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The facts hitherto collected upon this subject are so scanty and unsatisfactory, that I introduce its consideration in this place, rather with a wish to excite farther enquiry, than with any hope of imparting much additional information.

There can be little doubt, but that Soil, Culture, Climate, and Season,^[118] may very materially influence the active properties of a medicinal plant; while the two latter of these causes may as essentially change the type and character of a disease, and modify the vital susceptibility of the patient; the natives of the south of Europe, for instance, do not bear bleeding, and other modes of depletion, so well as those of the north. This must be admitted to its full extent, or it will be extremely difficult to explain the contradictory and even opposite opinions, and to reconcile the conflicting testimonies of the physicians of different countries, with respect to the efficacy of the same remedy, in similar diseases.

The Influence of Soil may be exemplified by many well known facts; thus, strongly smelling plants lose their odour in a sandy soil, and do not again recover it by transplantation into a richer one; a fact upon which Rozier founded his proposal for the improvement of Rape oil; so again, no management could induce the Ricotia Ægyptiaca to flower, until Linnæus suggested the expediency of mixing clay with the earth in the pot; Assafœtida is one of those plants that vary much according to station and soil, not only in the shape of the leaves, but in the peculiar nauseous quality of the juice which impregnates them, and Dr. Woodville states that it is frequently so modified that the leaves are eaten by goats; Gmelin informs us, on the authority of Steller, that the effects of the Rhododendron have been found to vary materially according to the “solum natale;” for example, that produced in a certain spot has proved uniformly narcotic, that in another, cathartic, while a sense of suffocation has been the only symptom occasioned by a third. Rhubarb, as grown in England, will differ greatly in its purgative qualities, according to the soil in which it may have been cultivated; that produced in a dry gravel being more efficacious than that which is reared in a clayey one. Dr. Carter, in his account of the “Principal Hospitals of France, Italy, and Switzerland,” tells us that at Nice, the Digitalis is commonly given in doses of a scruple in powder, or in that of half an ounce of the infusion made according to the London Pharmacopœia, every hour, and without any sensible effect; this fact he explains by stating that the Digitalis, in the neighbourhood of Nice, is much smaller, and is probably less powerful than the same plant as it grows in England.

Climate also produces a powerful impression upon vegetable and animal life. It is probable that in southern countries some vegetables enjoy more energetic properties than in northern climes. The history of opium immediately countenances such an opinion; thus Egypt produces a stronger opium than any of the countries on the north side of the Mediterranean,—France, than England or Germany;—and Languedoc, than the northern parts of France;—while Smyrna, Natolia, Aleppo, and Apulia, furnish a juice far more narcotic than Languedoc: so again, Senna by transplantation from Arabia into the south of France (Provence) assumes a marked change in its physiognomy and virtues, its leaves are more obtuse, and its taste less bitter and nauseous than the pointed leaved variety, while its effects will be found to be less purgative. Cruciform plants degenerate within the tropics, but acquire increased energies, as Antiscorbutics, in cold regions; the Menthæ have not so penetrating an essential oil in the south of Europe as in England and in the north of France. The relative proportions of gluten vary in the wheat of different countries, and as in the south of Europe, its quantity greatly predominates over the other principles, we at once discover the cause that gives such excellence to the Maccaroni of Italy. Many species of plants secrete juices in warmer regions, which are unknown in their œconomy, in colder climates; thus the Ash yields Manna in Calabria, but loses that faculty as it advances towards the north. The influence of climate, in its relations to moisture and dryness, upon vegetable productions, is also worthy of investigation; in wet and cold seasons, our herbage is far less nutritive to cattle, and we accordingly find that they are constantly grazing, in order to compensate by quantity, for what is deficient in quality, whereas in dry seasons, a larger proportion of their time is consumed in rumination; the same causes, however, that diminish the nutritive powers of plants, frequently increase the energy of those principles upon which their medicinal value depends: it is obvious that many herbs are more rank and virulent in wet and gloomy seasons: this would appear to be a wise and provident law, in order to apportion the natural condiment of the vegetable, to the deteriorated state of its nutritive elements, when the digestive organs must require more than the ordinary stimulus for the due exercise of their functions. It is hardly necessary to observe that plants, which in temperate climates are merely shrubs, have been developed into trees, by the hot and humid plains of Africa and Asia; while in the arid deserts of Nubia or in the frigid plains of Siberia, vegetable life is confined to stunted shrubs and humble mosses: cold also suppresses the colour of flowers, and indeed even that of the leaves, as is witnessed in the Cyclamen, Amaranthus, and Ranunculus of Lapland and Siberia. But climate not only modifies the powers of a remedy by influencing its structure and composition, but it renders it more or less active, by increasing or diminishing the susceptibility of the body to its impression; can a more striking proof of this fact be adduced than the well known effects of perfumes at Rome? The inhabitants are unable to sustain the strong scent of flowers in that climate, without experiencing a sensation highly oppressive, and which in some cases is even succeeded by syncope,^[119] and thus realising the well known line of the poet,