Page:Popular Science Monthly Volume 41.djvu/213

 Owing to their freedom of motion they were at one time supposed to be animals. Now it is known that they are plants, as they can perform all the functions of plants, and no animal, with all his superiority, high nature, etc., is able to do this. They are found everywhere in all inhabited countries, and in fact all over the seas, so it may be readily granted that a plant so common and wide-spread as this should be quite familiar to every one.

Again, not only are the living plants so wide-spread and common, but the shells of the dead ones remain intact for many years; and in certain localities these tiny shells are so numerous as to form a large portion of the soil. Some of the best known of these localities are the sites of Richmond, Va., and Berlin in Germany. It is often said that the city of Berlin rests on a foundation of



diatom shells. The little plant dies and decays, leaving the shell, which retains its shape for many years. These cells are most beautifully marked with very delicate tracery. No tools can be made to perform such work as this. Some shells with the most regular forms of markings are used for testing lenses, such as Pleurosigma, shown in Fig. 1. Some of the most common forms are represented by Figs. 2, 3, and 4, while another less frequent and with more curious markings is shown in Fig. 5.

Now, though it is so easy to obtain large numbers of these plants—only a spoonful of mud from the bank of a stream or edge of a pool, a bit of sea-weed thrown up on the shore will contain