1090. Temperature has the most obvious influence on vegetation. Every one knows that the plants of hot countries cannot in general live in such as are cold, and the contrary. The wheat and barley of Europe will not grow within the tropics; the same remark applies to plants of still higher latitudes, such as those within the polar circles, which cannot be made to vegetate in warmer latitudes; nor can the plants of hot latitudes be made to vegetate in colder ones, without the aid of artificial heat. In this respect, not only the medium temperature of a country ought to be studied, but the temperature of different seasons, and especially of winter. Countries where it never freezes ; those where it never freezes so strongly as to stagnate the sap in the stems of plants; and those where it freezes sufficiently to penetrate into the cellular tissue; form three classes of regions in which vegetation ought to differ. But this difference is somewhat modified by the effect of vegetable structure, which resists, in different degrees, the action of frost; thus, in general, trees which lose their leaves during winter resist the cold better than such as retain them; resinous trees more easily than such as are not so; herbs of which the shoots are annual and the root perennial, better than those where the stems and leaves are persistent; annuals which flower early, and whose seeds drop and germinate before winter, resist cold less easily than such as flower late, and whose seeds lie dormant in the soil till spring. Monocotyledonous trees, which have generally persistent leaves and a trunk without bark, as in palms, are less adapted to resist cold than dicotyledonous trees, which are more favourably organised for this purpose, not only by the nature of their proper juice, but by the disposition of the cortical and alburnous layers, and the habitual carbonisation of the outer bark. Plants of a dry nature resist cold better than such as are watery; all plants resist cold better in dry winters than in moist winters; and an attack of frost always does most injury in a moist country, in a humid season, or when the plant is too copiously supplied with water. Hence, after warm dry summers, when plants have ripened their wood properly and the watery particles have evaporated, exotic trees and shrubs will bear a much greater degree of frost than they can do after cold moist summers, during which the wood has not ripened. Some plants of firm texture, but natives of warm climates, will endure a frost of a few hours' continuance, as the orange at Genoa (Humboldt, De Distributione Plantarum); and the same thing is said of the palm and pine-apple, facts most important for the gardener. Plants of delicate texture, and natives of warm climates, are destroyed by the slightest attack of frost, as the Phaseolus, Tropï¾µ'olum, Pelargonium, Dahlia, &c. The temperature of spring has a material influence on the life of vegetables ; the injurious effects of late frosts are known to every cultivator. In general, vegetation is favoured in cold countries by exposing plants to the direct influence of the sun; but this excitement is injurious in a country subject to frosts late in the season: in such cases, it is better to retard than to accelerate vegetation. The temperature of summer, as it varies only by the intensity of heat, is not productive of so many injurious accidents as that of spring. Very hot dry summers, however, destroy many delicate plants, and especially those of cold climates. A very early summer is injurious to the germination and progress of seeds; a short summer to their ripening, and a prolonged one on the contrary. Autumn is an important season for vegetation, as it respects the ripening of seeds ; hence, where that season is cold and humid, annual plants, which naturally flower late, are never abundant, as in the polar regions; the effect is less injurious to perennial plants, which generally flower earlier. Frosts early in autumn are as injurious as those which happen late in spring. The conclusion, from these considerations, obviously is, that temperate climates are more favourable to vegetation than such as we either extremely cold or extremely hot. But the warmer climates, as Keith observes, are more favourable upon the whole to vegetation than the colder, and that nearly in proportion to their distance from the equator. The same plants, however, will grow in the same degree of latitude, throughout all degrees of longitude, and also in correspondent latitudes on different sides of the equator ; the same species of plants, as some of the palms and others, being found in Japan, India, Arabia, the West Indies, and part of South America, which are all in nearly the same latitudes; and the same species being also found in Kamschatka, Germany, Great Britain, and the coast of Labrador, which are all also in nearly the same latitudes. (Willdenow, p. 374.)
