WHAT ARE DIATOMS? 



201 



Owing to their freedom of motion they were at one time sup- 

 posed 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 com- 

 mon, 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 Rich- 

 mond, Va., and Berlin in Germany. It is often said 

 that the city of Berlin rests on a foundation of 



Fig. 1. Pleurosigma Fig. 2. Pinnularia 



FoRMOSUM. MAJOR. 



Fig. 3. Stauroneis 

 Phcenicenteron. 



Fig. 4. Navicula 



DlDYMA. 



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 



