Page:Journal of botany, British and foreign, Volume 9 (1871).djvu/107

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nortlierly winds at Malta are peculiarly characterized by excessive cold and dryness. On the other hand, its proximity to Africa exposes it in summer to scorching- blasts of heated air. These drawbacks are felt in their fullest extent, owing to the almost complete absence of trees on the island. The influence of forests on climate has been made a subject of investigation by meteorologists of late years, and though much still remains to be done, yet some important points have been established. The highest temperature of the air occurs in summer between two and three p.m., but trees do not attain their highest temperature till nine p.m. Changes of temperature take place slowly in the tree, but in the air they are rapid. Hence trees may be regarded, like the ocean, as powerful equalizers of temperature, in moderating the heat of the day, and in maintaining a higher temperature during the night. Since air is heated by contact with the soil, and since trees shield the soil from solar radia- tion, it is evident that trees diminish the force of the sun's rays, especially in the lower stratum of the atmosphere, which is breathed by man. Trees exhale moisture, and thus produce cold in the air by the latent heat ab- stracted from it. From this lowering of the temperature, and from the moisture which is exhaled, dry winds acquire greater relative humidity, and thus are deprived of much of their noxious influence; and since trees break the force of the wind, their beneficial influence is greatly augmented. During night the process of terrestrial radiation lowers the temperature of a tree at a slow rate. First the upper leaves are cooled, then tliose leaves immediately under, and so on, until the whole are cooled. Nov\^ in the earlier part of the day, before the tree is heated by the sun, its cool leaves present a very large surface to the air currents which pass through them. Hence the cooling influence of trees is very considerable, which all must have experienced in the deliciously cool breezes of well-planted parks on a warm summer day. This refrigerating influence of trees is sometimes well seen in the earlier part of the day, when the air is filled with fog. In such cases heavy drops of water fall from the trees and increase, on occasions, to the copiousness of a heavy shower; and, doubtless, when the air is saturated, the rainfall will be heavier when the wind advances on a forest, whose temperature is several degrees lower than that of the surrounding district, where there are no trees. Hence, then, it may fairly be inferred, if it has not been indeed proved, that trees bring about a different distribution of the rainfall, as respects the time of the day and the season of the year. Trees serve another important use. When rain falls on so dry and bare a soil as that of Malta, it runs ott" at once, and is lost in useless, if not destructive floods. But since the roots of trees penetrate the soil, and so loosen it, and render it porous, much of the ram is not only received and preserved by the trees, but what falls tq the ground is allowed to sink into the soil, and fill the reservoirs of the deep- seated springs. And since, owing to the stillness and greater dampness of the air among trees, the evaporation from forest soil is oidy about a fifth of what it is in an open country, woods ngulate the flow, and retard, if they do not altogether prevent the drying up of springs. — "Notes on the Structure and ]\Ieasurements of Cells in the Uepnilcfe. l>y James AVilliamson Edmond, M.B., CM. The author described the characters and measurements of the leaf-cells of twenty-six species of British llepa- tic(jP, he also gave the measurements of the spores and elaters of several of the species. He considered that, owing to the great variation

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