Page:Popular Science Monthly Volume 22.djvu/588

572 in height of the Milwaukee children is ascribed to the inferior density of population and the existence of fewer urban disadvantages in that city than in Boston; and the general hypothesis is drawn, from Peckham's tables, that the height of American-born men is more modified by the conditions accompanying density than by all other influences, race excepted, urban life as compared with rural life tending toward a decrease of stature. The rate of growth of Germans appears to be considerably modified by residence in this country through one generation; and, in intermarriage between Americans and Germans, the offspring seem to take the height of the taller parent.

Use of the Gummy Secretions of Plants.—Inquiry has often been made respecting the functions of the secretory apparatus of plants, or that which stores up special juices, such as the resins, gums, caoutchouc, milky juices, and the waxes. Sachs, even in the last edition of his botany, places these substances among those the office of which in the economy of the plant is wholly unknown. Because the secretions in question have been observed to be poor in oxygen and generally unassimilable, they have commonly been regarded as waste matter, useless to the organism. M. de Vries is of a different opinion, and regards these substances as a kind of protective salve, and considers them helpful in the healing of wounds. Of the resin of conifers, he remarks that, if it were simply a product of secretion, the accumulation of it would not cause the tree to suffer. The extraction of resin, however, weakens pines very considerably, and diminishes the growth of wood by about one third. Accidental wounds, moreover, and even normal wounds produced by the fall of limbs or by splitting of the bark, are very numerous in conifers. Whenever a wound is produced, it is forthwith covered over with a viscous and thick mass of resin, which gradually hardens in the air. Among non-resinous plants wounds become isolated by means of a pad of healing tissue which sometimes covers the wound completely over, but often too late to effect the purpose. From this point of view, M. de Vries suggests, the conifers are superior in organization to common angiospermous trees. The organism in coniferous trees seems in a manner to have foreseen possible wounds, and a system of canals designed solely to furnish a covering for wounds seems to have been differentiated in them. In a second part of his work, M. de Vries treats of the function of the juices analogous to the resins which are found in other plants, and seeks to assimilate to the resins, from different points of view, the latex, some of the gums, caoutchouc, and waxy matters, lie shows that these substances also exude for the occlusion of wounds, even in herbaceous plants like the northern chicories and spurges, and cites some recent experiments by M. Moll in favor of his view. It would, however, be a narrow judgment to conclude with him, from these experiments, that the sole object of the secretions is the healing of wounds. M. Raumhoff, criticising the work of M. de Vries, has shown that the considerations on which his theory rests do not furnish an adequate demonstration of it. It is evident, for instance, that the purpose of the lactiferous tissues can not be solely the healing of wounds, for these tissues in the spurges contain starch, a substance that does not assist in that office, and which is not a product of secretion. The studies of M. Treub on the tropical spurges furnish evidence that one of the probable offices of their lactiferous tissues is the conveyance of starch.

The Possible Annual Yield of a Forest.—The basis on which all sound forest management depends, says Colonel G. F. Pearson, is the revenue which any forest can be made to pay—that is to say, the income which it will produce in proportion to the volume of the standing trees, or, in other words, its capitalized value. To this end a forest should be considered as so much capital, represented by so many cubic feet of wood; while the amount of wood produced each year, by its growth, represents the interest thereon, and, in fact, is the revenue of the forest. It is evident that it is possible to cut and remove every year a quantity of timber equal to this annual increase of wood, without diminishing the volume of the standard crop. The possible annual yield of a forest may be estimated on the basis of a calculation that a tree, ten feet in