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Page:Popular Science Monthly Volume 18.djvu/151

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POPULAR MISCELLANY.

per cent, in the course of a few days. Professor Böhm's theory corresponds closely with that advanced by Professor Draper in his work on the circulation in plants and animals, and substantially agrees with the views of Herbert Spencer, as expressed in his paper on "Circulation and the Formation of Wood in Plants" ("Transactions of the Linnæan Society," March 1, 1866). It supposes that the sap-bearing cells in the whole plants are subject to a moderate pressure in consequence of the resistance which the water meets on its way from the root to the assimilating leaf. If, however, the branch can take up water through a cut end with little difficulty, a partial absorption of the contents of the vessels into the sap bearing cells will follow, new water will pass through the cut end from without into the vessels, and the limb will become heavier. In this process, the ducts of the willow do not serve as air-tubes, but as water-canals which pour their contents into the pump-system of the sap-bearing cells. These canals become obstructed, after the cuttings have stood for some time in the water, by the growth of cells across the tubes. As soon as the flow of water through the vessels to the higher part of the limb is thus interrupted, the rapid increase of weight ceases. That the cutting does not perish at this stage, but continues to live for several months without any considerable increase of weight, is due to the fact that after the ducts have been closed the circulation of water takes place only through the sap-bearing cells and is greatly retarded. In another series of plants, as the oaks, acacias, catalpas, amorphas, etc., the ducts of the new wood have been found to be penetrable to the air, but neither air nor water could pass through the old wood, because the older veins were closed by transverse cells or gummy substances. The vessels of these plants were really air-vessels, for they held only air of the tension of the atmosphere and were destitute of sap. Yet an uninterrupted stream of sap must be kept up in such plants from the root to the top. It takes place in the same manner as in willow-cuttings, the vessels of which have been closed by transverse cells—that is, the sap is filtrated from cell to cell, so that the balance in the pressure of the contents of the adjoining cells which has been disturbed by transpiration is restored. It follows from this that the tension of the air in the upper sap-bearing cells must be very slight to make a rising of the sap possible. The exhaustion of the air finally reaches its extreme degree at an appointed age of the cells, the air in the cells is cut off from the neighboring vessels, and the factor which produces the rise of sap is thereby eliminated. The wood, which was a living sapwood, becomes a dead heart-wood. This process is accomplished with different degrees of rapidity in different kinds of plants; even in individuals of the same species, circumstances cause many differences in the formation of heart-wood. The final result is, however, always the same, the natural death of the tree by debilitation. The thin outside layer of living wood is no longer sufficient to supply the expanded top with fluid food, no formation of new wood worthy of the name takes place, the limbs die out year by year, and finally only a feeble shoot here and there, with a few leaves of a strangely light color, indicates that there is still a little life in the stem, and this is destined soon to be extinguished. Those trees whose vessels continue to be filled with water in their old age, as the willows, birches, lindens, horse-chestnuts, etc., do not die in this manner, but through a dissolution of their sap-bearing vessels and wood-cells, opening the way for the introduction of fungi-, which settle within them and attack their substance. The process of decomposition spreads and the wood is gradually reduced to dirt, till the tree finally falls or is blown to the ground.

 

The French Association.—The French Association for the Advancement of the Sciences held its ninth annual meeting at Rheims. The opening address was delivered by the President, M. Krantz, who referred to the growth of the Association since its organization, just after the close of the Franco-German war, and to the results of the Great Exposition of 1878. The progress of the Association has been continuous and marked from year to year, and it now numbers thirty-one hundred and fifty-six adherents. It has a capital exceeding three hundred thousand francs, and