it there were 80 grains of pollen, whereas a similar glass, exposed at the margin of the water, showed no pollen nor any organic form. Whence came this pollen collected on the upper glass? Probably from Holland or Denmark. Possibly from some point nearer the center of Europe.
POTATO DISEASE.
A study of the terrible disease which so often attacks the potato crop in this country will serve, I think, to bring forcibly before you certain untoward conditions which may be called climatic, and which are attributable to fungoid spores in the air.
With the potato disease you are all, probably, more or less practically acquainted. When summer is at its height, and when the gardeners and farmers are all looking anxiously to the progress of their crops, how often have we heard the congratulatory remark of "How well and strong those potatoes look!" Such a remark is most common at the end of July or the beginning of August, when the green part, or haulm, of the plant is looking its best, and when the rows of potatoes, with their elegant rich foliage and bunches of blossom, have an appearance which would almost merit their admission to the flower border. The same evening, it may be, there comes a prolonged thunder storm, followed by a period of hot, close, moist, muggy weather. Four-and-twenty hours later, the hapless gardener notices that certain of his potato plants have dark spots upon some of their leaves. This, he knows too well, is the "plague spot," and if he examine his plants carefully, he will perhaps find that there is scarcely a plant which is not spotted. If the thunder shower which we have imagined be followed by a long period of drought, the plague may be stayed and the potatoes saved; but if the damp weather continue, the number of spotted leaves among the potatoes increases day by day, until the spotted leaves are the majority; and then the haulm dies, gets slimy, and emits a characteristic odor; and it will be found that the tubers beneath the soil are but half developed, and impregnated with the disease to an extent which destroys their value.
Now, the essential cause of the potato disease is perfectly well understood. It is parasitical, the parasite being a fungus, the Peronospora infestans, which grows at the expense of the leaves, stems, and tubers of the plant until it destroys their vitality. If a diseased potato leaf be examined with the naked eye, it will be seen that, on the upper surface, there is an irregular brownish black spot, and if the under surface of the leaf be looked at carefully, the brown spot is also visible, but it will be seen to be covered with a very faint white bloom, due to the growth of the fungus from the microscopic openings or "stomata," which exist in large numbers on the under surface of most green leaves. The microscope shows this "bloom" to be due to the protrusion of the fungus in the manner stated, and on the free ends of the minute branches are developed tiny egg shaped vessels, called "conidia," in which are developed countless "spores," each one of which is theoretically capable of infecting neighboring plants.
Now, it is right to say that, with respect to the mode of spread of the disease, scientific men are not quite agreed. All admit that it may be conveyed by contact, that one leaf may infect its neighbors, and that birds, flies, rabbits, and other ground game may carry the disease from one plant to another and from one crop to another. This is insufficient to account for the sudden onset and the wide extent of potato "epidemics," which usually attack whole districts at "one fell swoop." Some of those best qualified to judge believe that the spores are carried through the air, and I am myself inclined to trust in the opinion expressed by Mr. William Carruthers, F.R.S., before the select committee on the potato crop, in 1880. Mr. Carruthers' great scientific attainments, and his position as the head of the botanical department of the British Museum, and as the consulting naturalist of the Royal Agricultural Society, at least demand that his opinion should be received with the greatest respect and consideration. Mr. Carruthers said (report on the potato crop, presented to the House of Commons, July 9, 1880, question 143 et seq.): "The disease, I believe, did not exist at all in Europe before 1844.... Many diseases had been observed; many injuries to potatoes had been observed and carefully described before 1844; but this particular disease had not. It is due to a species of plant, and although that species is small, it is as easily separated from allied plants as species of flowering plants can be separated from each other. This plant was known in South America before it made its appearance in this country. It has been traced from South America to North America, and to Australia, and it made its first appearance in Europe in Belgium, in 1844, and within a very few days after it appeared in Belgium, it was noticed in the Isle of Wight, and then within almost a few hours after that it spread over the whole of the south of England and over Scotland.... When the disease begins to make its appearance, the fungus produces these large oblong bodies (conidia), and the question is how these bodies are spread, and the disease scattered.... I believe that these bodies, which are produced in immense quantities, and very speedily, within a few hours after the disease attacks the potato, are floating in the atmosphere, and are easily transplanted by the wind all over the country. I believe this is the explanation of the spread of the disease in 1844, when it made its appearance in Belgium. The spores produced in myriads were brought over in the wind, and first attacked the potato crops in the Isle of Wight, and then spread over the south of England. The course of the disease is clearly traced from the south of England toward the midland counties, and all over the island, and into Scotland and Ireland. It was a progress northward.... This plant, the Peronospora infestans, will only grow on the Solanum tuberosum, that is, the cultivated potato.... Just as plants of higher organization choose their soils, some growing in the water and some on land, so the Peronospora infestans chooses its host plant; and its soil is this species, the Solatium tuberosum. It will not grow if it falls on the leaves of the oak or the beech, or on grass, because that is not its soil, so to speak. Now, the process of growth is simply this: When the conidia fall on the leaf, they remain there perfectly innocent and harmless unless they get a supply of water to enable them to germinate.... The disease makes its appearance in the end of July or the beginning of August, when we have, generally, very hot weather. The temperature of the atmosphere is very high, and we have heavy showers of rain."