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Page:Popular Science Monthly Volume 18.djvu/586

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THE POPULAR SCIENCE MONTHLY.

cessions of gypsum and repeated heavy dews the accumulation of plaster goes on increasing, the cracks are enlarged, and in time the stone is split.

 

Variations in the Forms of Water-Plants.—Dr. W. Behrens, of Brunswick, has published a preliminary report of researches which he has been making into the influence of the movement, and other physical relations, of water upon the plants growing in it. He observes that water-plants, both those that grow submerged and those which appear on the surface, are subject to a variety of modifications in the forms of their stems, leaves, and other organs, according as the water in which they grow is in more or less lively motion. Plants which grow in a stream, and are rooted in the ground, seem to receive a kind of pull from the moving force of the water, which is proportioned to the speed of the current. If plants which grow indifferently in standing, moderately moving, and swift waters, exhibit variations which are constant for the same kind of waters, the conclusion is allowable that the variations are produced by the kinetic influence of the waters. Plants which are met in only one kind of waters do not exhibit equivalent variations. The common frogbit, of Europe, which floats on the surface of still waters, has its leaves always of the same broad kidney-form. The pond-weed, which grows both in still and running waters, exhibits, on the other hand, manifold variations. The most common form has floating, oval leaves, the diameters of which are to each other as 1 to 1·5. The leaves of those varieties that grow in running water are longer and narrower in proportion to the swiftness of the stream; and one form is mentioned in which the diameters of the leaves are as 1 to 3. The water-ranunculuses (Batrachium) afford excellent examples of the influence of running water on the forms of plants. Several species are found in all kinds of waters, the most of them having leaves of two forms, peltate floating leaves, and much divided submerged leaves. They grow in still and running, fresh and brackish waters, and even on the land after the water has dried away. Of the species common in Europe, Batrachium aquatile is the most interesting object of study in respect to its variations. Its forms are numerous, but they may be arranged under two general heads. The first includes those varieties which have plain floating leaves and also divided, submerged leaves, and the second those which have only divided leaves under water. The forms of the former class occur principally in waters having little motion; the floating leaves are round and peltate, with fine slight scallops on the borders. In swifter waters the leaves are cut up into pointed lobes, the distinctness of which increases with the speed of the current. Dr. Behrens has distinguished about thirty varieties in different waters, most of which he has described and named in his paper. In the swiftest currents, the floating leaves disappear, and only the varieties of the second form—those having exclusively divided, submerged leaves—are found. Still another change comes over the Batrachium when the water is taken away from it. It grows up with a short stem, and is thickly covered with geeen leaves divided into numerous short, firm, succulent lobes. Dr. Behrens is preparing a work in which variations of this sort will be fully discussed, with illustrations.

 

Progress in Photography.—"Some Recent Advances in Photography" is the subject of a paper recently read before the Society of Arts by Captain Abney, R. E., F. R. S., in which reference was made to the more important improvements in the art that have been put in practice since 1875. A new negative process, called the gelatino-bromide process, offers decided advantages. It consists in the use of a gelatine emulsion of silver bromide for the sensitive surface. With a plate thus prepared, a photograph may now be taken in one second of time which it formerly took thirty seconds to secure; and a plate can be prepared which needs an exposure of only one sixtieth of a second, when a view is fairly lighted to secure a soft and harmonious negative. It makes instantaneous views possible under circumstances which were impossible, in illustration of which the speaker exhibited a view in which the shadow and reflection of a swallow passing in the air over a pond were perfectly represented. The plates and the development of pictures